KEYPROV Working Group A. Doherty Internet-Draft RSA, The Security Division of EMC Intended status: Standards Track M. Pei Expires: January 27, 2008 VeriSign, Inc. M. Nystroem RSA, The Security Division of EMC S. Machani Diversinet Corp. July 26, 2007 Dynamic Symmetric Key Provisioning Protocol (DSKPP) draft-ietf-keyprov-dskpp-00.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 January 27, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). 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 Doherty, et al. Expires January 27, 2008 [Page 1] Internet-Draft DSKPP July 2007 private-key capabilities in the cryptographic modules, and with or without an established public-key infrastructure. Three variations of the protocol support multiple usage scenarios. The four-pass (i.e., two round-trip) variant enables key generation in near real-time. 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. Two- and one-pass variants enable secure and efficient download and installation of symmetric keys to a cryptographic module in environments where near real-time communication may not be possible. This document builds on information contained in [RFC4758], adding specific enhancements in response to implementation experience and liaison requests. It is intended, therefore, 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. Doherty, et al. Expires January 27, 2008 [Page 2] Internet-Draft DSKPP July 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7 1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 7 2. Notation and Terminology . . . . . . . . . . . . . . . . . . 8 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. A cryptographic module obtains a symmetric key . . . . . 9 3.2. A cryptographic module acquires multiple symmetric keys of different types . . . . . . . . . . . . . . . . . 9 3.3. A provisioning server imposes a validity period policy for provisioning sessions . . . . . . . . . . . . . . . . 10 3.4. A symmetric key issuer uses a third party provisioning service provider . . . . . . . . . . . . . . . . . . . . 10 3.5. A cryptographic module renews its symmetric key with the same key ID . . . . . . . . . . . . . . . . . . . . . 10 3.6. An administrator initiates a symmetric key replacement before it can be used . . . . . . . . . . . . . . . . . . 10 3.7. A cryptographic module hosted by a smart card uses a pre-shared transport key to communicate with the provisioning server . . . . . . . . . . . . . . . . . . . 11 3.8. A cryptographic module hosted by a mobile device downloads a symmetric key through SMS . . . . . . . . . . 11 3.9. A cryptographic module acquires a symmetric key over a transport protocol that does not ensure data confidentiality . . . . . . . . . . . . . . . . . . . . . 12 3.10. A cryptographic module acquires a symmetric key over a transport protocol that does not provide authentication . 12 4. DSKPP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Entities . . . . . . . . . . . . . . . . . . . . . . . . 12 4.2. Principles of Operation . . . . . . . . . . . . . . . . . 14 4.2.1. Four-pass DSKPP . . . . . . . . . . . . . . . . . . . 15 4.2.2. Two-pass DSKPP . . . . . . . . . . . . . . . . . . . 19 4.2.3. One-pass DSKPP . . . . . . . . . . . . . . . . . . . 21 4.3. Authentication . . . . . . . . . . . . . . . . . . . . . 22 4.3.1. Client Authentication (Applicable to Four- and Two-Pass DSKPP) . . . . . . . . . . . . . . . . . . . 22 4.3.2. Server Authentication . . . . . . . . . . . . . . . . 25 4.4. Symmetric Key Container Format . . . . . . . . . . . . . 25 4.5. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF . . . 25 4.5.1. Introduction . . . . . . . . . . . . . . . . . . . . 25 4.5.2. Declaration . . . . . . . . . . . . . . . . . . . . . 26 4.6. Generation of Symmetric Keys for Cryptographic Modules . 26 4.7. Encryption of Pseudorandom Nonces Sent from the DSKPP Client . . . . . . . . . . . . . . . . . . . . . . . . . 27 4.8. MAC calculations . . . . . . . . . . . . . . . . . . . . 27 4.8.1. Four-pass DSKPP . . . . . . . . . . . . . . . . . . . 27 4.8.2. Two-pass DSKPP . . . . . . . . . . . . . . . . . . . 28 Doherty, et al. Expires January 27, 2008 [Page 3] Internet-Draft DSKPP July 2007 4.8.3. One-pass DSKPP . . . . . . . . . . . . . . . . . . . 29 4.9. DSKPP Schema Basics . . . . . . . . . . . . . . . . . . . 30 4.9.1. The AbstractRequestType Type . . . . . . . . . . . . 31 4.9.2. The AbstractResponseType Type . . . . . . . . . . . . 31 4.9.3. The VersionType Type . . . . . . . . . . . . . . . . 32 4.9.4. The IdentifierType Type . . . . . . . . . . . . . . . 32 4.9.5. The StatusCode Type . . . . . . . . . . . . . . . . . 32 4.9.6. The DeviceIdentifierDataType Type . . . . . . . . . . 34 4.9.7. The TokenPlatformInfoType and PlatformType Types . . 35 4.9.8. The NonceType Type . . . . . . . . . . . . . . . . . 35 4.9.9. The AlgorithmsType Type . . . . . . . . . . . . . . . 36 4.9.10. The ProtocolVariantsType and the TwoPassSupportType Types . . . . . . . . . . . . . . 36 4.9.11. The KeyContainersFormatTypeType . . . . . . . . . . . 37 4.9.12. The AuthenticationDataType Type . . . . . . . . . . . 38 4.9.13. The PayloadType Type . . . . . . . . . . . . . . . . 40 4.9.14. The MacType Type . . . . . . . . . . . . . . . . . . 40 4.9.15. The KeyContainerType Type . . . . . . . . . . . . . . 40 4.9.16. The ExtensionsType and the AbstractExtensionType Types . . . . . . . . . . . . . . . . . . . . . . . . 41 4.10. DSKPP Messages . . . . . . . . . . . . . . . . . . . . . 41 4.10.1. Introduction . . . . . . . . . . . . . . . . . . . . 41 4.10.2. DSKPP Initialization (OPTIONAL) . . . . . . . . . . . 41 4.10.3. The DSKPP Client's Initial PDU (2- and 4-Pass) . . . 43 4.10.4. The DSKPP Server's Initial PDU (4-Pass Only) . . . . 46 4.10.5. The DSKPP Client's Second PDU (4-Pass Only) . . . . . 47 4.10.6. The DSKPP Server's Final PDU (1-, 2-, and 4-Pass) . . 48 4.11. Protocol Extensions . . . . . . . . . . . . . . . . . . . 50 4.11.1. The ClientInfoType Type . . . . . . . . . . . . . . . 50 4.11.2. The ServerInfoType Type . . . . . . . . . . . . . . . 50 4.11.3. The KeyInitializationDataType Type . . . . . . . . . 51 5. Protocol Bindings . . . . . . . . . . . . . . . . . . . . . . 52 5.1. General Requirements . . . . . . . . . . . . . . . . . . 52 5.2. HTTP/1.1 Binding for DSKPP . . . . . . . . . . . . . . . 52 5.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 52 5.2.2. Identification of DSKPP Messages . . . . . . . . . . 53 5.2.3. HTTP Headers . . . . . . . . . . . . . . . . . . . . 53 5.2.4. HTTP Operations . . . . . . . . . . . . . . . . . . . 53 5.2.5. HTTP Status Codes . . . . . . . . . . . . . . . . . . 53 5.2.6. HTTP Authentication . . . . . . . . . . . . . . . . . 54 5.2.7. Initialization of DSKPP . . . . . . . . . . . . . . . 54 5.2.8. Example Messages . . . . . . . . . . . . . . . . . . 54 6. DSKPP Schema . . . . . . . . . . . . . . . . . . . . . . . . 55 7. Security Considerations . . . . . . . . . . . . . . . . . . . 63 7.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 63 7.2. Active Attacks . . . . . . . . . . . . . . . . . . . . . 63 7.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 63 7.2.2. Message Modifications . . . . . . . . . . . . . . . . 64 Doherty, et al. Expires January 27, 2008 [Page 4] Internet-Draft DSKPP July 2007 7.2.3. Message Deletion . . . . . . . . . . . . . . . . . . 65 7.2.4. Message Insertion . . . . . . . . . . . . . . . . . . 65 7.2.5. Message Replay . . . . . . . . . . . . . . . . . . . 66 7.2.6. Message Reordering . . . . . . . . . . . . . . . . . 66 7.2.7. Man-in-the-Middle . . . . . . . . . . . . . . . . . . 66 7.3. Passive Attacks . . . . . . . . . . . . . . . . . . . . . 66 7.4. Cryptographic Attacks . . . . . . . . . . . . . . . . . . 67 7.5. Attacks on the Interaction between DSKPP and User Authentication . . . . . . . . . . . . . . . . . . . . . 67 7.6. Additional Considerations Specific to 2- and 1-pass DSKPP . . . . . . . . . . . . . . . . . . . . . . . . . . 68 7.6.1. Client Contributions to K_TOKEN Entropy . . . . . . . 68 7.6.2. Key Confirmation . . . . . . . . . . . . . . . . . . 68 7.6.3. Server Authentication . . . . . . . . . . . . . . . . 68 7.6.4. Client Authentication . . . . . . . . . . . . . . . . 68 7.6.5. Key Protection in the Passphrase Profile . . . . . . 69 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 70 9. Intellectual Property Considerations . . . . . . . . . . . . 70 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 70 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 70 11.1. Normative references . . . . . . . . . . . . . . . . . . 70 11.2. Informative references . . . . . . . . . . . . . . . . . 71 Appendix A. Key Initialization Profiles of DSKPP . . . . . . . . 72 A.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 73 A.2. Key Transport Profile . . . . . . . . . . . . . . . . . . 73 A.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 73 A.2.2. Identification . . . . . . . . . . . . . . . . . . . 73 A.2.3. Payloads . . . . . . . . . . . . . . . . . . . . . . 73 A.3. Key wrap profile . . . . . . . . . . . . . . . . . . . . 74 A.3.1. Introduction . . . . . . . . . . . . . . . . . . . . 74 A.3.2. Identification . . . . . . . . . . . . . . . . . . . 74 A.3.3. Payloads . . . . . . . . . . . . . . . . . . . . . . 74 A.4. Passphrase-based key wrap profile . . . . . . . . . . . . 76 A.4.1. Introduction . . . . . . . . . . . . . . . . . . . . 76 A.4.2. Identification . . . . . . . . . . . . . . . . . . . 76 A.4.3. Payloads . . . . . . . . . . . . . . . . . . . . . . 76 Appendix B. Example Messages . . . . . . . . . . . . . . . . . . 77 B.1. Example Messages in a Four-pass Exchange . . . . . . . . 77 B.1.1. Example of a DSKPP Initialization (Trigger) Message . 78 B.1.2. Example of a Message . . . . . . . . . 79 B.1.3. Example of a Message . . . . . . . . . 80 B.1.4. Example of a Message . . . . . . . . . 80 B.1.5. Example of a Message . . . . . . . . 80 B.2. Example Messages in a Two- or One-pass Exchange . . . . . 81 B.2.1. Example of a Message Indicating Support for Two-pass DSKPP . . . . . . . . . . . . . 81 B.2.2. Example of a Message Using the Key Transport Profile . . . . . . . . . . . . . . . . 83 Doherty, et al. Expires January 27, 2008 [Page 5] Internet-Draft DSKPP July 2007 B.2.3. Example of a Message Using the Key Wrap Profile . . . . . . . . . . . . . . . . . . 85 B.2.4. Example of a Message using the Passphrase-based Key Wrap Profile . . . . . . . . . . 86 Appendix C. Requirements . . . . . . . . . . . . . . . . . . . . 88 Appendix D. Integration with PKCS #11 . . . . . . . . . . . . . 90 D.1. The 4-pass Variant . . . . . . . . . . . . . . . . . . . 91 D.2. The 2-pass Variant . . . . . . . . . . . . . . . . . . . 91 D.3. The 1-pass Variant . . . . . . . . . . . . . . . . . . . 93 Appendix E. Example of DSKPP-PRF Realizations . . . . . . . . . 95 E.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 96 E.2. DSKPP-PRF-AES . . . . . . . . . . . . . . . . . . . . . . 96 E.2.1. Identification . . . . . . . . . . . . . . . . . . . 96 E.2.2. Definition . . . . . . . . . . . . . . . . . . . . . 96 E.2.3. Example . . . . . . . . . . . . . . . . . . . . . . . 97 E.3. DSKPP-PRF-SHA256 . . . . . . . . . . . . . . . . . . . . 97 E.3.1. Identification . . . . . . . . . . . . . . . . . . . 97 E.3.2. Definition . . . . . . . . . . . . . . . . . . . . . 98 E.3.3. Example . . . . . . . . . . . . . . . . . . . . . . . 99 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 99 Intellectual Property and Copyright Statements . . . . . . . . . 100 Doherty, et al. Expires January 27, 2008 [Page 6] Internet-Draft DSKPP July 2007 1. Introduction 1.1. Scope This document describes 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. The objectives of this protocol are to: o Provide a secure method of initializing cryptographic modules with symmetric keys without exposing generated, secret material to any other entities than the server and the cryptographic module itself. o Provide a secure method of generating and transporting symmetric keys to a cryptographic module in environments where near real-time communication is not possible. o Provide a secure method of transporting pre-generated (i.e., legacy) keys to a cryptographic module. o Provide a solution that is easy to administer and scales well. The mechanism is intended for general use within computer and communications systems employing symmetric cryptographic modules that are locally (i.e., over-the-wire) or remotely (i.e., over-the-air) accessible. 1.2. Background A symmetric cryptographic module may be hosted by a hand-held hardware device (e.g., a mobile phone), a hardware device connected to a personal computer through an electronic interface, such as USB, or a software application resident on a personal computer. The cryptographic module offers symmetric cryptographic functionality that may be used to authenticate a user towards some service, perform data encryption, etc. Increasingly, these modules enable their programmatic initialization as well as programmatic retrieval of their output values. This document intends to meet the need for an open and inter-operable mechanism to programmatically initialize and configure symmetric keys to locally and remotely accessible cryptographic modules. The target mechanism addressed herein is a symmetric key provisioning server. In an ideal deployment scenario, near real-time communication is possible between the provisioning server and the cryptographic module. In such an environment, it is possible for the cryptographic module and provisioning server to mutually generate a symmetric key, and to ensure that keys are not transported between Doherty, et al. Expires January 27, 2008 [Page 7] Internet-Draft DSKPP July 2007 them. There are, however, several deployment scenarios that make mutual key generation less suitable. Specifically, scenarios where near real- time communication between the symmetric key provisioning server and the cryptographic module is not possible, and scenarios with significant design constraints. Examples include work-flow constraints (e.g., policies that require incremental administrative approval), network design constraints that create network latency, and budget constraints that sustain reliance upon legacy systems that already have supplies of pre-generated keys. In these situations, the cryptographic module is required to download and install a symmetric key from the provisioning server in a secure and efficient manner. This document tries to meet the needs of these scenarios by describing three variations to DSKPP for the provisioning of symmetric keys in two round trips or less. The four-pass (i.e., two round-trip) variant enables key generation in near real-time. With this variant, keys are mutually generated by the provisioning server and cryptographic module; provisioned keys are not transferred over- the-wire or over-the-air. In contrast, two- and one-pass variants enable secure and efficient download and installation of symmetric keys to a cryptographic module in environments where near real-time communication is not possible. 2. Notation and Terminology 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]. The following notations are used in this document: || String concatenation [x] Optional element x A ^ B Exclusive-OR operation on strings A and B (where A and B are of equal length) DSKPP client Manages communication between the symmetric cryptographic module and the DSKPP server DSKPP server The symmetric key provisioning server that participates in the DSKPP protocol run ID_C Identifier for DSKPP client Doherty, et al. Expires January 27, 2008 [Page 8] Internet-Draft DSKPP July 2007 ID_S Identifier for DSKPP server K Key used to encrypt R_C (either K_SERVER or K_SHARED) K_AUTH Secret key used for server authentication purposes in 4-pass DSKPP K_CLIENT Public key of the DSKPP client K_DERIVED Secret key derived from a passphrase that is known to both the DSKPP client or user and the DSKPP server K_MAC Secret key used for key confirmation and server authentication purposes, and generated in DSKPP K_MAC' A second secret key used for server authentication purposes in 2- and 1-pass DSKPP K_SERVER Public key of the DSKPP server K_SHARED Secret key shared between the DSKPP client and the DSKPP server K_TOKEN Secret key used for cryptographic module computations, and generated in DSKPP R Pseudorandom value chosen by the DSKPP client and used for MAC computations, which is mandatory for 2-pass DSKPP and optional for 4-pass 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 The following typographical convention is used in the body of the text: . 3. Use Cases This section describes typical use cases. 3.1. A cryptographic module obtains a symmetric key A cryptographic module hosted by a device, such as a mobile phone, makes a request for a symmetric key from a provisioning server. Depending upon how the system is deployed, 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. 3.2. A cryptographic module acquires multiple symmetric keys of different types A cryptographic module makes multiple requests for symmetric keys from the same provisioning server. The symmetric keys may or may not be of the same type, i.e., the keys may be used with different Doherty, et al. Expires January 27, 2008 [Page 9] Internet-Draft DSKPP July 2007 symmetric cryptographic algorithms, including the HMAC-Based One-Time Password (HOTP), RSA SecurID, challenge-response, etc. 3.3. A provisioning server imposes a validity period policy for provisioning sessions Once a cryptographic module initiates a symmetric key request, the provisioning server may require that any subsequent actions to complete the provisioning cycle occur within a certain time window. For example, an issuer may provide a time-limited authentication code to a user during registration, which the user will input into the cryptographic module to authenticate themselves with the provisioning server. As long as the user inputs a valid authentication code within the fixed time period established by the issuer, the server will provision a key to the cryptographic module hosted by the user's device. 3.4. A symmetric key issuer uses a third party provisioning service provider A symmetric key issuer outsources its key provisioning to a third party key provisioning server provider. The issuer is responsible for authenticating and granting rights to users to acquire keys while acting as a proxy to the cryptographic module to acquire symmetric keys from the provisioning server; the cryptographic module communicates with the issuer proxy server, which forwards provisioning requests to the provisioning server. 3.5. A cryptographic module renews its symmetric key with the same key ID 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. 3.6. An administrator initiates a symmetric key replacement before it can be used This use case 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 an 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. Doherty, et al. Expires January 27, 2008 [Page 10] Internet-Draft DSKPP July 2007 Bulk initialization under controlled conditions, e.g., during manufacture, is likely to meet the security needs of most applications. However, reliance on a pre-disclosed secret is unacceptable in some circumstances. One such circumstance is when cryptographic modules are issued for classified government use or high security applications. In such cases, the issuer requires the ability to remove all secret information already installed on the cryptographic module and replace it with symmetric keys established under conditions controlled by the issuer. Another variation of this use case is the issuer who recycles devices. In this case, an 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 use case is essentially the same as the last use case wherein the same key ID is used for renewal. 3.7. A cryptographic module hosted by a smart card uses a pre-shared transport key to communicate with the provisioning server 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-shared transport 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 use case are for the protocol to be tunneled and the provisioning server to know the correct pre- established transport key. 3.8. A cryptographic module hosted by a mobile device downloads a symmetric key through SMS A mobile device supports Short Message Service (SMS) but is not able to support a data service allowing for HTTP or HTTPS transports. In addition, the cryptographic module can ensure that SMS will provide an acceptable level of protection for download of the symmetric key. In such a case, the cryptographic module hosted by the mobile device may initiate a symmetric key request from a desktop computer and ask the server to send the key to the mobile device through SMS. User authentication is carried out via the online communication established between the desktop computer and the provisioning server. Doherty, et al. Expires January 27, 2008 [Page 11] Internet-Draft DSKPP July 2007 3.9. A cryptographic module acquires a symmetric key over a transport protocol that does not ensure data confidentiality Some devices are not able to support a secure transport channel such as SSL or TLS to provide data confidentiality. A cryptographic module hosted by such a device requests a symmetric key from the provisioning server. It is up to DSKPP to ensure data confidentiality over non-secure networks. 3.10. A cryptographic module acquires a symmetric key over a transport protocol that does not provide authentication Some devices are not able to use a transport protocol that provides server authentication such as SSL or TLS. A cryptographic module hosted by such a device wants to be sure that it sends a request for a symmetric key to a legitimate provisioning server. It is up to DSKPP to provide proper client and server authentication. 4. DSKPP 4.1. 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. The DSKPP server herein 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 represents the following: User The person or client to whom devices are issued UserID A unique identifier for the user or client Device A physical piece of hardware that hosts symmetric cryptographic modules DeviceID A unique identifier for the device Cryptographic Module A low-level component of an application, which enables symmetric cryptographic functionality CryptoModuleID A unique identifier for an instance of the cryptographic module Encryption Algorithms Encryption algorithms supported by the cryptographic module Doherty, et al. Expires January 27, 2008 [Page 12] Internet-Draft DSKPP July 2007 MAC Algorithms MAC algorithms supported by the cryptographic module Key Container An object that encapsulates a symmetric key and its configuration data KeyID A unique identifier for the symmetric key Key Type The type of symmetric cryptographic methods for which the key will be used (e.g., OATH HOTP or RSA SecurID authentication, AES encryption, etc.) ----------- ------------- | User | | Device | |---------|* owns *|-----------| | UserID |--------->| DeviceID | | ... | | ... | ----------- ------------- | 1 | | contains | | * V ----------------------- |Cryptographic Module | |---------------------| |CryptoModuleID |Encryption Algorithms| |MAC Algorithms | |... | ----------------------- | 1 | | contains | | * V ----------------------- |Key Container | |---------------------| |KeyID | |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 Doherty, et al. Expires January 27, 2008 [Page 13] Internet-Draft DSKPP July 2007 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]. 4.2. Principles of Operation To initiate a DSKPP session, a user may use a browser to connect to a web server. The user may then identify and optionally authenticate herself and possibly indicate how the DSKPP client has to contact the DSKPP server. There are also other alternatives for DSKPP session initiation, such as the DSKPP client being pre-configured to contact a certain DSKPP server, or the user being informed out-of-band about the address 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, 2-pass, or 1-pass protocol. 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. Two- and one-pass variants enable secure and efficient download and installation of symmetric keys to a cryptographic module in environments where near real-time communication may not be possible. DSKPP protocol variants may be applied to the use cases described in Section 3, as shown below: Doherty, et al. Expires January 27, 2008 [Page 14] Internet-Draft DSKPP July 2007 ========================================================== Protocol Applicable Applicable Variant Use Cases Deployment Scenarios ========================================================== 4-pass All but 3.6 and Near real-time 3.8 if mutual key communication is generation is desired; possible none if transport of a pre-generated key is required ----------------------------------------------------------- 2-pass All Either near real-time or non real-time communication may be possible ----------------------------------------------------------- 1-pass All but 3.8 Either near real-time or non real-time communication may be possible ========================================================== Figure 2: Mapping of use cases to protocol variants 4.2.1. Four-pass DSKPP The 4-pass protocol flow is suitable for environments wherein there is near real-time communication possible between the DSKPP client and DSKPP server. It is not suitable for environments wherein administrative approval is a required step in the flow, nor for provisioning of pre-generated keys. The 4-pass protocol flow, shown in Figure 3 and expanded in Figure 4, consists of two round trips between the DSKPP client and server. Doherty, et al. Expires January 27, 2008 [Page 15] Internet-Draft DSKPP July 2007 +---------------+ +---------------+ | | | | | DSKPP client | | DSKPP server | | | | | +---------------+ +---------------+ | | | [ <---- DSKPP trigger ----- ] | | | | ------- Client Hello -------> | | | | <------ Server Hello -------- | | | | ------- Client Nonce -------> | | | | <----- Server Finished ------ | | | Figure 3: The 4-pass DSKPP protocol (with OPTIONAL preceeding trigger) a. The DSKPP client sends a message to the DSKPP server. The message provides information to the DSKPP server about the DSKPP versions, protocol variants, key types, encryption and MAC algorithms supported by the cryptographic module for the purposes of this protocol. The message may also include client authentication data, such as a certificate or authentication code. b. The DSKPP server responds to the DSKPP client with a message, whose content includes a random nonce, R_S, along with information about the type of key to generate, and the encryption algorithm chosen to protect sensitive data sent in the protocol. The length of the nonce R_S may depend on the selected key type. The message also provides information about either a shared secret key to use for encrypting the cryptographic module's random nonce (see description of below), or its own public key. Optionally, may include a MAC that the DSKPP client may use for server authentication. c. Based on information contained in the message, the cryptographic module generates a random nonce, R_C. The length of the nonce R_C may 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. If K is equivalent to K_SERVER, then the cryptographic module SHOULD verify the server's certificate before using it to encrypt R_C. The DSKPP client then sends the encrypted random nonce to the DSKPP Doherty, et al. Expires January 27, 2008 [Page 16] Internet-Draft DSKPP July 2007 server in a message, and may include client authentication data, such as a certificate or authentication code. Finally, the cryptographic module calculates a symmetric key, K_TOKEN, of the selected type from the combination of the two random nonces R_S and R_C, the encryption key K, and possibly some other data, using the DSKPP-PRF function defined in Section 4.5. d. 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, using the DSKPP-PRF function defined in Section 4.5. 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. e. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . The confirmation message includes a key container 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. Optionally, may include a MAC that the DSKPP client may use for server authentication. f. Upon receipt of the DSKPP server's confirmation message, the cryptographic module associates the provided key container with the generated key K_TOKEN, and stores any provided configuration data. 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 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 Doherty, et al. Expires January 27, 2008 [Page 17] Internet-Draft DSKPP July 2007 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 synch" 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. by using a connection independent from the one used for the key generation). +----------------------+ +-------+ +----------------------+ | +------------+ | | | | | | | 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 4: Principal data flow for DSKPP key generation - using public server key Doherty, et al. Expires January 27, 2008 [Page 18] Internet-Draft DSKPP July 2007 4.2.2. Two-pass DSKPP The 2-pass protocol flow is suitable for environments wherein near real-time communication between the DSKPP client and server may not be possible. It is also suitable for environments wherein administrative approval is a required step in the flow, and for provisioning of pre-generated keys. In the 2-pass protocol flow, shown in Figure 5, the client's initial message is directly followed by a message. There is no exchange of the message or the message. However, as the two-pass variant 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 than by including R_C in , the DSKPP client is able to ensure the server is alive before "commiting" the key. Also note that the DSKPP "trigger" message MAY be used to trigger the client's sending of the message. Essentially, two-pass DSKPP is a transport of key material from the DSKPP server to the DSKPP client. Two-pass DSKPP supports multiple key initialization methods that ensure K_TOKEN is not exposed to any other entity than the DSKPP server and the cryptographic module itself. Currently, three such key initialization methods are defined (refer to Appendix A), each supporting a different usage of 2-pass DSKPP: Key Transport This profile is intended for PKI-capable devices. Key transport is carried out using a public key, K_CLIENT, whose private key part resides in the cryptographic module as the transport key. Key Wrap This profile is ideal for pre-keyed devices, e.g., SIM cards. Key wrap is carried out using a symmetric key- wrapping key, K_SHARED, 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, K_DERIVED, which is known in advance by both the cryptographic module and DSKPP server. Doherty, et al. Expires January 27, 2008 [Page 19] Internet-Draft DSKPP July 2007 +---------------+ +---------------+ | | | | | DSKPP client | | DSKPP server | | | | | +---------------+ +---------------+ | | | [ <---- DSKPP trigger ----- ] | | | | ------- Client Hello -------> | | | | <----- Server Finished ------ | | | Figure 5: The 2-pass DSKPP protocol (with OPTIONAL preceding trigger) a. The DSKPP client sends a message to the DSKPP server. The message provides the client nonce, R_C, and information about the DSKPP versions, protocol variants, key types, encryption and MAC algorithms supported by the cryptographic module for the purposes of this protocol. The message may also include client authentication data, such as a certificate or authentication code. Unlike 4-pass DSKPP, 2-pass DSKPP client uses the message to declare which key initialization method it supports, providing required payload information, e.g., K_CLIENT for the Key Transport Profile. b. The DSKPP server generates a key K from which two keys, K_TOKEN and K_MAC are derived. K is either transported or wrapped in accordance with the key initialization method specified by the DSKPP client in the message. 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. c. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . The confirmation message includes a key container that holds an identifier for the key, the key K from which K_TOKEN and K_MAC are derived, and additional configuration information (note that the latter MUST include the identity of the DSKPP server for authentication purposes). In addition, MUST include two MACs 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 authentication before "commiting" the key. Doherty, et al. Expires January 27, 2008 [Page 20] Internet-Draft DSKPP July 2007 d. Upon receipt of the DSKPP server's confirmation message, the cryptographic module extracts the key data from the provided key container, uses the two MAC values to perform key confirmation and server authentication, and stores the key material locally. 4.2.3. One-pass DSKPP The one-pass protocol flow is suitable for environments wherein near real-time communication between the DSKPP client and server may not be possible. It is also suitable for environments wherein administrative approval is a required step in the flow, and for provisioning of pre-generated keys. In one-pass DSKPP, shown in Figure 6, the server simply sends a message to the DSKPP client. In this case, there is no exchange of the , , and DSKPP messages, and hence there is no way for the client to express supported algorithms or key types. Before attempting one-pass DSKPP, the server MUST therefore have prior knowledge not only that the client is able and willing to accept this variant of DSKPP, but also of algorithms and key types supported by the client. Essentially, one-pass DSKPP is a transport of key material from the DSKPP server to the DSKPP client. As with two-pass DSKPP, the one- pass variant relies on key initialization methods that ensure K_TOKEN is not exposed to any other entity than the DSKPP server and the cryptographic module itself. The same key initialization profiles are defined as described in Section 4.2.2 and Appendix A. Outside the specific cases where one-pass DSKPP is desired, clients SHOULD be constructed and configured to only accept DSKPP server messages in response to client-initiated transactions. +---------------+ +---------------+ | | | | | DSKPP client | | DSKPP server | | | | | +---------------+ +---------------+ | | | <----- Server Finished ------ | | | Figure 6: The 1-pass DSKPP protocol Doherty, et al. Expires January 27, 2008 [Page 21] Internet-Draft DSKPP July 2007 a. The DSKPP server generates a key K from which two keys, K_TOKEN and K_MAC are derived. K is either transported or wrapped in accordance with the key initialization method known in advance by the DSKPP server. 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. b. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . The confirmation message includes a key container that holds an identifier for the key, the key K from which K_TOKEN and K_MAC are derived, and additional configuration information (note that the latter MUST include the identity of the DSKPP server for authentication purposes). In addition, MUST include two MACs, which will allow the cryptographic module to perform key confirmation and server authentication before "commiting" the key. Note that unlike two-pass DSKPP, in the one-pass variant, the server does not have the client nonce, R_C, and therefore the MACs values are calculated with contribution from an unsigned integer, I, generated by the server during the protocol run. c. Upon receipt of the DSKPP server's confirmation message, the cryptographic module extracts the key data from the provided key container, uses the two MAC values to perform key confirmation and server authentication, and stores the key material locally. 4.3. Authentication 4.3.1. Client Authentication (Applicable to Four- and Two-Pass DSKPP) To ensure that a generated 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 in several ways, as discussed in the following sub-sections. Whatever the method, the DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user. For a further discussion of this, and threats related to man-in-the- middle attacks in this context, see Section 7. 4.3.1.1. Device Certificate Instead of requiring an Authentication Code for in-band authentication, a device certificate could be used, which was supplied with the cryptographic module by its issuer. Doherty, et al. Expires January 27, 2008 [Page 22] Internet-Draft DSKPP July 2007 4.3.1.2. Device Identifier The provisioning server could be pre-configured with a device identifier. The DSKPP server MAY then include this identifier in the DSKPP initialization trigger, and the DSKPP client would 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 protocol. 4.3.1.3. One-time Use Authentication Code A key issuer may provide a one-time value, called an Authentication Code, to the user or device out-of-band and require this value to be used by the DSKPP client when contacting the DSKPP server. The DSKPP client MAY include the authentication data in its (and for four-pass) message, and the DSKPP server MUST verify the data before continuing with the protocol run. Note: An alternate method for getting the Authentication Code to the client, is for the DSKPP server to place the value in the element of the DSKPP initialization trigger (if triggers are used; see Section 5.2.7) . +------------+ Get Authentication Code +------------+ | User |<------------------------->| Issuer | +------------+ +------------+ | | | | | | V V +--------------+ +--------------+ | Provisioning | Authentication Data | Provisioning | | Client |----------------------->| Server | +--------------+ +--------------+ Figure 7: User Authentication with One-Time Code Considering an Authentication Code as a special form of shared secret between a user and a provisioning server, Authentication Data can have one of the following forms: o AuthenticationData = Hash (Authentication Code) Doherty, et al. Expires January 27, 2008 [Page 23] Internet-Draft DSKPP July 2007 When an Authentication Code is used to initiate the protocol run, the Authentication Code MUST be sent to the DSKPP server in a secure manner. If the underlying transport channel is secure, the authentication data MAY contain the plaintext format or the hashed format of the Authentication Code using a hash function. o AuthenticationData = HMAC(Authentication Code, K_AUTH) If the underlying transport is not secure, the client MUST use a key K_AUTH and the Authentication Code to derive authentication data. For example, if the Authentication Code has a fixed format, e.g., AuthenticationCode = passwordLength || ID || password || checksum then AuthenticationData MAY be calculated as follows: AuthenticationData = AuthenticationCode->ID || B64(Digest) where for four-pass DSKPP, the cryptographic module uses the server nonce R_S in combination with the server URL to calculate the Digest: Digest = DSKPP-PRF-AES(K_AUTH, AuthCode->ID || serverURL || R_S, 16) Refer to Section 4.5 for a description of DSKPP-PRF in general and Appendix E for a description of DSKPP-PRF-AES. For two-pass DSKPP, the cryptographic module does not have access to the server nonce R_S in combination and so: Digest = DSKPP-PRF-AES(K_AUTH, AuthenticationCode->ID || serverURL, 16) In either case, K_AUTH MAY be derived AES key from AuthenticationCode->password as in: K_AUTH = truncate( Hash( Hash(...n times...( AuthCode->password ) ) ) ) where truncate() returns the first 16 bytes from the result of the last hash iteration, and n is the number of hash iterations (set to fixed values, e.g., between 10 and 100). Doherty, et al. Expires January 27, 2008 [Page 24] Internet-Draft DSKPP July 2007 o AuthenticationData = When a certificate is used for authentication, the authentication data MAY be client-signed. Authentication data MAY be omitted if client certificate authentication has been provided by the transport channel such as TLS. When an issuer delegates symmetric key provisioning to a third party provisioning service provider, both client authentication and issuer authentication are required by the provisioning server. Client authentication to the Issuer MAY be in-band or out-of-band as described above. The issuer acts as a proxy for the provisioning server. The issuer authenticates to the provisioning service provider either using a certificate or a pre-established secret key. 4.3.2. Server Authentication A DSKPP server MUST authenticate itself to avoid a false "Commit" of a symmetric key that which could cause the cryptographic module to end up in an initialized state for which the server does not know the stored key. To do this, the DSKPP server authenticates itself by including a MAC in each of its responses to the client. In 2-pass and 1-pass DSKPP, servers authenticate themselves by including a second MAC value in the response message. In addition, a DSKPP server can leverage transport layer authentication if it is available. 4.4. Symmetric Key Container Format The default symmetric key container format that is used in the message is based on the Portable Symmetric Key Container (PSKC) defined in [PSKC]. Alternative formats MAY include PKCS#12 [PKCS-12] or PKCS#5 XML [PKCS-5-XML] format. 4.5. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF 4.5.1. Introduction 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 Doherty, et al. Expires January 27, 2008 [Page 25] Internet-Draft DSKPP July 2007 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 E contains two example realizations of DSKPP-PRF. 4.5.2. 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 16 octets long. 4.6. Generation of Symmetric Keys for Cryptographic Modules In DSKPP, keys are generated using the DSKPP-PRF function, 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, and the input parameter dsLen is set to the desired length of the key, K_TOKEN (the length of K_TOKEN is given by the key's type): dsLen = (desired length of K_TOKEN) K_TOKEN = DSKPP-PRF (R_C, "Key generation" || k || R_S, dsLen) When computing K_TOKEN above, the output of DSKPP-PRF 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 January 27, 2008 [Page 26] Internet-Draft DSKPP July 2007 4.7. 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: Enc-R_C = DS ^ R_C The DSKPP server will then perform the reverse operation to extract R_C from Enc-R_C. Note: It may appear that an attacker, who learns a previous value of R_C, may be able to replay the corresponding R_S and, hence, learn a new R_C as well. However, this attack is mitigated by the requirement for a server to show knowledge of K_AUTH (see below) in order to successfully complete a key re-generation. 4.8. MAC calculations 4.8.1. Four-pass DSKPP 4.8.1.1. Server Authentication: The MAC value MUST be computed on the (ASCII) string "MAC 1 computation", the client's nonce R (if sent), and the server's nonce R_S using an authentication key K_AUTH that SHOULD be a special authentication key used only for this purpose but MAY be the current K_TOKEN. The MAC value MAY be computed by using the DSKPP-PRF function of Section 4.5, 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 K_AUTH. The input Doherty, et al. Expires January 27, 2008 [Page 27] Internet-Draft DSKPP July 2007 parameter dsLen MUST be set to the length of R_S: dsLen = len(R_S) MAC = DSKPP-PRF (K_AUTH, "MAC 1 computation" || [R ||] R_S, dsLen) 4.8.1.2. Server Authentication: The MAC value MUST be computed on the (ASCII) string "MAC 2 computation" and R_C using an authentication key K_AUTH. Again, this SHOULD be a special authentication key used only for this purpose, but MAY also be an existing K_TOKEN. (In this case, implementations MUST protect against attacks where K_TOKEN is used to pre-compute MAC values.) If no authentication key is present in the cryptographic module, and no K_TOKEN existed before the DSKPP run, K_AUTH MUST be the newly generated K_TOKEN. 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", R_C, the parameter dsLen MUST be set to the length of R_C: dsLen = len(R_C) MAC = DSKPP-PRF (K_AUTH, "MAC 2 computation" || R_C, dsLen) 4.8.2. Two-pass DSKPP 4.8.2.1. Key Confirmation In two-pass DSKPP, the client is REQUIRED to include a nonce R in the message. Further, the server is REQUIRED to include an identifier, ID_S, for itself (via the key container) in the message. The MAC value in the message MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and R using a MAC key K_MAC. Again, in contrast with the MAC calculation in the four-pass DSKPP, this key MUST be provided together with K_TOKEN to the cryptographic module, and hence there is no need for a K_AUTH for key confirmation purposes. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation" and R, and the parameter dsLen MUST be set to the length of R: dsLen = len(R) Doherty, et al. Expires January 27, 2008 [Page 28] Internet-Draft DSKPP July 2007 MAC = DSKPP-PRF (K_MAC, "MAC 1 computation" || ID_S || R, dsLen) 4.8.2.2. Server Authentication As discussed in Section 4.3.2, servers need to authenticate themselves when attempting to replace an existing K_TOKEN. In 2-pass DSKPP, servers authenticate themselves by including a second MAC value in the AuthenticationDataType element. The MAC value in the AuthenticationDataType element MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and R, using the existing MAC key K_MAC' (the MAC key that existed before this protocol run). The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation" ID_S, 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 1 computation" || ID_S || R, dsLen) 4.8.3. One-pass DSKPP 4.8.3.1. Key Confirmation In one-pass DSKPP, the server MUST include an identifier, ID_S, for itself (via the key container) in the message. The MAC value in the message MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and an unsigned integer value I, using a MAC key K_MAC. The value I MUST be monotonically increasing and guaranteed not to be used again by this server towards this cryptographic module. It could for example be the number of seconds since some point in time with sufficient granularity, a counter value, or a combination of the two where the counter value is reset for each new time value. In contrast to the MAC calculation in four-pass DSKPP, the MAC key K_MAC MUST be provided together with K_TOKEN to the cryptographic module, and hence there is no need for a K_AUTH for key confirmation purposes. Note: The integer I does not necessarily need to be maintained per cryptographic module by the DSKPP server (it is enough if the server can guarantee that the same value is never being sent twice to the same cryptographic module). If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 Doherty, et al. Expires January 27, 2008 [Page 29] Internet-Draft DSKPP July 2007 computation", ID_S, and I. 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 1 computation" || ID_S || I, dsLen) The server MUST provide I to the client in the Nonce attribute of the element of the message using the AuthenticationCodeMacType defined in Section 4.9.12. 4.8.3.2. Server Authentication As discussed in Section 4.3.2, servers need to authenticate themselves when attempting to replace an existing K_TOKEN. In 1-pass DSKPP, servers authenticate themselves by including a second MAC value in the AuthenticationDataType element. The MAC value in the AuthenticationDataType element MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and a new value I', I' > I, using the existing MAC key K_MAC' (the MAC key that existed before this protocol run). The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation" ID_S, and I'. 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 1 computation" || ID_S || I', dsLen) The server MUST provide I' to the client in the Nonce attribute of the element of the AuthenticationDataType extension. If the protocol run is successful, the client stores I' as the new value of I for this server. 4.9. DSKPP Schema Basics This section describes the schema used by DSKPP. The DSKPP XML schema itself can be found in Section 6. Specific protocol message elements are defined in Section 4.10. Examples can be found in Appendix B. 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 Doherty, et al. Expires January 27, 2008 [Page 30] Internet-Draft DSKPP July 2007 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. No collation or sorting order for attributes or element values is defined. Therefore, DSKPP implementations MUST NOT depend on specific sorting orders for values. 4.9.1. The AbstractRequestType Type All DSKPP requests are defined as extensions to the abstract AbstractRequestType type. The elements of the AbstractRequestType, therefore, apply to all DSKPP requests. All DSKPP requests MUST contain a Version attribute. For this version of this specification, Version MUST be set to "1.0". 4.9.2. The AbstractResponseType Type All DSKPP responses are defined as extensions to the abstract AbstractResponseType type. The elements of the AbstractResponseType, therefore, apply to all DSKPP responses. All DSKPP responses contain a Version attribute indicating the version that was used. A Status attribute, which indicates whether the preceding request was successful or not MUST also be present. Finally, all responses MAY contain a SessionID attribute identifying the particular DSKPP session. The SessionID attribute needs only be present if more than one roundtrip is REQUIRED for a successful protocol run (this is the case with the protocol version described herein). Doherty, et al. Expires January 27, 2008 [Page 31] Internet-Draft DSKPP July 2007 4.9.3. The VersionType Type The VersionType type is used within DSKPP messages to identify the highest version of this protocol supported by the DSKPP client and server. 4.9.4. The IdentifierType Type The IdentifierType type is used to identify various DSKPP elements, such as sessions, users, and services. Identifiers MUST NOT be longer than 128 octets. 4.9.5. The StatusCode Type The StatusCode type enumerates all possible return codes: Doherty, et al. Expires January 27, 2008 [Page 32] Internet-Draft DSKPP July 2007 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: 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. Doherty, et al. Expires January 27, 2008 [Page 33] Internet-Draft DSKPP July 2007 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. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested encryption algorithms. 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. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested MAC algorithms. o "NoProtocolVariants" indicates that the DSKPP client only suggested a protocol variant (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. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested protocol variants. o "NoSupportedKeyContainers" indicates that the DSKPP client only suggested key container formats that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested key container formats. o "AuthenticationDataInvalid" indicates that the DSKPP client supplied user or device 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. 4.9.6. The DeviceIdentifierDataType Type The DeviceIdentifierDataType type is used to uniquely identify the device that houses the cryptographic module, e.g., a mobile phone. Doherty, et al. Expires January 27, 2008 [Page 34] Internet-Draft DSKPP July 2007 The device identifier allows the DSKPP server to find, e.g., a pre- shared transport key for 2-pass DSKPP and/or the correct shared secret for MAC'ing purposes. The default DeviceIdentifierDataType is defined in [PSKC]. 4.9.7. The TokenPlatformInfoType and PlatformType Types The TokenPlatformInfoType type is used to carry characteristics of the intended cryptographic module platform, and applies in the public-key variant of DSKPP in situations when the client potentially needs to select a cryptographic module to initialize. 4.9.8. The NonceType Type The NonceType type is used to carry pseudorandom values in DSKPP messages. A nonce, as the name implies, MUST be used only once. For each DSKPP message that requires a nonce element to be sent, a fresh nonce MUST be generated each time. Nonce values MUST be at least 16 octets long. Doherty, et al. Expires January 27, 2008 [Page 35] Internet-Draft DSKPP July 2007 4.9.9. The AlgorithmsType Type The AlgorithmsType type is a list of type-value pairs that define algorithms supported by a DSKPP client or server. Algorithms are identified through URIs. 4.9.10. The ProtocolVariantsType and the TwoPassSupportType Types The ProtocolVariantsType type is OPTIONALLY used by the DSKPP client to indicate the number of passes of the DSKPP protocol that it supports (see Section 4.2). The ProtocolVariantsType MAY be used to indicate support for 4-pass or 2-pass DSKPP. Because 1-pass DSKPP does not include a client request to the server, the ProtocolVariantsType type MAY NOT be used to indicate support for 1-pass DSKPP. If the ProtocolVariantsType is not used, then the DSKPP server will proceed with ordinary 4-pass DSKPP. However, it does not support 4-pass DSKPP, then the server MUST find a suitable two-pass variant or else the protocol run will fail. Doherty, et al. Expires January 27, 2008 [Page 36] Internet-Draft DSKPP July 2007 The TwoPassSupportType type signals client support for the 2-pass version of DSKPP, informs the server of supported two-pass variants, 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 two-pass variant the payload is associated with. The elements of this type have the following meaning: o : A two-pass key initialization 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 initialization 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). 4.9.11. The KeyContainersFormatTypeType The KeyContainersFormatType type is a list of type-value pairs that are OPTIONALLY used to define key container formats supported by a DSKPP client or server. Key container formats are identified through URIs, e.g., the PSKC URI "http://www.openauthentication.org/OATH/2006/10/PSKC#KeyContainer" (see [PSKC]. Doherty, et al. Expires January 27, 2008 [Page 37] Internet-Draft DSKPP July 2007 4.9.12. The AuthenticationDataType Type The AuthenticationDataType type is OPTIONALLY used to carry client or server authentication values in DSKPP messages (see Section 4.3). The element MAY be used as follows: a. A DSKPP client MAY include a one-time use AuthenticationCode that was given by the issuer to the user for acquiring a symmetric key. An AuthenticationCode MAY or MAY NOT contain alphanumeric characters in addition to numeric digits depending on the device type and policy of the issuer. For example, if the device is a mobile phone, a code that the user enters on the keypad would typically be restricted to numeric digits for ease of use. An activation code can be sent to the DSKPP server in plaintext form, hashed data form, or keyed hash data form depending on the underlying transport protocol. b. A DSKPP client MAY include an AuthenticationCertificate that contains a certificate issued with the device by the issuer. c. A DSKPP server MAY use the AuthenticationDataType element AuthenticationCodeMac to carry a MAC for authenticating itself to the client. For example, when a successful 1- or 2-pass DSKPP protocol run will result in an existing key being replaced, then the DSKPP server MUST include a MAC proving to the DSKPP client that the server knows the value of the key it is about to replace. Doherty, et al. Expires January 27, 2008 [Page 38] Internet-Draft DSKPP July 2007 The element of the AuthenticationDataType type have the following meaning: o : A requestor's identifier. The value MAY be a user ID, a device ID, or a keyID associated with the requestor's authentication value. When the authentication data is based on a certificate, can be omitted, as the certificate itself Doherty, et al. Expires January 27, 2008 [Page 39] Internet-Draft DSKPP July 2007 is typically sufficient to identify the requestor. Also, if a message was provided by the server to initiate the DSKPP protocol run, can be omitted, as the DeviceID, KeyID, and/or nonce provided in the element ought to be sufficient to identify the requestor. o : A one-time use value sent in the clear to the DSKPP server. o : A one-time use value sent in digest form to the DSKPP server. o : An authentication MAC and OPTIONAL additional information (e.g., MAC algorithm). The value could be a one-time use value sent as a MAC value to the DSKPP server; or, it could be a MAC value sent to the DSKPP client, where the MAC is calculated as described in Section 4.8. o : A device certificate sent to the DSKPP server. 4.9.13. The PayloadType Type The PayloadType type is used to carry data in a DSKPP client or server message. For this version of the protocol, only one payload is defined, the pseudorandom string R_S, for one message, the DSKPP . 4.9.14. The MacType Type The MacType type is used by the DSKPP server to carry a MAC value that the DSKPP server uses to authenticate itself to the client. 4.9.15. The KeyContainerType Type The KeyContainerType type is used by the DSKPP server in its final message to carry symmetric key(s) (in the 2- and 1-pass exchanges) Doherty, et al. Expires January 27, 2008 [Page 40] Internet-Draft DSKPP July 2007 and configuration data. The default element defined for the KeyContainerType is contained in the namespace defined in the PSKC namespace as KeyContainerType (see [PSKC]. 4.9.16. The ExtensionsType and the AbstractExtensionType Types The ExtensionsType type is a list of type-value pairs that define OPTIONAL DSKPP features supported by a DSKPP client or server. Extensions MAY be sent with any DSKPP message. Please see the description of individual DSKPP messages in Section 4.11 of this document for applicable extensions. All DSKPP extensions are defined as extensions to the AbstractExtensionType type. The elements of the AbstractExtensionType, therefore, apply to all DSKPP extensions. Unless an extension is marked as Critical, a receiving party need not be able to interpret it. A receiving party is always free to disregard any (non-critical) extensions. 4.10. DSKPP Messages 4.10.1. Introduction In this section, DSKPP messages, including their parameters, encoding and semantics are defined. 4.10.2. DSKPP Initialization (OPTIONAL) 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. Doherty, et al. Expires January 27, 2008 [Page 41] Internet-Draft DSKPP July 2007 Message used to trigger the device to initiate a DSKPP protocol run. 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 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. Doherty, et al. Expires January 27, 2008 [Page 42] Internet-Draft DSKPP July 2007 The Version attribute MUST be set to "1.0" for this version of DSKPP. 4.10.3. The DSKPP Client's Initial PDU (2- and 4-Pass) This message is the initial message sent from the DSKPP client to the DSKPP server. Doherty, et al. Expires January 27, 2008 [Page 43] Internet-Draft DSKPP July 2007 Message sent from DSKPP client to DSKPP server to initiate a DSKPP session. 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 4.3.1 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 5.2.7 below). In the latter case, it MUST have the same Doherty, et al. Expires January 27, 2008 [Page 44] Internet-Draft DSKPP July 2007 value as the identifier provided in that element. 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 was provided by the server in a element (see Section 5.2.7 below). In the latter case, it MUST have the same value as the identifier provided in that element. 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 5.2.7 below), 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 URIs indicating the key types for which the cryptographic module is willing to generate keys through DSKPP. o : A sequence of URIs 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 URIs 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., urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes defined in Appendix E). 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 URIs indicating the key container formats supported by the DSKPP client. If this element is not provided, then the DSKPP server MUST proceed with "http://www.openauthentication.org/OATH/2006/10/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 4.3.1. Doherty, et al. Expires January 27, 2008 [Page 45] Internet-Draft DSKPP July 2007 o : A sequence of extensions. One extension is defined for this message in this version of DSKPP: the ClientInfoType (see Section 4.11). 4.10.4. The DSKPP Server's Initial PDU (4-Pass Only) This message is the first message sent from 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. Message sent from DSKPP server to DSKPP client in response to a received ClientHello PDU. The components of this message have the following meaning: Doherty, et al. Expires January 27, 2008 [Page 46] Internet-Draft DSKPP July 2007 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. 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. 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 container format type to be used by the DSKPP server. The default setting relies on the KeyContainerType 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 4.11). 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 Second message sent from DSKPP client to DSKPP server in a DSKPP session. 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 : MUST have the same value as the SessionID attribute in the received message. 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 4.5. o : The authentication data value, which MAY OPTIONALLY be the same as provided in the , MUST be set as specified in Section 4.3.1. o : A list of extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 4.11). 4.10.6. The DSKPP Server's Final PDU (1-, 2-, and 4-Pass) 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. In a Doherty, et al. Expires January 27, 2008 [Page 48] Internet-Draft DSKPP July 2007 1-pass exchange, the DSKPP server sends only this message to the client. Final message sent from DSKPP server to DSKPP client in a DSKPP session. 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. 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 container containing symmetric key values (in the case of a 2- or 1-pass exchange) and configuration data. The default container 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 4.11). Doherty, et al. Expires January 27, 2008 [Page 49] Internet-Draft DSKPP July 2007 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. The MAC value MUST be computed as specified in Section 4.8.1.2. When receiving a message with Status="Success" for which the MAC verifies, the DSKPP client MUST associate the generated key K_TOKEN with the provided key identifier 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. The DSKPP client MUST verify the MAC. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and MUST, in this case, also delete any nonces, keys, and/or secrets associated with the failed run of the DSKPP protocol. The MacType's MacAlgorithm attribute MUST, when present, identify the negotiated MAC algorithm. 4.11. Protocol Extensions 4.11.1. The ClientInfoType Type When present in a or a message, the OPTIONAL ClientInfoType extension contains DSKPP client-specific information. DSKPP servers MUST support this extension. DSKPP servers MUST NOT attempt to interpret the data it carries and, if received, MUST include it unmodified in the current protocol run's next server response. Servers need not retain the ClientInfoType's data after that response has been generated. 4.11.2. The ServerInfoType Type When present, the OPTIONAL ServerInfoType extension contains DSKPP server-specific information. This extension is only valid in messages for which Status = "Continue". DSKPP clients Doherty, et al. Expires January 27, 2008 [Page 50] Internet-Draft DSKPP July 2007 MUST support this extension. DSKPP clients MUST NOT attempt to interpret the data it carries and, if received, MUST include it unmodified in the current protocol run's next client request (i.e., the message). DSKPP clients need not retain the ServerInfoType's data after that request has been generated. This extension MAY be used, e.g., for state management in the DSKPP server. 4.11.3. The KeyInitializationDataType Type This extension is used for 2- and 1-pass DSKPP exchange; it carries an identifier for the selected key initialization method as well as key initialization method-dependent payload data. Servers MAY include this extension in a message that is being sent in response to a received message if and only if that message selected TwoPassSupport as the ProtocolVariantType and the client indicated support for the selected key initialization method. Servers MUST include this extension in a message that is sent as part of a 1-pass DSKPP. Doherty, et al. Expires January 27, 2008 [Page 51] Internet-Draft DSKPP July 2007 This extension is only valid in ServerFinished PDUs. It contains key initialization data and its presence results in a two-pass (or one-pass, if no ClientHello was sent) DSKPP exchange. The elements of this type have the following meaning: o : A two-pass key initialization method supported by the DSKPP client. o : A payload associated with the key initialization method. Since the syntax is a shorthand for , any well-formed payloads can be carried in this element. 5. Protocol Bindings 5.1. General Requirements DSKPP assumes a reliable transport. 5.2. HTTP/1.1 Binding for DSKPP 5.2.1. Introduction This section presents a binding of the previous messages to HTTP/1.1 [RFC2616]. Note that the HTTP client normally will be different from the DSKPP client, i.e., the HTTP client will only exist to "proxy" DSKPP messages from the DSKPP client to the DSKPP server. Likewise, on the HTTP server side, the DSKPP server MAY receive DSKPP PDUs from a "front-end" HTTP server. Doherty, et al. Expires January 27, 2008 [Page 52] Internet-Draft DSKPP July 2007 5.2.2. Identification of DSKPP Messages The MIME-type for all DSKPP messages MUST be application/vnd.ietf.keyprov.dskpp+xml 5.2.3. HTTP Headers HTTP proxies MUST NOT cache responses carrying DSKPP messages. For this reason, the following holds: o When using HTTP/1.1, requesters SHOULD: * Include a Cache-Control header field set to "no-cache, no- store". * Include a Pragma header field set to "no-cache". o When using HTTP/1.1, responders SHOULD: * Include a Cache-Control header field set to "no-cache, no-must- revalidate, private". * Include a Pragma header field set to "no-cache". * NOT include a Validator, such as a Last-Modified or ETag header. There are no other restrictions on HTTP headers, besides the requirement to set the Content-Type header value according to Section 5.2.2. 5.2.4. HTTP Operations Persistent connections as defined in HTTP/1.1 are assumed but not required. DSKPP requests are mapped to HTTP POST operations. DSKPP responses are mapped to HTTP responses. 5.2.5. HTTP Status Codes A DSKPP HTTP responder that refuses to perform a message exchange with a DSKPP HTTP requester SHOULD return a 403 (Forbidden) response. In this case, the content of the HTTP body is not significant. In the case of an HTTP error while processing a DSKPP request, the HTTP server MUST return a 500 (Internal Server Error) response. This type of error SHOULD be returned for HTTP-related errors detected before control is passed to the DSKPP processor, or when the DSKPP processor reports an internal error (for example, the DSKPP XML namespace is incorrect, or the DSKPP schema cannot be located). If the type of a DSKPP request cannot be determined, the DSKPP responder MUST return a 400 (Bad request) response. In these cases (i.e., when the HTTP response code is 4xx or 5xx), the content of the HTTP body is not significant. Redirection status codes (3xx) apply as usual. Doherty, et al. Expires January 27, 2008 [Page 53] Internet-Draft DSKPP July 2007 Whenever the HTTP POST is successfully invoked, the DSKPP HTTP responder MUST use the 200 status code and provide a suitable DSKPP message (possibly with DSKPP error information included) in the HTTP body. 5.2.6. HTTP Authentication No support for HTTP/1.1 authentication is assumed. 5.2.7. Initialization of DSKPP The DSKPP server MAY initialize the DSKPP protocol by sending an HTTP response with Content-Type set according to Section 5.2.2 and response code set to 200 (OK). This message MAY, e.g., be sent in response to a user requesting key initialization in a browsing session. The initialization message MAY carry data in its body. If this is the case, the data MUST be a valid instance of a element. 5.2.8. Example Messages a. Initialization from DSKPP server: HTTP/1.1 200 OK Cache-Control: no-store Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: DSKPP initialization data in XML form... b. Initial request from DSKPP client: POST http://example.com/cgi-bin/DSKPP-server HTTP/1.1 Cache-Control: no-store Pragma: no-cache Host: example.com Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: DSKPP data in XML form (supported version, supported algorithms...) c. Initial response from DSKPP server: HTTP/1.1 200 OK Cache-Control: no-store Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: Doherty, et al. Expires January 27, 2008 [Page 54] Internet-Draft DSKPP July 2007 DSKPP data in XML form (server random nonce, server public key, ...) 6. DSKPP Schema Doherty, et al. Expires January 27, 2008 [Page 55] Internet-Draft DSKPP July 2007 Doherty, et al. Expires January 27, 2008 [Page 56] Internet-Draft DSKPP July 2007