KEYPROV Working Group A. Doherty Internet-Draft RSA, The Security Division of EMC Intended status: Standards Track M. Pei Expires: December 24, 2008 Verisign, Inc. S. Machani Diversinet Corp. M. Nystrom RSA, The Security Division of EMC June 22, 2008 Dynamic Symmetric Key Provisioning Protocol (DSKPP) draft-ietf-keyprov-dskpp-04.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 24, 2008. Abstract DSKPP is a client-server protocol for initialization (and configuration) of symmetric keys to locally and remotely accessible cryptographic modules. The protocol can be run with or without private-key capabilities in the cryptographic modules, and with or without an established public-key infrastructure. Two variations of the protocol support multiple usage scenarios. Doherty, et al. Expires December 24, 2008 [Page 1] Internet-Draft DSKPP June 2008 With the four-pass variant, keys are mutually generated by the provisioning server and cryptographic module; provisioned keys are not transferred over-the-wire or over-the-air. The two-pass variant enables secure and efficient download and installation of pre- generated symmetric keys to a cryptographic module. This document builds on information contained in [RFC4758], adding specific enhancements in response to implementation experience and liaison requests. It is intended that this document or a successor version thereto will become the basis for subsequent progression of a symmetric key provisioning protocol specification on the standards track. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . 7 1.1.1. Single Key Request . . . . . . . . . . . . . . . . . 7 1.1.2. Multiple Key Requests . . . . . . . . . . . . . . . . 7 1.1.3. User Authentication . . . . . . . . . . . . . . . . . 7 1.1.4. Provisioning Time-Out Policy . . . . . . . . . . . . 7 1.1.5. Key Renewal . . . . . . . . . . . . . . . . . . . . . 8 1.1.6. Pre-Loaded Key Replacement . . . . . . . . . . . . . 8 1.1.7. Pre-Shared Manufacturing Key . . . . . . . . . . . . 8 1.1.8. End-to-End Protection of Key Material . . . . . . . . 9 1.2. Protocol Entities . . . . . . . . . . . . . . . . . . . . 9 1.3. Initiating DSKPP . . . . . . . . . . . . . . . . . . . . 10 1.4. Determining Which Protocol Variant to Use . . . . . . . . 11 1.4.1. Criteria for Using the Four-Pass Protocol . . . . . . 11 1.4.2. Criteria for Using the Two-Pass Protocol . . . . . . 12 1.5. Presentation Syntax . . . . . . . . . . . . . . . . . . . 12 1.5.1. Versions . . . . . . . . . . . . . . . . . . . . . . 12 1.5.2. Namespaces . . . . . . . . . . . . . . . . . . . . . 12 1.5.3. Identifiers . . . . . . . . . . . . . . . . . . . . . 13 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 13 2.3. Notation . . . . . . . . . . . . . . . . . . . . . . . . 15 2.4. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 16 3. DSKPP Protocol Details . . . . . . . . . . . . . . . . . . . 17 3.1. Four-Pass Protocol Usage . . . . . . . . . . . . . . . . 19 3.1.1. Message Flow . . . . . . . . . . . . . . . . . . . . 19 3.1.2. Generation of Symmetric Keys for Cryptographic Modules . . . . . . . . . . . . . . . . . . . . . . . 22 3.1.3. Encryption of Pseudorandom Nonces Sent from the DSKPP Client . . . . . . . . . . . . . . . . . . . . 25 3.1.4. MAC Calculations . . . . . . . . . . . . . . . . . . 25 Doherty, et al. Expires December 24, 2008 [Page 2] Internet-Draft DSKPP June 2008 3.2. Two-Pass Protocol Usage . . . . . . . . . . . . . . . . . 27 3.2.1. Message Flow . . . . . . . . . . . . . . . . . . . . 27 3.2.2. Key Protection Profiles . . . . . . . . . . . . . . . 30 3.2.3. MAC Calculations . . . . . . . . . . . . . . . . . . 34 3.3. Device Identification . . . . . . . . . . . . . . . . . . 35 3.4. User Authentication . . . . . . . . . . . . . . . . . . . 36 3.4.1. Authentication Data . . . . . . . . . . . . . . . . . 36 3.4.2. Authentication Code Format . . . . . . . . . . . . . 37 3.4.3. Authentication Data Calculation . . . . . . . . . . . 39 3.5. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF . . . 40 3.5.1. Introduction . . . . . . . . . . . . . . . . . . . . 40 3.5.2. Declaration . . . . . . . . . . . . . . . . . . . . . 41 4. DSKPP Message Formats . . . . . . . . . . . . . . . . . . . . 41 4.1. General XML Schema Requirements . . . . . . . . . . . . . 41 4.2. Components of the Message . . . . . . . 42 4.3. Components of the Request . . . . . 43 4.3.1. The DeviceIdentifierDataType Type . . . . . . . . . . 46 4.3.2. The ProtocolVariantsType Type . . . . . . . . . . . . 46 4.3.3. The KeyPackagesFormatType Type . . . . . . . . . . . 47 4.3.4. The AuthenticationDataType Type . . . . . . . . . . . 48 4.4. Components of the Response (Used Only in Four-Pass DSKPP) . . . . . . . . . . . . . . . . 48 4.5. Components of a Request (Used Only in Four-Pass DSKPP) . . . . . . . . . . . . . . . . 50 4.6. Components of a Response . . . . 51 4.7. The StatusCode Type . . . . . . . . . . . . . . . . . . . 53 5. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 55 5.1. The ClientInfoType Type . . . . . . . . . . . . . . . . . 55 5.2. The ServerInfoType Type . . . . . . . . . . . . . . . . . 55 6. Protocol Bindings . . . . . . . . . . . . . . . . . . . . . . 55 6.1. General Requirements . . . . . . . . . . . . . . . . . . 55 6.2. HTTP/1.1 Binding for DSKPP . . . . . . . . . . . . . . . 55 6.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 55 6.2.2. Identification of DSKPP Messages . . . . . . . . . . 56 6.2.3. HTTP Headers . . . . . . . . . . . . . . . . . . . . 56 6.2.4. HTTP Operations . . . . . . . . . . . . . . . . . . . 56 6.2.5. HTTP Status Codes . . . . . . . . . . . . . . . . . . 57 6.2.6. HTTP Authentication . . . . . . . . . . . . . . . . . 57 6.2.7. Initialization of DSKPP . . . . . . . . . . . . . . . 57 6.2.8. Example Messages . . . . . . . . . . . . . . . . . . 58 7. DSKPP Schema . . . . . . . . . . . . . . . . . . . . . . . . 58 8. Conformance Requirements . . . . . . . . . . . . . . . . . . 66 9. Security Considerations . . . . . . . . . . . . . . . . . . . 68 9.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 68 9.2. Active Attacks . . . . . . . . . . . . . . . . . . . . . 68 9.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 68 9.2.2. Message Modifications . . . . . . . . . . . . . . . . 68 9.2.3. Message Deletion . . . . . . . . . . . . . . . . . . 70 Doherty, et al. Expires December 24, 2008 [Page 3] Internet-Draft DSKPP June 2008 9.2.4. Message Insertion . . . . . . . . . . . . . . . . . . 70 9.2.5. Message Replay . . . . . . . . . . . . . . . . . . . 70 9.2.6. Message Reordering . . . . . . . . . . . . . . . . . 71 9.2.7. Man-in-the-Middle . . . . . . . . . . . . . . . . . . 71 9.3. Passive Attacks . . . . . . . . . . . . . . . . . . . . . 71 9.4. Cryptographic Attacks . . . . . . . . . . . . . . . . . . 71 9.5. Attacks on the Interaction between DSKPP and User Authentication . . . . . . . . . . . . . . . . . . . . . 71 9.6. Miscellaneous Considerations . . . . . . . . . . . . . . 72 9.6.1. Client Contributions to K_TOKEN Entropy . . . . . . . 72 9.6.2. Key Confirmation . . . . . . . . . . . . . . . . . . 73 9.6.3. Server Authentication . . . . . . . . . . . . . . . . 73 9.6.4. User Authentication . . . . . . . . . . . . . . . . . 73 9.6.5. Key Protection in Two-Pass DSKPP . . . . . . . . . . 74 10. Internationalization Considerations . . . . . . . . . . . . . 74 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 75 11.1. URN Sub-Namespace Registration . . . . . . . . . . . . . 75 11.2. XML Schema Registration . . . . . . . . . . . . . . . . . 75 11.3. MIME Media Type Registration . . . . . . . . . . . . . . 76 12. Intellectual Property Considerations . . . . . . . . . . . . 76 13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 77 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 77 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 78 15.1. Normative references . . . . . . . . . . . . . . . . . . 78 15.2. Informative references . . . . . . . . . . . . . . . . . 78 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 80 A.1. Trigger Message . . . . . . . . . . . . . . . . . . . . . 81 A.2. Four-Pass Protocol . . . . . . . . . . . . . . . . . . . 81 A.2.1. Without a Preceding Trigger . . 82 A.2.2. Assuming a Preceding Trigger . . 83 A.2.3. Without a Preceding Trigger . . 84 A.2.4. Assuming a Preceding Trigger . . 85 A.2.5. Using Default Encryption . . . . 85 A.2.6. Using Default Encryption . . 87 A.3. Two-Pass Protocol . . . . . . . . . . . . . . . . . . . . 87 A.3.1. Example Using the Key Transport Profile . . . . . . . 87 A.3.2. Example Using the Key Wrap Profile . . . . . . . . . 90 A.3.3. Example Using the Passphrase-Based Key Wrap Profile . 93 Appendix B. Integration with PKCS #11 . . . . . . . . . . . . . 96 B.1. The 4-pass Variant . . . . . . . . . . . . . . . . . . . 96 B.2. The 2-pass Variant . . . . . . . . . . . . . . . . . . . 96 Appendix C. Example of DSKPP-PRF Realizations . . . . . . . . . 99 C.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 99 C.2. DSKPP-PRF-AES . . . . . . . . . . . . . . . . . . . . . . 99 C.2.1. Identification . . . . . . . . . . . . . . . . . . . 99 C.2.2. Definition . . . . . . . . . . . . . . . . . . . . . 99 C.2.3. Example . . . . . . . . . . . . . . . . . . . . . . . 100 C.3. DSKPP-PRF-SHA256 . . . . . . . . . . . . . . . . . . . . 101 Doherty, et al. Expires December 24, 2008 [Page 4] Internet-Draft DSKPP June 2008 C.3.1. Identification . . . . . . . . . . . . . . . . . . . 101 C.3.2. Definition . . . . . . . . . . . . . . . . . . . . . 101 C.3.3. Example . . . . . . . . . . . . . . . . . . . . . . . 102 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 102 Intellectual Property and Copyright Statements . . . . . . . . . 104 Doherty, et al. Expires December 24, 2008 [Page 5] Internet-Draft DSKPP June 2008 1. Introduction A symmetric key cryptographic module provides data authentication and encryption services to software (or firmware) applications hosted on hardware devices, such as personal computers, handheld mobile phones, one-time password tokens, USB flash drives, tape drives, etc. Until recently, provisioning symmetric keys to these modules has been labor intensive, involving manual operations that are device-specific, and inherently error-prone. Fortunately, an increasing number of hardware devices enable programmatic initialization of their applications. For example, a U3-ready thumb drive lets users load and configure applications locally through a USB port on their PC. Other hardware devices, such as Personal Digital Assistant (PDA) phones, allow users to load and configure applications over-the-air. Likewise, programmable cryptographic modules enable key issuers to provision symmetric keys via the Internet, whether over-the-wire or over-the-air. This document describes the Dynamic Symmetric Key Provisioning Protocol (DSKPP), which leverages these recent technological developments. DSKPP provides an open and interoperable mechanism for initializing and configuring symmetric keys to cryptographic modules that are accessible over the Internet. The description is based on the information contained in RFC4758, and contains specific enhancements, such as User Authentication and support for the [PSKC] format for transmission of keying material. DSKPP is a client-server protocol with two variations. One variation establishes a symmetric key by mutually authenticated key agreement. The other variation relies on key distribution. In the former case, key agreement enables two parties (a cryptographic module and key provisioning server) to establish a symmetric cryptographic key using an exchange of four messages, such that the key is not transported over the Internet. In the latter case, key distribution enables a key provisioning server to transport a symmetric key to a cryptographic module over the Internet using an exchange of two messages. In either case, DSKPP is flexible enough to be run with or without private-key capability in the cryptographic module, and with or without an established public-key infrastructure. All DSKPP communications consist of pairs of messages: a request and a response. Each pair is called an "exchange", and each message sent in an exchange is called a "pass". Thus, an implementation of DSKPP that relies on mutually authenticated key agreement is called the "four-pass protocol"; an implementation of DSKPP that relies on key distribution is called the "two-pass protocol". Doherty, et al. Expires December 24, 2008 [Page 6] Internet-Draft DSKPP June 2008 DSKPP message flow always consists of a request followed by a response. It is the responsibility of the client to ensure reliability. If the response is not received with a timeout interval, the client needs to retransmit the request (or abandon the connection). Number of retries and lengths of timeouts are not covered in this document because they do not affect interoperability. 1.1. Usage Scenarios DSKPP is expected to be used to provision symmetric keys to cryptographic modules in a number of different scenarios, each with its own special requirements. 1.1.1. Single Key Request The usual scenario is that a cryptographic module makes a request for a symmetric key from a provisioning server that is located on the local network or somewhere on the Internet. Depending upon the deployment scenario, the provisioning server may generate a new key on-the-fly or use a pre-generated key, e.g., one provided by a legacy back-end issuance server. The provisioning server assigns a unique key ID to the symmetric key and provisions it to the cryptographic module. 1.1.2. Multiple Key Requests A cryptographic module makes multiple requests for symmetric keys from the same provisioning server. The symmetric keys need not be of the same type, i.e., the keys may be used with different symmetric key cryptographic algorithms, including one-time password authentication algorithms, and the AES encryption algorithm. 1.1.3. User Authentication In some deployment scenarios, a key issuer may rely on a third party provisioning service. In this case, the issuer directs provisioning requests from the cryptographic module to the provisioning service. As such, it is the responsibility of the issuer to authenticate the user through some out-of-band means before granting him rights to acquire keys. Once the issuer has granted those rights, the issuer provides an authentication code to the user and makes it available to the provisioning service, so that the user can prove that he is authorized to acquire keys. 1.1.4. Provisioning Time-Out Policy An issuer may provide a time-limited authentication code to a user during registration, which the user will input into the cryptographic Doherty, et al. Expires December 24, 2008 [Page 7] Internet-Draft DSKPP June 2008 module to authenticate themselves with the provisioning server. The server will allow a key to be provisioned to the cryptographic module hosted by the user's device when user authentication is required only if the user inputs a valid authentication code within the fixed time period established by the issuer. 1.1.5. Key Renewal A cryptographic module requests renewal of a symmetric key using the same key ID already associated with the key. Such a need may occur in the case when a user wants to upgrade her device that houses the cryptographic module or when a key has expired. When a user uses the same cryptographic module to, for example, perform strong authentication at multiple Web login sites, keeping the same key ID removes the need for the user to register a new key ID at each site. 1.1.6. Pre-Loaded Key Replacement This scenario represents a special case of symmetric key renewal in which a local administrator can authenticate the user procedurally before initiating the provisioning process. It also allows for a device issuer to pre-load a key onto a cryptographic module with a restriction that the key is replaced with a new key prior to use of the cryptographic module. Another variation of this scenario is the organization who recycles devices. In this case, a key issuer would provision a new symmetric key to a cryptographic module hosted on a device that was previously owned by another user. Note that this usage scenario is essentially the same as the last scenario wherein the same key ID is used for renewal. 1.1.7. Pre-Shared Manufacturing Key A cryptographic module is loaded onto a smart card after the card is issued to a user. The symmetric key for the cryptographic module will then be provisioned using a secure channel mechanism present in many smart card platforms. This allows a direct secure channel to be established between the smart card chip and the provisioning server. For example, the card commands (i.e., Application Protocol Data Units, or APDUs) are encrypted with a pre-issued card manufacturer's key and sent directly to the smart card chip, allowing secure post- issuance in-the-field provisioning. This secure flow can pass Transport Layer Security (TLS) and other transport security boundaries. Note that two pre-conditions for this usage scenario are for the protocol to be tunneled and the provisioning server to know the correct pre-established manufacturer's key. Doherty, et al. Expires December 24, 2008 [Page 8] Internet-Draft DSKPP June 2008 1.1.8. End-to-End Protection of Key Material In this scenario, transport layer security does not provide end-to- end protection of keying material transported from the provisioning server to the cryptographic module. For example, TLS may terminate at an application hosted on a PC rather than at the cryptographic module (i.e., the endpoint) located on a data storage device. Mutually authenticated key agreement provides end-to-end protection, which TLS cannot provide. 1.2. Protocol Entities In principle, the protocol involves a DSKPP client and a DSKPP server. The DSKPP client manages communication between the cryptographic module and the provisioning server. In this document, the DSKPP server represents the provisioning server. A high-level object model that describes the client-side entities and how they relate to each other is shown in Figure 1. Conceptually, each entity is represented by the definitions found in Section 2.2. Doherty, et al. Expires December 24, 2008 [Page 9] Internet-Draft DSKPP June 2008 ----------- ------------- | User | | Device | |---------|* owns *|-----------| | UserID |--------->| DeviceID | | ... | | ... | ----------- ------------- | 1 | | contains | | * V -------------------------- |Cryptographic Module | |------------------------| |Crypto Module ID | |Security Attribute List | |... | -------------------------- | 1 | | contains | | * V ----------------------- |Key Package | |---------------------| |Key ID | |Key Type | |... | ----------------------- Figure 1: Object Model It is assumed that a device will host an application layered above the cryptographic module, and this application will manage communication between the DSKPP client and cryptographic module. The manner in which the communicating application will transfer DSKPP protocol elements to and from the cryptographic module is transparent to the DSKPP server. One method for this transfer is described in [CT-KIP-P11]. 1.3. Initiating DSKPP To initiate DSKPP: Doherty, et al. Expires December 24, 2008 [Page 10] Internet-Draft DSKPP June 2008 1. A server may first send a DSKPP trigger message to a client application (e.g., in response to a user browsing to a Web site that requires a symmetric key for authentication), although this step is optional. 2. A client application calls on the DSKPP client to send a symmetric key request to a DSKPP server, thus beginning a DSKPP protocol run. One of the following actions may be used to contact a DSKPP server: 1. A user may indicate how the DSKPP client is to contact a certain DSKPP server during a browsing session. 2. A DSKPP client may be pre-configured to contact a certain DSKPP server. 3. A user may be informed out-of-band about the location of the DSKPP server. Once the location of the DSKPP server is known, the DSKPP client and the DSKPP server engage in a 4-pass or 2-pass protocol. 1.4. Determining Which Protocol Variant to Use The four-pass and two-pass protocols are appropriate in different deployment scenarios, as described in the sub-sections below. The biggest differentiator between the two is that the two-pass protocol supports transport of an existing key to a cryptographic module, while the four-pass involves key generation on-the-fly via key agreement. In either case, both protocol variants support algorithm agility through negotiation of encryption mechanisms and key types at the beginning of each protocol run. 1.4.1. Criteria for Using the Four-Pass Protocol The four-pass protocol is needed under one or more of the following conditions: o Policy requires that both parties engaged in the protocol jointly contribute entropy to the key. Enforcing this policy mitigates the risk of exposing a key during the provisioning process as the key is generated through mutual agreement without being transferred over-the-air or over-the-wire. It also mitigates risk of exposure after the key is provisioned, as the key will be not be vulnerable to a single point of attack in the system. o A cryptographic module does not have private-key capabilities. o The cryptographic module is hosted by a device that was neither pre-issued with a manufacturer's key or other form of pre-shared key (as might be the case with a smart card or SIM card) nor has a keypad that can be used for entering a passphrase (such as present Doherty, et al. Expires December 24, 2008 [Page 11] Internet-Draft DSKPP June 2008 on a mobile phone). 1.4.2. Criteria for Using the Two-Pass Protocol The two-pass protocol is needed under one or more of the following conditions: o Pre-existing (i.e., legacy) keys must be provisioned via transport to the cryptographic module. o The cryptographic module is hosted on a device that was pre-issued with a manufacturer's key (such as may exist on a smart card), or other form of pre-shared key (such as may exist on a SIM-card), and is capable of performing private-key operations. o The cryptographic module is hosted by a device that has a built-in keypad with which a user may enter a passphrase, useful for deriving a key wrapping key for distribution of keying material. 1.5. Presentation Syntax This documents presents DSKPP message formats and data elements using XML syntax. The main goal in using this syntax is to document DSKPP. Application of the syntax beyond this goal is OPTIONAL. 1.5.1. Versions There is a provision made in the syntax for an explicit version number. Only version "1.0" is currently specified. 1.5.2. Namespaces The XML namespace [XMLNS] URN that MUST be used by implementations of this syntax is: xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:dskpp:1.0" References to qualified elements in the DSKPP schema defined herein use the prefix "dskpp". This document relies on qualified elements already defined in the Portable Symmetric Key Container [PSKC] namespace, which is represented by the prefix "pskc" and declared as: xmlns:pskc="urn:ietf:params:xml:ns:keyprov:container:1.0" Finally, the DSKPP syntax presented in this document relies on algorithm identifiers defined in the XML Signature [XMLDSIG] namespace: Doherty, et al. Expires December 24, 2008 [Page 12] Internet-Draft DSKPP June 2008 xmlns:ds="http://www.w3.org/2000/09/xmldsig#" References to algorithm identifiers in the XML Signature namespace are represented by the prefix "ds". 1.5.3. Identifiers This document uses Uniform Resource Identifiers [RFC2396] to identify resources, algorithms, and semantics. 2. Terminology 2.1. Key Words The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2.2. Definitions The definitions provided below are defined as used in this document. The same terms may be defined differently in other documents. Authentication Code (AC): Client Authentication Code comprised of a string of numeric characters known to the device and the server and containing an identifier and a password Authentication Data (AD): Client Authentication Data that may be derived from the Authentication Code (AC) Cryptographic Module: A component of an application, which enables symmetric key cryptographic functionality CryptoModule ID: A unique identifier for an instance of the cryptographic module Device: A physical piece of hardware, or a software framework, that hosts symmetric key cryptographic modules Device ID (DeviceID): A unique identifier for the device DSKPP Client: Manages communication between the symmetric key cryptographic module and the DSKPP server Doherty, et al. Expires December 24, 2008 [Page 13] Internet-Draft DSKPP June 2008 DSKPP Server: The symmetric key provisioning server that participates in the DSKPP protocol run DSKPP Server ID (ServerID): The unique identifier of a DSKPP server Issuer: See "Key Issuer" Key Issuer: An organization that issues symmetric keys to end-users Key Package (KP): An object that encapsulates a symmetric key and its configuration data Key Package Header (KPH): Information about the Key Package, useful for two-pass DSKPP, e.g., the passing the ServerID and the Key Protection Method Key ID (KeyID): A unique identifier for the symmetric key Key Protection Method (KPM): The key transport method used during two-pass DSKPP Key Protection Method List (KPML): The list of key protection methods supported by a cryptographic module Key Provisioning Server: A lifecycle management system that provides a key issuer with the ability to provision keys to cryptographic modules hosted on end-users' devices Key Transport: A key establishment procedure whereby the DSKPP server selects and encrypts the keying material and then sends the material to the DSKPP client [NIST-SP800-57] Key Transport Key: The private key that resides on the cryptographic module. This key is paired with the DSKPP client's public key, which the DSKPP server uses to encrypt keying material during key transport [NIST-SP800-57] Key Type: The type of symmetric key cryptographic methods for which the key will be used (e.g., OATH HOTP or RSA SecurID authentication, AES encryption, etc.) Key Wrapping: A method of encrypting keys for key transport [NIST-SP800-57] Doherty, et al. Expires December 24, 2008 [Page 14] Internet-Draft DSKPP June 2008 Key Wrapping Key: A symmetric key encrypting key used for key wrapping [NIST-SP800-57] Keying Material: The data necessary (e.g., keys and key configuration data) necessary to establish and maintain cryptographic keying relationships [NIST-SP800-57] Manufacturer's Key A unique master key pre-issued to a hardware device, e.g., a smart card, during the manufacturing process. If present, this key may be used by a cryptographic module to derive secret keys Provisioning Service: See "Key Provisioning Server" Security Attribute List (SAL): A payload that contains the DSKPP version, DSKPP variation (four- or two-pass), key package formats, key types, and cryptographic algorithms that the cryptographic module is capable of supporting Security Context (SC): A payload that contains the DSKPP version, DSKPP variation (four- or two-pass), key package format, key type, and cryptographic algorithms relevant to the current protocol run User: The person or client to whom devices are issued User ID: A unique identifier for the user or client 2.3. Notation || String concatenation [x] Optional element x A ^ B Exclusive-OR operation on strings A and B (where A and B are of equal length) A typographical convention used in the body of the text DSKPP-PRF(k,x,l) A keyed pseudo-random function (see Section 3.5) E(k,m) Encryption of m with the key k Doherty, et al. Expires December 24, 2008 [Page 15] Internet-Draft DSKPP June 2008 K Key used to encrypt R_C (either K_SERVER or K_SHARED), or in MAC or DSKPP_PRF computations K_AC Secret key that is derived from the Authentication Code and used for user authentication purposes K_MAC Secret key derived during a DSKPP exchange for use with key confirmation K_MAC' A second secret key used for server authentication K_PROV A provisioning master key from which two keys are derived: K_TOKEN and K_MAC K_SERVER Public key of the DSKPP server; used for encrypting R_C in the four-pass protocol variant K_SHARED Secret key that is pre-shared between the DSKPP client and the DSKPP server; used for encrypting R_C in the four-pass protocol variant K_TOKEN Secret key that is established in a cryptographic module using DSKPP R Pseudorandom value chosen by the DSKPP client and used for MAC computations R_C Pseudorandom value chosen by the DSKPP client and used as input to the generation of K_TOKEN R_S Pseudorandom value chosen by the DSKPP server and used as input to the generation of K_TOKEN R_TRIGGER Pseudorandom value chosen by the DSKPP server and used as input in a trigger message. URL_S DSKPP server address, as a URL 2.4. Abbreviations AC Authentication Code AD Authentication Data DSKPP Dynamic Symmetric Key Provisioning Protocol Doherty, et al. Expires December 24, 2008 [Page 16] Internet-Draft DSKPP June 2008 HTTP Hypertext Transfer Protocol KP Key Package KPH Key Package Header KPM Key Protection Method KPML Key Protection Method List MAC Message Authentication Code PC Personal Computer PDU Protocol Data Unit PKCS Public-Key Cryptography Standards PRF Pseudo-Random Function PSKC Portable Symmetric Key Container SAL Security Attribute List (see Section 2.2) SC Security Context (see Section 2.2) TLS Transport Layer Security URL Uniform Resource Locator USB Universal Serial Bus XML eXtensible Markup Language 3. DSKPP Protocol Details DSKPP enables symmetric key provisioning between a DSKPP server and DSKPP client. The DSKPP protocol supports the request and response messages shown in Figure 2. These messages are described below. +---------------+ +---------------+ | | | | | DSKPP Client | | DSKPP Server | | | | | +---------------+ +---------------+ | | | [ <--------- --------- ] | | | | ------- -------> | | (Applicable to 4- and 2-pass) | | | | <------ -------- | | (Applicable to 4-pass only) | | | | ------- -------> | | (Applicable to 4-pass only) | | | | <---- ------- | | (Applicable to 4- and 2-pass) | | | Figure 2: The DSKPP protocol (with OPTIONAL preceding trigger) Doherty, et al. Expires December 24, 2008 [Page 17] Internet-Draft DSKPP June 2008 []: A DSKPP server may initiate the DSKPP protocol by sending a message. For example, this message may be sent in response to a user requesting a symmetric key in a browsing session. The trigger message always contains a nonce to allow the server to couple the trigger with a later request. : With this request, a DSKPP client initiates contact with the DSKPP server, indicating which protocol versions and variations (four-pass or two-pass), key types, encryption and MAC algorithms that it supports. In addition, the request may include client authentication data that the DSKPP server uses to verify proof-of-possession of the device. : Upon receiving a request, the DSKPP server uses the response to specify which protocol version and variation, key type, encryption algorithm, and MAC algorithm that will be used by the DSKPP server and DSKPP client during the protocol run. The decision of which variation, key type, and cryptographic algorithms to pick is policy- and implementation-dependent and therefore outside the scope of this document. The response includes the DSKPP server's random nonce, R_S. The response also consists of information about either a shared secret key, or its own public key, that the DSKPP client uses when sending its protected random nonce, R_C, in the request (see below). Optionally, the DSKPP server may provide a MAC that the DSKPP client may use for server authentication. : With this request, a DSKPP client and DSKPP server securely exchange protected data, e.g., the protected random nonce R_C. In addition, the request may include user authentication data that the DSKPP server uses to verify proof- of-possession of the device. : The response is a confirmation message that includes a key package that holds configuration data, and may also contain protected keying material (this depends on the protocol variation, as discussed below). Optionally, the DSKPP server may provide a MAC that the DSKPP client may use for server authentication. Doherty, et al. Expires December 24, 2008 [Page 18] Internet-Draft DSKPP June 2008 3.1. Four-Pass Protocol Usage This section describes the message flow and methods that comprise the four-pass protocol variant. 3.1.1. Message Flow The four-pass protocol flow consists of two message exchanges: 1: Pass 1 = , Pass 2 = 2: Pass 3 = , Pass 4 = The first pair of messages negotiate cryptographic algorithms and exchange nonces. The second pair of messages establishes a symmetric key using mutually authenticated key agreement. The DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user. To do this, the DSKPP server MAY couple an initial user authentication to the DSKPP execution using one of the mechanisms described in Section 3.4. The purpose and content of each message are described below, including the optional . DSKPP Client DSKPP Server ------------ ------------ [<---] R_TRIGGER, [DeviceID], [KeyID], [URL_S] The DSKPP server optionally sends a message to the DSKPP client. The trigger message MUST contain a nonce, R_TRIGGER, to allow the server to couple the trigger with a later request. MAY include a DeviceID to allow the client to select the device with which it will communicate (for more information about device identification, refer to Section 3.3). In the case of key renewal, MAY include the identifier for the key, KeyID, that is being replaced. Finally, the trigger MAY contain a URL for the DSKPP client to use when contacting the DSKPP server. DSKPP Client DSKPP Server ------------ ------------ SAL, [R_TRIGGER], [DeviceID], [KeyID] ---> The DSKPP client sends a message to the DSKPP server. This message MUST contain a Security Attribute List (SAL), Doherty, et al. Expires December 24, 2008 [Page 19] Internet-Draft DSKPP June 2008 identifying which DSKPP versions, protocol variations (in this case "four-pass"), key package formats, key types, encryption and MAC algorithms that the client supports. In addition, if a trigger message preceded , then it passes the parameters received in back to the DSKPP Server. In particular, it MUST include R_TRIGGER so that the DSKPP server can associate the client with the trigger message, and SHOULD include DeviceID and KeyID. DSKPP Client DSKPP Server ------------ ------------ <--- SC, R_S, [K], [MAC] The DSKPP server responds to the DSKPP client with a message, whose Status attribute is set to a return code for . If Status is not "Continue", only the Status and Version attributes will be present, and the DSKPP client MUST abort the protocol. If Status is set to "Continue", then the message MUST include a Security Context (SC). The DSKPP client will use the SC to select the DSKPP version and variation (e.g., four-pass), type of key to generate, and cryptographic algorithms that it will use for the remainder of the protocol run. MUST also include the server's random nonce, R_S, whose length may depend on the selected key type. In addition, the message MAY provide K, which represents its own public key (K_SERVER) or information about a shared secret key (K_SHARED) to use for encrypting the cryptographic module's random nonce (see description of below). Optionally, MAY include a MAC that the DSKPP client can use for server authentication in the case of key renewal (Section 3.1.4.1 describes how to calculate the MAC). DSKPP Client DSKPP Server ------------ ------------ E(K,R_C), [AD] ---> Based on the Security Context (SC) provided in the message, the cryptographic module generates a random nonce, R_C. The length of the nonce R_C will depend on the selected key type. The cryptographic module encrypts R_C using the selected encryption algorithm and with a key, K, that is either the DSKPP server's public key, K_SERVER, or a shared secret key, K_SHARED, as indicated by the DSKPP server. Note: If K is equivalent to K_SERVER, then the cryptographic module SHOULD verify the server's certificate before using it to encrypt R_C in accordance with [RFC3280]. Doherty, et al. Expires December 24, 2008 [Page 20] Internet-Draft DSKPP June 2008 Note: If successful execution of the protocol will result in the replacement of an existing key with a newly generated one, the DSKPP client MUST verify the MAC provided in the message. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and MUST delete any nonces, keys, and/or secrets associated with the failed run. The DSKPP client MUST send the encrypted random nonce to the DSKPP server in a message, and MAY include client Authentication Data (AD), such as a MAC derived from an authentication code and R_C (refer to Section 3.4.1). Finally, the cryptographic module calculates and stores a symmetric key, K_TOKEN, of the key type specified in the SC received in (refer to Section 3.1.2.2. for a description of how K_TOKEN is generated). DSKPP Client DSKPP Server ------------ ------------ <--- KP, MAC If Authentication Data (AD) was received in the message, then the DSKPP server MUST authenticate the user in accordance with Section 3.4.1. If authentication fails, then DSKPP server MUST abort. Otherwise, the DSKPP server decrypts R_C, calculates K_TOKEN from the combination of the two random nonces R_S and R_C, the encryption key K, and possibly some other data (refer to Section 3.1.2.2 for a description of how K_TOKEN is generated). The server then associates K_TOKEN with the cryptographic module in a server-side data store. The intent is that the data store later on will be used by some service that needs to verify or decrypt data produced by the cryptographic module and the key. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . The confirmation message MUST include a Key Package (KP) that holds an identifier for the generated key (but not the key itself) and additional configuration information, e.g., the identity of the DSKPP server. The default symmetric key package format is based on the Portable Symmetric Key Container (PSKC) defined in [PSKC]. Alternative formats MAY include [SKPC-ASN.1], PKCS#12 [PKCS-12], or PKCS#5 XML [PKCS-5-XML] format. In addition to a Key Package, MUST also include a MAC that the DSKPP client will use to authenticate the message before committing K_TOKEN. After receiving a message with Status = "Success", the DSKPP client MUST verify the MAC. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and Doherty, et al. Expires December 24, 2008 [Page 21] Internet-Draft DSKPP June 2008 MUST, in this case, also delete any nonces, keys, and/or secrets associated with the failed run of the protocol. If has Status = "Success" and the MAC was verified, then the DSKPP client MUST associate the provided key package with the generated key K_TOKEN, and store this data permanently. After this operation, it MUST NOT be possible to overwrite the key unless knowledge of an authorizing key is proven through a MAC on a later (and ) message. 3.1.2. Generation of Symmetric Keys for Cryptographic Modules With 4-pass DSKPP, the symmetric key that is the target of provisioning, is generated on-the-fly without being transferred between the DSKPP client and DSKPP server. A sample data flow depicting how this works followed by computational information are provided in the subsections below. 3.1.2.1. Data Flow A sample data flow showing key generation during the 4-pass protocol is shown in Figure 3. Doherty, et al. Expires December 24, 2008 [Page 22] Internet-Draft DSKPP June 2008 +----------------------+ +-------+ +----------------------+ | +------------+ | | | | | | | Server key | | | | | | | +<-| Public |------>------------->-------------+---------+ | | | | Private | | | | | | | | | | +------------+ | | | | | | | | | | | | | | | | | | V V | | | | V V | | | +---------+ | | | | +---------+ | | | | | Decrypt |<-------<-------------<-----------| Encrypt | | | | | +---------+ | | | | +---------+ | | | | | +--------+ | | | | ^ | | | | | | Server | | | | | | | | | | | | Random |--->------------->------+ +----------+ | | | | | +--------+ | | | | | | Client | | | | | | | | | | | | | Random | | | | | | | | | | | | +----------+ | | | | | | | | | | | | | | | | V V | | | | V V | | | | +------------+ | | | | +------------+ | | | +-->| DSKPP PRF | | | | | | DSKPP PRF |<----+ | | +------------+ | | | | +------------+ | | | | | | | | | | V | | | | V | | +-------+ | | | | +-------+ | | | Key | | | | | | Key | | | +-------+ | | | | +-------+ | | +-------+ | | | | +-------+ | | |Key Id |-------->------------->------|Key Id | | | +-------+ | | | | +-------+ | +----------------------+ +-------+ +----------------------+ DSKPP Server DSKPP Client DSKPP Client (PC Host) (cryptographic module) Figure 3: Principal data flow for DSKPP key generation - using public server key Note: Conceptually, although R_C is one pseudorandom string, it may be viewed as consisting of two components, R_C1 and R_C2, where R_C1 is generated during the protocol run, and R_C2 can be pre-generated and loaded on the cryptographic module before the device is issued to the user. In that case, the latter string, R_C2, SHOULD be unique for each cryptographic module. The inclusion of the two random nonces R_S and R_C in the key generation provides assurance to both sides (the cryptographic module and the DSKPP server) that they have contributed to the key's randomness and that the key is unique. The inclusion of the Doherty, et al. Expires December 24, 2008 [Page 23] Internet-Draft DSKPP June 2008 encryption key K ensures that no man-in-the-middle may be present, or else the cryptographic module will end up with a key different from the one stored by the legitimate DSKPP server. Note: A man-in-the-middle (in the form of corrupt client software or a mistakenly contacted server) may present his own public key to the cryptographic module. This will enable the attacker to learn the client's version of K_TOKEN. However, the attacker is not able to persuade the legitimate server to derive the same value for K_TOKEN, since K_TOKEN is a function of the public key involved, and the attacker's public key must be different than the correct server's (or else the attacker would not be able to decrypt the information received from the client). Therefore, once the attacker is no longer "in the middle," the client and server will detect that they are "out of sync" when they try to use their keys. In the case of encrypting R_C with K_SERVER, it is therefore important to verify that K_SERVER really is the legitimate server's key. One way to do this is to independently validate a newly generated K_TOKEN against some validation service at the server (e.g. using a connection independent from the one used for the key generation). 3.1.2.2. Computing the Symmetric Key In DSKPP, K_TOKEN and K_MAC are generated using the DSKPP-PRF function defined in Section 3.5, a secret random value R_C chosen by the DSKPP client, a random value R_S chosen by the DSKPP server, and the key K used to encrypt R_C. The input parameter s of DSKPP-PRF is set to the concatenation of the (ASCII) string "Key generation", K, and R_S. The input parameter dsLen is set to the desired length of the key, K_PROV, whose first half constitutes K_MAC and second half constitutes K_TOKEN. The combined length is determined by the type of K_TOKEN and K_MAC: dsLen = (desired length of K_PROV, i.e., the combined length of K_TOKEN and K_MAC) K_PROV = DSKPP-PRF (R_C, "Key generation" || K || R_S, dsLen) Then K_TOKEN and K_MAC derived from K_PROV, where K_PROV = K_MAC || K_TOKEN When computing K_PROV, the derived keys, K_MAC and K_TOKEN, MAY be subject to an algorithm-dependent transform before being adopted as a key of the selected type. One example of this is the need for parity in DES keys. Doherty, et al. Expires December 24, 2008 [Page 24] Internet-Draft DSKPP June 2008 3.1.3. Encryption of Pseudorandom Nonces Sent from the DSKPP Client DSKPP client random nonce(s) are either encrypted with the public key provided by the DSKPP server or by a shared secret key. For example, in the case of a public RSA key, an RSA encryption scheme from PKCS #1 [PKCS-1] MAY be used. In the case of a shared secret key, to avoid dependence on other algorithms, the DSKPP client MAY use the DSKPP-PRF function described herein with the shared secret key K_SHARED as input parameter K (in this case, K_SHARED SHOULD be used solely for this purpose), the concatenation of the (ASCII) string "Encryption" and the server's nonce R_S as input parameter s, and dsLen set to the length of R_C: dsLen = len(R_C) DS = DSKPP-PRF(K_SHARED, "Encryption" || R_S, dsLen) This will produce a pseudorandom string DS of length equal to R_C. Encryption of R_C MAY then be achieved by XOR-ing DS with R_C: E(DS, R_C) = DS ^ R_C The DSKPP server will then perform the reverse operation to extract R_C from E(DS, R_C). 3.1.4. MAC Calculations 3.1.4.1. Server Authentication in the Case of Key Renewal A MAC MUST be present in the message if the DSKPP run will result in the replacement of an existing key with a new one, as proof that the DSKPP server is authenticated to perform the action. When the MAC value is used for server authentication, the value MAY be computed by using the DSKPP-PRF function of Section 3.5, in which case the input parameter s MUST be set to the concatenation of the (ASCII) string "MAC 1 computation", R (if sent by the client), and R_S, and K MUST be set to the existing MAC key K_MAC' (i.e., the value of the MAC key that existed before this protocol run). Note that the implementation may specify K_MAC' to be the value of the K_TOKEN that is being replaced, or a version of K_MAC from the previous protocol run. The input parameter dsLen MUST be set to the length of R_S: dsLen = len(R_S) MAC = DSKPP-PRF (K_MAC', "MAC 1 computation" || [R ||] R_S, dsLen) Doherty, et al. Expires December 24, 2008 [Page 25] Internet-Draft DSKPP June 2008 The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes. 3.1.4.2. Key Confirmation To avoid a false "Commit" message causing the cryptographic module to end up in an initialized state in which the server does not recognize the stored key, messages MUST be authenticated with a MAC, calculated as follows: msg_hash = SHA-256(msg_1, ..., msg_n) dsLen = len(msg_hash) MAC = DSKPP-PRF (K_MAC, "MAC 2 computation" || msg_hash, dsLen) where MAC The MAC MUST be calculated using the already established MAC algorithm and MUST be computed on the (ASCII) string "MAC 2 computation" and msg_hash using the existing the MAC key K_MAC. K_MAC The key derived from K_PROV, as described in Section 3.1.2.2. msg_hash The message hash, defined below, of messages msg_1, ..., msg_n. If DSKPP-PRF (defined in Section 3.5) is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 2 computation" and msg_hash, and the parameter dsLen MUST be set to the length of msg_hash. 3.1.4.3. Message Hash Algorithm To compute a message hash for a MAC, given a sequence of DSKPP messages msg_1, ..., msg_n, the following operations MUST be carried out: a. The sequence of messages contains all DSKPP Request and Response messages up to but not including this message. b. Re-transmitted messages are removed from the sequence of messages. Note: The resulting sequence of messages MUST be an alternating sequence of DSKPP Request and DSKPP Response messages Doherty, et al. Expires December 24, 2008 [Page 26] Internet-Draft DSKPP June 2008 c. The contents of each message is concatenated together. d. The resulting string is hashed using SHA-256 in accordance with [FIPS180-SHA]. 3.2. Two-Pass Protocol Usage This section describes the message flow and methods that comprise the two-pass protocol variant. Two-pass DSKPP is essentially a transport of keying material from the DSKPP server to the DSKPP client. The keying material is contained in a package that is formatted in such a way that ensures that the symmetric key that is being established, K_TOKEN, is not exposed to any other entity than the DSKPP server and the cryptographic module itself. To ensure the keying material is adequately protected for all two-pass usage scenarios, the key package format MUST support the following key protection methods, as defined in Section 3.2.2: Key Transport This profile is intended for PKI-capable devices. Key transport is carried out using the public key of the DSKPP client, whose private key part resides in the cryptographic module as the key transport key. Key Wrap This profile is ideal for pre-keyed devices, e.g., SIM cards. Key wrap is carried out using a key wrapping key, which is known in advance by both the cryptographic module and the DSKPP server. Passphrase-Based Key Wrap This profile is a variation of the Key Wrap Profile. It is applicable to constrained devices with keypads, e.g., mobile phones. Key wrap is carried out using a passphrase-derived key wrapping key, known in advance by both the cryptographic module and DSKPP server. Key package formats that satisfy this criteria are [PSKC] and [SKPC-ASN.1]. 3.2.1. Message Flow The two-pass protocol flow consists of one exchange: Doherty, et al. Expires December 24, 2008 [Page 27] Internet-Draft DSKPP June 2008 1: Pass 1 = , Pass 2 = The client's initial message is directly followed by a message (unlike the four-pass variant, there is no exchange of the and messages). However, as the two-pass variation of DSKPP consists of one round trip to the server, the client is still able to include its random nonce, R_C, algorithm preferences and supported key types in the message. Note that by including R_C in , the DSKPP client is able to ensure the server is alive before "committing" the key. The DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user. To ensure that the key K_TOKEN ends up associated with the correct cryptographic module and user, the DSKPP server MAY couple an initial user authentication to the DSKPP execution as described in Section 3.4. The purpose and content of each message are described below, including the optional . DSKPP Client DSKPP Server ------------ ------------ [<---] R_TRIGGER, [DeviceID], [KeyID], [URL_S] The DSKPP server optionally sends a message to the DSKPP client. The trigger message MUST contain a nonce, R_TRIGGER, to allow the server to couple the trigger with a later request. MAY include a DeviceID to allow the client to select the device with which it will communicate (for more information about device identification, refer to Section 3.3). In the case of key renewal, SHOULD include the identifier for the key, KeyID, that is being replaced. Finally, the trigger MAY contain a URL for the DSKPP client to use when contacting the DSKPP server. DSKPP Client DSKPP Server ------------ ------------ R_C, SAL, KPML, [AD], [R_TRIGGER], [DeviceID], [KeyID] ---> The DSKPP client sends a message to the DSKPP server. MUST include client nonce, R_C, and a Security Attribute List (SAL), identifying which DSKPP versions, protocol variations (in this case "two-pass"), key package formats, Doherty, et al. Expires December 24, 2008 [Page 28] Internet-Draft DSKPP June 2008 key types, encryption and MAC algorithms that the client supports. Unlike 4-pass DSKPP, the 2-pass DSKPP client uses the message to declare the list of Key Protection Methods (KPML) it supports, providing required payload information in accordance with Section 3.2.2. Optionally, the message MAY include client Authentication Data (AD), such as a MAC derived from an authentication code and R_C (refer to Section 3.4.1). In addition, if a trigger message preceded , then it passes the parameters received in back to the DSKPP Server. In particular, it MUST include R_TRIGGER so that the DSKPP server can associate the client with the trigger message, and SHOULD include DeviceID and KeyID. DSKPP Client DSKPP Server ------------ ------------ <--- KPH, KP, E(K,K_PROV), MAC, AD If Authentication Data (AD) was received, then the DSKPP server MUST authenticate the user in accordance with Section 3.4.1. If authentication fails, then DSKPP server MUST abort. Otherwise, the DSKPP server generates a key K_PROV from which two keys, K_TOKEN and K_MAC, are derived. (Alternatively, the key K_PROV may have been pre-generated as described in Section 1.1.1. The DSKPP server selects a Key Protection Method (KPM) and applies it to K_PROV in accordance with Section 3.2.2. The server then associates K_TOKEN with the cryptographic module in a server-side data store. The intent is that the data store later will be used by some service that needs to verify or decrypt data produced by the cryptographic module and the key. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . For two-pass DSKPP, the confirmation message MUST include a Key Package Header (KPH) that contains the DSKPP Server's ID and KPM. The ServerID is used for authentication purposes, and the KPM informs the DSKPP client of the security context in which it will operate. In addition to the KPH, the confirmation message MUST include the Key Package (KP) that holds the KeyID, K_PROV from which K_TOKEN and K_MAC are derived, and additional configuration information. The default symmetric key package format is based on the Portable Symmetric Key Container (PSKC) defined in [PSKC]. Alternative formats MAY include [SKPC-ASN.1], PKCS#12 [PKCS-12], or PKCS#5 XML [PKCS-5-XML]. Finally, MUST include two MACs (MAC and AD) whose values are calculated with contribution from the client nonce, R_C, provided in the message. The MAC values will allow the cryptographic module to perform key confirmation and server Doherty, et al. Expires December 24, 2008 [Page 29] Internet-Draft DSKPP June 2008 authentication before "committing" the key (see Section 3.2.3 for more information). After receiving a message with Status = "Success", the DSKPP client MUST verify both MAC values (MAC and AD). The DSKPP client MUST terminate the DSKPP session if either MAC does not verify, and MUST, in this case, also delete any nonces, keys, and/or secrets associated with the failed run of the protocol. If has Status = "Success" and the MACs were verified, then the DSKPP client MUST extract the key data from the provided key package, and store data locally. After this operation, it MUST NOT be possible to overwrite the key unless knowledge of an authorizing key is proven through a MAC on a later message. 3.2.2. Key Protection Profiles This section introduces three profiles of two-pass DSKPP for key protection. Further profiles MAY be defined by external entities or through the IETF process. 3.2.2.1. Key Transport Profile This profile establishes a symmetric key, K_TOKEN, in the cryptographic module using key transport and key derivation. Key transport is carried out using a public key whose private key part resides in the cryptographic module as the key transport key. A provisioning master key, K_PROV, MUST be transported from the DSKPP server to the client. From K_PROV, two keys are derived: the symmetric key to be established, K_TOKEN, and a key used to compute MACs, K_MAC. This profile MUST be identified with the following URN: urn:ietf:params:xml:schema:keyprov:protocol#transport In the two-pass version of DSKPP, the client MUST send a payload associated with this key protection method. This payload MUST be of type ([XMLDSIG]), and only those choices of that identify a public key are allowed. The option of the alternative is RECOMMENDED when the public key corresponding to the private key on the cryptographic module has been certified. The server payload associated with this key protection method MUST be of type xenc:EncryptedKeyType ([XMLENC]), and only those encryption methods utilizing a public key that are supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP) are allowed Doherty, et al. Expires December 24, 2008 [Page 30] Internet-Draft DSKPP June 2008 as values for the . Further, in the case of 2-pass DSKPP, MUST contain the same value (i.e. identify the same public key) as the of the corresponding supported key protection method in the message that triggered the response. MAY be present, but MUST, when present, contain the same value as the element of the message. The Type attribute of the xenc: EncryptedKeyType MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP). The transported key, K_PROV, MUST consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN. DSKPP servers and cryptographic modules supporting this profile MUST support the http://www.w3.org/2001/04/xmlenc#rsa-1_5 key wrapping mechanism defined in [XMLENC]. When this profile is used, the MacAlgorithm attribute of the element of the message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP). The MAC MUST be calculated as described in Section 3.2.3 for two-pass DSKPP. In addition, DSKPP servers MUST include the AuthenticationDataType element in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. 3.2.2.2. Key Wrap Profile This profile establishes a symmetric key, K_TOKEN, in the cryptographic module through key wrap and key derivation. Key wrap is carried out using a symmetric key wrapping key, known in advance by both the cryptographic module and the DSKPP server. A provisioning master key, K_PROV, MUST be transported from the DSKPP server to the client. From K_PROV, two keys are derived: the symmetric key to be established, K_TOKEN, and a key used to compute MACs, K_MAC. This profile MUST be identified with the following URI: urn:ietf:params:xml:schema:keyprov:protocol#wrap In the 2-pass version of DSKPP, the client MUST send a payload Doherty, et al. Expires December 24, 2008 [Page 31] Internet-Draft DSKPP June 2008 associated with this key protection method. This payload MUST be of type ([XMLDSIG]), and only those choices of that identify a symmetric key are allowed. The alternative is RECOMMENDED. The server payload associated with this key protection method MUST be of type xenc:EncryptedKeyType ([XMLENC]), and only those encryption methods utilizing a symmetric key that are supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP) are allowed as values for the . Further, in the case of 2-pass DSKPP, MUST contain the same value (i.e. identify the same symmetric key) as the of the corresponding supported key protection method in the message that triggered the response. MAY be present, and MUST, when present, contain the same value as the element of the message. The Type attribute of the xenc:EncryptedKeyType MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP). The wrapped key, K_PROV, MUST consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN. DSKPP servers and cryptographic modules supporting this profile MUST support the http://www.w3.org/2001/04/xmlenc#kw-aes128 key wrapping mechanism defined in [XMLENC]. When this profile is used, the MacAlgorithm attribute of the element of the message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP). The MAC MUST be calculated as described in Section 3.2.3. In addition, DSKPP servers MUST include the AuthenticationDataType element in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. 3.2.2.3. Passphrase-Based Key Wrap Profile This profile is a variation of the key wrap profile. It establishes a symmetric key, K_TOKEN, in the cryptographic module through key wrap and key derivation. Key wrap is carried out using a passphrase- Doherty, et al. Expires December 24, 2008 [Page 32] Internet-Draft DSKPP June 2008 derived key wrapping key. The passphrase is known in advance by both the user of the device and the DSKPP server. To preserve the property of not exposing K_TOKEN to any other entity than the DSKPP server and the cryptographic module itself, the method SHOULD be employed only when the device contains facilities (e.g. a keypad) for direct entry of the passphrase. A provisioning master key, K_PROV, MUST be transported from the DSKPP server to the client. From K_PROV, two keys are derived: the symmetric key to be established, K_TOKEN, and a key used to compute MACs, K_MAC. This profile MUST be identified with the following URI: urn:ietf:params:xml:schema:keyprov:protocol#passphrase-wrap In the 2-pass version of DSKPP, the client MUST send a payload associated with this key protection method. This payload MUST be of type ([XMLDSIG]). The option MUST be used and the key name MUST identify the passphrase that will be used by the server to generate the key wrapping key. As an example, the identifier could be a user identifier or a registration identifier issued by the server to the user during a session preceding the DSKPP protocol run. The server payload associated with this key protection method MUST be of type xenc:EncryptedKeyType ([XMLENC]), and only those encryption methods utilizing a passphrase to derive the key wrapping key that are supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP) are allowed as values for the . Further, in the case of 2-pass DSKPP, MUST contain the same value (i.e. identify the same passphrase) as the of the corresponding supported key protection method in the message that triggered the response. MAY be present, and MUST, when present, contain the same value as the element of the message. The Type attribute of the xenc: EncryptedKeyType MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP). The wrapped key, K_PROV, MUST consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN. DSKPP servers and cryptographic modules supporting this profile MUST support the PBES2 password based encryption scheme defined in [PKCS-5] (and identified as http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbes2 in Doherty, et al. Expires December 24, 2008 [Page 33] Internet-Draft DSKPP June 2008 [PKCS-5-XML]), the PBKDF2 passphrase-based key derivation function also defined in [PKCS-5] (and identified as http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbkdf2 in [PKCS-5-XML]), and the http://www.w3.org/2001/04/xmlenc#kw-aes128 key wrapping mechanism defined in [XMLENC]. When this profile is used, the MacAlgorithm attribute of the element of the message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP). The MAC MUST be calculated as described in Section 3.2.3. In addition, DSKPP servers MUST include the AuthenticationDataType element in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. 3.2.3. MAC Calculations 3.2.3.1. Key Confirmation The MAC value in the message MUST be calculated as follows: msg_hash = SHA-256(msg_1, ..., msg_n) dsLen = len(msg_hash) MAC = DSKPP-PRF (K_MAC, "MAC 1 computation" || msg_hash || ServerID, dsLen) where MAC The MAC MUST be calculated using the already established MAC algorithm and MUST be computed on the (ASCII) string "MAC 1 computation", msg_hash, and ServerID using the existing the MAC key K_MAC. K_MAC The key, along with K_TOKEN, that is derived from K_PROV which the DSKPP server MUST provide to the cryptographic module. Doherty, et al. Expires December 24, 2008 [Page 34] Internet-Draft DSKPP June 2008 msg_hash The message hash, defined in Section 3.1.4.3, of messages msg_1, ..., msg_n. ServerID The identifier that the DSKPP server MUST include in the element of . If DSKPP-PRF (defined in Section 3.5) is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation", msg_hash, and ServerID, and the parameter dsLen MUST be set to the length of msg_hash. 3.2.3.2. Server Authentication in the Case of Key Renewal A second MAC MUST be present in the message as proof that the DSKPP server is authorized to replace a key on the cryptographic module. In 2-pass DSKPP, servers provide the second MAC in the AuthenticationDataType element of . The MAC value in the AuthenticationDataType element MUST be computed on the (ASCII) string "MAC 2 computation", the server identifier ServerID, and R, using a pre-existing MAC key K_MAC' (the MAC key that existed before this protocol run). Note that the implementation may specify K_MAC' to be the value of the K_TOKEN that is being replaced, or a version of K_MAC from the previous protocol run. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 2 computation" ServerID, and R. The parameter dsLen MUST be set to at least 16 (i.e. the length of the MAC MUST be at least 16 octets): dsLen >= 16 MAC = DSKPP-PRF (K_MAC', "MAC 2 computation" || ServerID || R, dsLen) The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes. 3.3. Device Identification The DSKPP server MAY be pre-configured with a unique device identifier corresponding to a particular cryptographic module. The DSKPP server MAY then include this identifier in the DSKPP initialization trigger, in which case the DSKPP client MUST include it in its message(s) to the DSKPP server for authentication. Note that it is also legitimate for a DSKPP client to initiate the DSKPP protocol run without having received an initialization message from a server, but in this case any provided device identifier MUST NOT be accepted by the DSKPP server unless the server has access to a unique key for the identified device and that key will be used in the Doherty, et al. Expires December 24, 2008 [Page 35] Internet-Draft DSKPP June 2008 protocol. 3.4. User Authentication The DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user. If the user has not been authenticated by some out-of-band means, then the user SHOULD be authenticated within the DSKPP. When relying on DSKPP for user authentication, the DSKPP server SHOULD explicitly rely on client-provided Authentication Data (AD) to verify that a legitimate user is behind the wheel. For a further discussion of this, and threats related to man-in-the-middle attacks in this context, see Section 9.6.4. 3.4.1. Authentication Data As described in the message flows above (see Section 3.1.1 and Section 3.2.1), the DSKPP client MAY include Authentication Data (AD) in its request(s). Note that AD MAY be omitted if client certificate authentication has been provided by the transport channel such as TLS. Nonetheless, when AD is provided, the DSKPP server MUST verify the data before continuing with the protocol run. The data element that holds AD MUST include a Client ID and a value derived from an Authentication Code (AC). The Client ID represents a key request made by the user to the Provisioning Server. AC is a one-time use value that is a (potentially low entropy) shared secret between a user and the Provisioning Server. This secret is made available to the client before the DSKPP message exchange. Below are examples of how the DSKPP client may obtain the AC: a. A key issuer may deliver an AC to the user or device in response to a key request, which the user enters into an application hosted on their device. For example, a user runs an application that is resident on their device, e.g., a mobile phone. The application cannot proceed without a new symmetric key. The user is redirected to an issuer's Web site from where the user requests a key. The issuer's Web application processes the request, and returns an AC, which then appears on the user's display. The user then invokes a symmetric key-based application hosted on the device, which asks the user to input the AC using a keypad. The application invokes the DSKPP client, providing it with the AC. b. The provisioning server may send a trigger message, , to the DSKPP client, which sets the value of the trigger nonce, R_TRIGGER, to AC. When this method is used, a transport providing privacy and integrity MUST be used to deliver the DSKPP initialization trigger from the DSKPP server to the Doherty, et al. Expires December 24, 2008 [Page 36] Internet-Draft DSKPP June 2008 DSKPP client, e.g., HTTPS. A description of the AC and how it is used to derive AD is contained in the sub-sections below. 3.4.2. Authentication Code Format AC is encoded in Type-Length-Value (TLV) format. The format consists of a minimum of two TLVs and a variable number of additional TLVs, depending on implementation. See Figure 4 for TLV field layout. A 1 byte type field identifies the specific TLV, and a 1 byte length, in hexadecimal, indicates the length of the value field contained in the TLV. A TLV MUST start on a 4 byte boundary. Pad bytes MUST be placed at the end of the previous TLV in order to align the next TLV. These pad bytes are not counted in the length field of the TLV. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value[0] | ...Value[Length-1] +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: TLV Format The TLV fields are defined as follows: Type (1 byte) The integer value identifying the type of information contained in the value field. Length (1 byte) The length, in hexadecimal, of the value field to follow. Value (variable length) A variable-length hexadecimal value containing the instance-specific information for this TLV. Figure 5 summarizes the TLVs defined in this document. Optional TLVs are allowed for vendor-specific extensions with the constraint that the high bit MUST be set to indicate a vendor-specific type. Other TLVs are left for later revisions of this protocol. Doherty, et al. Expires December 24, 2008 [Page 37] Internet-Draft DSKPP June 2008 +------+------------+-------------------------------------------+ | Type | TLV Name | Conformance | Example Usage | +------+------------+-------------------------------------------+ | 1 | Client ID | Mandatory | { "AC00000A" } | +------+------------+-------------+-----------------------------+ | 2 | Password | Mandatory | { "3582" } | +------+------------+-------------+-----------------------------+ | 3 | Checksum | Optional | { 0x5F8D } | +------+------------+-------------+-----------------------------+ Figure 5: TLV Summary 3.4.2.1. Client ID (MANDATORY) The Client ID is a mandatory TLV that represents the user's key request. A summary of the Client ID TLV format is given in Figure 6. The fields are transmitted from left to right. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x1 | Length | clientID ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: ClientID TLV Format clientID is an ASCII string that identifies the key request. The clientID MUST be HEX encoded. For example, suppose clientID is set to "AC00000A", the hexadecimal equivalent is 0x4143303030303041, resulting in a TLV of {0x1, 0x8, 0x4143303030303041}. 3.4.2.2. Password (MANDATORY) The Password is a mandatory TLV the contains a one-time use shared secret known by the user and the Provisioning Server. A summary of the Password TLV format is given in Figure 7. The fields are transmitted from left to right. Doherty, et al. Expires December 24, 2008 [Page 38] Internet-Draft DSKPP June 2008 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x2 | Length | password ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Password TLV Format Password is a unique value that SHOULD be a random string to make AC more difficult to guess. The string MUST be UTF-8 encoded in accordance with [RFC3629]. For example, suppose password is set to "3582", then the TLV would be {0x2, 0x4, UTF-8("3582")}. 3.4.2.3. Checksum (OPTIONAL) The Checksum is an OPTIONAL TLV, which is generated by the issuing server and sent to the user as part of the AC. A summary of the Checksum TLV format is given in Figure 8. The fields are transmitted from left to right. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x3 | Length | checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Checksum TLV Format If included, the checksum MUST be computed using the CRC16 algorithm [ISO3309]. When the user enters the AC, the typed password is verified with the checksum to ensure it is correctly entered by the user. For example, suppose the Password is set to "3582", then the CRC16 calculation would generate a checksum of 0x5F8D, resulting in TLV {0x3, 0x2, 0x5F8D}. 3.4.3. Authentication Data Calculation The Authentication Data consists of a Client ID (extracted from the AC) and a value, which is derived from AC as follows (refer to Section 3.5 for a description of DSKPP-PRF in general and Appendix C for a description of DSKPP-PRF-AES): MAC = DSKPP-PRF(K_AC, AC->clientID||URL_S||R_C||[R_S], 16) Doherty, et al. Expires December 24, 2008 [Page 39] Internet-Draft DSKPP June 2008 In four-pass DSKPP, the cryptographic module uses R_C, R_S, and URL_S to calculate the MAC, where URL_S is the URL the DSKPP client uses when contacting the DSKPP server. In two-pass DSKPP, the cryptographic module does not have access to R_S, therefore only R_C is used in combination with URL_S to produce the MAC. In either case, K_AC MUST be derived from AC>password as follows [PKCS-5]: K_AC = PBKDF2(AC->password, R_C || [K], iter_count, 16) K is OPTIONAL only in four-pass where no K_SHARED is used. In all other cases one of the following values for K MUST be used: a. The public key of the DSKPP client, or the public key of the device when a device certificate is available b. The pre-shared key between the client and the server c. A passphrase-derived key The iteration count, iter_count, MUST be set to at least 100,000 except for case (b) and (c), above, in which case it MUST be set to 1. 3.5. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF 3.5.1. Introduction All of the protocol variations depend on DSKPP-PRF. The general requirements on DSKPP-PRF are the same as on keyed hash functions: It MUST take an arbitrary length input, and be one-way and collision- free (for a definition of these terms, see, e.g., [FAQ]). Further, the DSKPP-PRF function MUST be capable of generating a variable- length output, and its output MUST be unpredictable even if other outputs for the same key are known. It is assumed that any realization of DSKPP-PRF takes three input parameters: A secret key k, some combination of variable data, and the desired length of the output. The combination of variable data can, without loss of generalization, be considered as a salt value (see PKCS#5 Version 2.0 [PKCS-5], Section 4), and this characterization of DSKPP-PRF SHOULD fit all actual PRF algorithms implemented by cryptographic modules. From the point of view of this specification, DSKPP-PRF is a "black-box" function that, given the inputs, generates a pseudorandom value. Separate specifications MAY define the implementation of DSKPP-PRF for various types of cryptographic modules. Appendix C contains two example realizations of DSKPP-PRF. Doherty, et al. Expires December 24, 2008 [Page 40] Internet-Draft DSKPP June 2008 3.5.2. Declaration DSKPP-PRF (k, s, dsLen) Input: k secret key in octet string format s octet string of varying length consisting of variable data distinguishing the particular string being derived dsLen desired length of the output Output: DS pseudorandom string, dsLen-octets long For the purposes of this document, the secret key k MUST be at least 16 octets long. 4. DSKPP Message Formats The message formats from the DSKPP XML schema, found in Section 7, are explained in this section. Examples can be found in Appendix A. The XML format for DSKPP messages has been designed to be extensible. However, it is possible that the use of extensions will harm interoperability; therefore, any use of extensions SHOULD be carefully considered. For example, if a particular implementation relies on the presence of a proprietary extension, then it may not be able to interoperate with independent implementations that have no knowledge of this extension. 4.1. General XML Schema Requirements Some DSKPP elements rely on the parties being able to compare received values with stored values. Unless otherwise noted, all elements in this document that have the XML Schema "xs:string" type, or a type derived from it, MUST be compared using an exact binary comparison. In particular, DSKPP implementations MUST NOT depend on case-insensitive string comparisons, normalization or trimming of white space, or conversion of locale-specific formats such as numbers. Implementations that compare values that are represented using different character encodings MUST use a comparison method that returns the same result as converting both values to the Unicode character encoding, Normalization Form C [UNICODE], and then performing an exact binary comparison. Doherty, et al. Expires December 24, 2008 [Page 41] Internet-Draft DSKPP June 2008 No collation or sorting order for attributes or element values is defined. Therefore, DSKPP implementations MUST NOT depend on specific sorting orders for values. 4.2. Components of the Message The DSKPP server MAY initialize the DSKPP protocol by sending a message. This message MAY, e.g., be sent in response to a user requesting key initialization in a browsing session. The element is intended for the DSKPP client and MAY inform the DSKPP client about the identifier for the device that houses the cryptographic module to be initialized, and optionally of the identifier for the key on that module. The latter would apply to key renewal. The trigger always contains a nonce to allow the DSKPP server to couple the trigger with a later DSKPP request. Finally, the trigger MAY contain a URL to use when contacting the DSKPP server. The elements are for future extensibility. Any provided or values MUST be used by the DSKPP client in the subsequent Doherty, et al. Expires December 24, 2008 [Page 42] Internet-Draft DSKPP June 2008 request. The OPTIONAL element informs the DSKPP client about the characteristics of the intended cryptographic module platform, and applies in the public-key variant of DSKPP in situations when the client potentially needs to decide which one of several modules to initialize. 4.3. Components of the Request This message is the initial message sent from the DSKPP client to the DSKPP server in both variations of the DSKPP. Doherty, et al. Expires December 24, 2008 [Page 43] Internet-Draft DSKPP June 2008 The components of this message have the following meaning: o Version: (attribute inherited from the AbstractRequestType type) The highest version of this protocol the client supports. Only version one ("1.0") is currently specified. o : An identifier for the cryptographic module as defined in Section 3.4 above. The identifier MUST only be present if such shared secrets exist or if the identifier was provided by the server in a element (see Section 6.2.7). In the latter case, it MUST have the same value as the identifier provided in that element. Doherty, et al. Expires December 24, 2008 [Page 44] Internet-Draft DSKPP June 2008 o : An identifier for the key that will be overwritten if the protocol run is successful. The identifier MUST only be present if the key exists or if the identifier was provided by the server in a element, in which case, it MUST have the same value as the identifier provided in that element (see a (Section 4.2) and Section 6.2.7). o : This is the nonce R, which, when present, MUST be used by the server when calculating MAC values (see below). It is RECOMMENDED that clients include this element whenever the element is present. o : This OPTIONAL element MUST be present if and only if the DSKPP run was initialized with a message (see Section 6.2.7), and MUST, in that case, have the same value as the child of that message. A server using nonces in this way MUST verify that the nonce is valid and that any device or key identifier values provided in the message match the corresponding identifier values in the message. o : A sequence of container elements that in turn contain URLs indicating the key types for which the cryptographic module is willing to generate keys through DSKPP. o : A sequence of container elements that in turn contain URLs indicating the encryption algorithms supported by the cryptographic module for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as a supported key type and as an encryption algorithm. o : A sequence of container elements that in turn contain URLs indicating the MAC algorithms supported by the cryptographic module for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as an encryption algorithm and as a MAC algorithm (e.g., http://www.ietf.org/keyprov/dskpp#dskpp-prf-aes, which is defined in Appendix C). o : This OPTIONAL element is used by the DSKPP client to indicate support for four-pass or two-pass DSKPP. If two-pass support is specified, then MUST be set to nonce R in the message unless is already present. o : This OPTIONAL element is a sequence of container elements that in turn contain URLs indicating the key package formats supported by the DSKPP client. If this element is not provided, then the DSKPP server MUST proceed with "http://www.ietf.org/keyprov/pskc#KeyContainer" (see [PSKC]). o : This OPTIONAL element contains data that the DSKPP client uses to authenticate the user or device to the DSKPP server. The element is set as specified in Section 3.4. Doherty, et al. Expires December 24, 2008 [Page 45] Internet-Draft DSKPP June 2008 o : A sequence of extensions. One extension is defined for this message in this version of DSKPP: the ClientInfoType (see Section 5). Some of the core elements of the message are described below. 4.3.1. The DeviceIdentifierDataType Type The DeviceIdentifierDataType type is used to uniquely identify the device that houses the cryptographic module, e.g., a mobile phone. The device identifier allows the DSKPP server to find, e.g., a pre- shared key transport key for 2-pass DSKPP and/or the correct shared secret for MAC'ing purposes. The default DeviceIdentifierDataType is defined in [PSKC]. 4.3.2. The ProtocolVariantsType Type The ProtocolVariantsType is a complex type that is a sequence of elements, each describing a DSKPP protocol variant. The DSKPP client MAY use the ProtocolVariantsType to identify which protocol variants it supports, i.e., by providing within a message. Selecting the element signals client support for 4-pass DSKPP as described in Section 3.1.1. Selecting the element signals client support for the 2-pass version of DSKPP as described in Section 3.2.1. The element is of type KeyProtectionDataType, which carries information that informs the server of supported two-pass key protection methods as described in Section 3.2.2, and provides OPTIONAL payload data to the DSKPP server. The payload is sent in an opportunistic fashion, and MAY be discarded by the DSKPP server if the server does not support the key protection method with which the payload is associated. If the DSKPP client does not include in the message, then the DSKPP server MUST proceed by using the 4-pass DSKPP variant. If the DSKPP server does not support 4-pass DSKPP, then the server MUST use the two-pass protocol variant. If it cannot support the two-pass protocol variant, then Doherty, et al. Expires December 24, 2008 [Page 46] Internet-Draft DSKPP June 2008 the protocol run MUST fail. The elements of this type have the following meaning: o : A two-pass key protection method supported by the DSKPP client. Multiple supported methods MAY be present, in which case they MUST be listed in order of precedence. o : An OPTIONAL payload associated with each supported key protection method. A DSKPP client that indicates support for two-pass DSKPP MUST also include the nonce R in its message (this will enable the client to verify that the DSKPP server it is communicating with is alive). 4.3.3. The KeyPackagesFormatType Type The OPTIONAL KeyPackagesFormatType type is a list of type-value pairs that a DSKPP client or server MAY use to define key package formats it supports. Key package formats are identified through URLs, e.g., the PSKC KeyContainer URL "http://www.ietf.org/keyprov/pskc#KeyContainer" (see [PSKC]). Doherty, et al. Expires December 24, 2008 [Page 47] Internet-Draft DSKPP June 2008 4.3.4. The AuthenticationDataType Type The OPTIONAL AuthenticationDataType type is used by DSKPP clients to carry authentication values in DSKPP messages as described in Section 3.4. The elements of the AuthenticationDataType type have the following meaning: o : A requester's identifier of maximum length 128. The value MAY be a user ID, a device ID, or a keyID associated with the requester's authentication value. o : An authentication MAC and additional information (e.g., MAC algorithm), derived as described in Section 3.4.3. 4.4. Components of the Response (Used Only in Four-Pass DSKPP) In a four-pass exchange, this message is the first message sent from Doherty, et al. Expires December 24, 2008 [Page 48] Internet-Draft DSKPP June 2008 the DSKPP server to the DSKPP client (assuming a trigger message has not been sent to initiate the protocol, in which case, this message is the second message sent from the DSKPP server to the DSKPP client). It is sent upon reception of a message. The components of this message have the following meaning: o Version: (attribute inherited from the AbstractResponseType type) The version selected by the DSKPP server. MAY be lower than the version indicated by the DSKPP client, in which case, local policy at the client MUST determine whether or not to continue the session. o SessionID: (attribute inherited from the AbstractResponseType type) An identifier for this session. The SessionID has a maximum length of 128. o Status: (attribute inherited from the AbstractResponseType type) Return code for the . If Status is not "Continue", only the Status and Version attributes will be present; otherwise, all the other element MUST be present as well. o : The type of the key to be generated. o : The encryption algorithm to use when protecting R_C. Doherty, et al. Expires December 24, 2008 [Page 49] Internet-Draft DSKPP June 2008 o : The MAC algorithm to be used by the DSKPP server. o : Information about the key to use when encrypting R_C. It will either be the server's public key (the alternative of ds:KeyInfoType) or an identifier for a shared secret key (the alternative of ds:KeyInfoType). o : The key package format type to be used by the DSKPP server. The default setting relies on the KeyPackageType element defined in "urn:ietf:params:xml:schema:keyprov:container" [PSKC]. o : The actual payload. For this version of the protocol, only one payload is defined: the pseudorandom string R_S. o : A list of server extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 5). o : The MAC MUST be present if the DSKPP run will result in the replacement of an existing symmetric key with a new one (i.e., if the element was present in the Request (Used Only in Four- Pass DSKPP) In a four-pass DSKPP exchange, this message contains the nonce R_C that was chosen by the cryptographic module, and encrypted by the negotiated encryption key and encryption algorithm Doherty, et al. Expires December 24, 2008 [Page 50] Internet-Draft DSKPP June 2008 The components of this message have the following meaning: o Version: (inherited from the AbstractRequestType type) MUST be the same version as in the message. o : (attribute inherited from the AbstractResponseType type) MUST have the same value as the SessionID attribute in the received message. SessionID has maximum length of 128. o : The nonce generated and encrypted by the cryptographic module. The encryption MUST be made using the selected encryption algorithm and identified key, and as specified in Section 3.5. o : IThe authentication data value MUST be set as specified in Section 3.4 and Section 4.3.4. o : A list of extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 5) 4.6. Components of a Response This message is the last message of the DSKPP protocol run. In a 4-pass exchange, the DSKPP server sends this message in response to a message, whereas in a 2-pass exchange, the DSKPP server sends this message in response to a message. Doherty, et al. Expires December 24, 2008 [Page 51] Internet-Draft DSKPP June 2008 The components of this message have the following meaning: o Version: (inherited from the AbstractResponseType type) The DSKPP version used in this session. o SessionID: (inherited from the AbstractResponseType type) The previously established identifier for this session. The SessionID is of maximum length 128. o Status: (inherited from the AbstractResponseType type) Return code for the message. If Status is not "Success", only the Status, SessionID, and Version attributes will be present (the presence of the SessionID attribute is dependent on the type of reported error); otherwise, all the other elements MUST be present as well. In this latter case, the message can be seen as a "Commit" message, instructing the cryptographic module to store the generated key and associate the given key identifier with this key. o : The key package containing keying material in accordance with four- and two-pass DSKPP usage (see Section 3.1 and Section 3.2). The default package format is based on the KeyContainerType type from PSKC, as defined in [PSKC]. o : A list of extensions chosen by the DSKPP server. For this message, this version of DSKPP defines one extension, the ClientInfoType (see Section 5). o : To avoid a false "Commit" message causing the cryptographic module to end up in an initialized state for which the server does not know the stored key, messages MUST always be authenticated with a MAC. The MAC MUST be made using the already established MAC algorithm. Doherty, et al. Expires December 24, 2008 [Page 52] Internet-Draft DSKPP June 2008 o : This OPTIONAL element contains a MAC value that the DSKPP server provides in a two-pass message exchange as proof that the server is authorized to replace a key on the cryptographic module. The MAC MUST be calculated as specified in Section 3.2.3.2. 4.7. The StatusCode Type The StatusCode type enumerates all possible return codes: Upon transmission or receipt of a message for which the Status attribute's value is not "Success" or "Continue", the default behavior, unless explicitly stated otherwise below, is that both the DSKPP server and the DSKPP client MUST immediately terminate the DSKPP session. DSKPP servers and DSKPP clients MUST delete any secret values generated as a result of failed runs of the DSKPP protocol. Session identifiers MAY be retained from successful or failed protocol runs for replay detection purposes, but such retained identifiers MUST NOT be reused for subsequent runs of the protocol. When possible, the DSKPP client SHOULD present an appropriate error message to the user. These status codes are valid in all DSKPP Response messages unless explicitly stated otherwise: Doherty, et al. Expires December 24, 2008 [Page 53] Internet-Draft DSKPP June 2008 o "Continue" indicates that the DSKPP server is ready for a subsequent request from the DSKPP client. It cannot be sent in the server's final message. o "Success" indicates successful completion of the DSKPP session. It can only be sent in the server's final message. o "Abort" indicates that the DSKPP server rejected the DSKPP client's request for unspecified reasons. o "AccessDenied" indicates that the DSKPP client is not authorized to contact this DSKPP server. o "MalformedRequest" indicates that the DSKPP server failed to parse the DSKPP client's request. o "UnknownRequest" indicates that the DSKPP client made a request that is unknown to the DSKPP server. o "UnknownCriticalExtension" indicates that a critical DSKPP extension (see below) used by the DSKPP client was not supported or recognized by the DSKPP server. o "UnsupportedVersion" indicates that the DSKPP client used a DSKPP protocol version not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoSupportedKeyTypes" indicates that the DSKPP client only suggested key types that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoSupportedEncryptionAlgorithms" indicates that the DSKPP client only suggested encryption algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoSupportedMacAlgorithms" indicates that the DSKPP client only suggested MAC algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoProtocolVariants" indicates that the DSKPP client only suggested a protocol variation (either 2-pass or 4-pass) that is not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoSupportedKeyPackages" indicates that the DSKPP client only suggested key package formats that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "AuthenticationDataMissing" indicates that the DSKPP client didn't provide authentication data that the DSKPP server required. o "AuthenticationDataInvalid" indicates that the DSKPP client supplied user authentication data that the DSKPP server failed to validate. o "InitializationFailed" indicates that the DSKPP server could not generate a valid key given the provided data. When this status code is received, the DSKPP client SHOULD try to restart DSKPP, as it is possible that a new run will succeed. Doherty, et al. Expires December 24, 2008 [Page 54] Internet-Draft DSKPP June 2008 o "ProvisioningPeriodExpired" indicates that the provisioning period set by the DSKPP server has expired. When the status code is received, the DSKPP client SHOULD report the reason for key initialization failure to the user and the user MUST register with the DSKPP server to initialize a new key. 5. Protocol Extensions 5.1. The ClientInfoType Type Present in a or a message, the OPTIONAL ClientInfoType extension contains DSKPP client-specific information that is custom to an implementation. DSKPP servers MUST support this extension. DSKPP servers MUST NOT attempt to interpret the data it carries and, if rece