Internet Engineering Task Force M. Lorlacks Internet-Draft Independent Intended status: Experimental 12 September 2021 Expires: 16 March 2022 License Activation Protocol draft-lorlacks-license-activation-protocol-04 Abstract This document defines an experimental method for uniform license activation mechanism for use in digital rights management (DRM). Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on 16 March 2022. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Lorlacks Expires 16 March 2022 [Page 1] Internet-Draft License Activation Protocol September 2021 3.1. Provisioning . . . . . . . . . . . . . . . . . . . . . . 3 3.2. Service Discovery . . . . . . . . . . . . . . . . . . . . 5 3.3. Request . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.4. Response . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Security Considerations . . . . . . . . . . . . . . . . . . . 10 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 Document History [RFC Editor: Please remove this section] . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Normative References . . . . . . . . . . . . . . . . . . . . . 15 Informative References . . . . . . . . . . . . . . . . . . . . 17 Appendix A. Implementation Notes . . . . . . . . . . . . . . . . 17 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction A common issue with on-premises software licensing is ensuring that licensing limitations are enforced. Digital rights management (DRM) is an umbrella term for modeling legal licensing requirements in the form of executable code. Part of DRM implementation is often communication with a server to ensure central knowledge of licenses in circulation. DRM implementations are in practice almost always one-off solutions for a particular product. No observable efforts to standardize DRM have been made. While DRM necessarily relies on either hardware or on security by obscurity, another use for DRM is to simplify license compliance for honest customers. Consolidating the license information on a single server within an organization brings obvious benefits for keeping track of software inventory, obviating the need of manually triangulating licenses in use. It therefore seems beneficial to standardize a license management protocol that is both suitable for obfuscation without giving away too much information to active adversaries observing traffic and at the same time has reasonable implementation semantics for simplified cases where active adversaries are ignored, such as when it is too costly to spend great effort on software protection; modern obfuscation techniques are complex and accordingly expensive to implement, see e.g. [virtsc]. The license activation protocol presented herein aims to satisfy both of these needs. 2. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [BCP14] when, and only when, they appear in all capitals, as shown here. Lorlacks Expires 16 March 2022 [Page 2] Internet-Draft License Activation Protocol September 2021 A "byte" refers to an octet of bits. It is generally assumed that fixed-szie integer types for 8-, 16-, 32-bit integers are available. A "UUID" is a Universally Unique IDentifier in the sense of [RFC4122]. It is always respresented as a sequence of bytes. The byte sequences use _big-endian_ encoding for all numerical components of the UUID. For example, this means that the UUID "00112233-4455-6677-8899-aabbccddeeff" is encoded as the following sequence of bytes: 0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff. This is mandated by [RFC4122]; _all other integer values in this document use little- endian encoding_. | Blocks marked up like this are "asides". They contain | rationale or examples to clarify as well as help with | interpretation of the main text, but are not normative | sections. 3. Protocol The protocol operates between a client and a server in a request- response fashion. The client sends a request and the server sends a response. Both request and response are each UDP datagrams[RFC0768]. The protocol can both be used in a direct setup (customer client and vendor server communicate directly over the Internet) and an indirect setup (customer client and customer intermediate server communicate directly; customer intermediate server and vendor server communicate with each other using this same protocol). Practical considerations may necessitate one or the other setup. The choice between the setups must be made at provisioning time. 3.1. Provisioning Prior to executing the protocol, every client must first be "provisioned". Provisioning means equipping the client with: 1. a UUID that remains static for the installation time of the client (client base ID), 2. a UUID that remains static for the installation time of the client and SKU (client add-on ID), 3. unique, hard-to-predict information (client seed), 4. information about the expected server. Lorlacks Expires 16 March 2022 [Page 3] Internet-Draft License Activation Protocol September 2021 The client IDs MUST be generated at installation time. It is RECOMMENDED that a version 4 UUID using a cryptographically secure random number generator is used. The client base ID refers to a "base installation"; if there are add- ons to a product, the client add-on ID can be used to distinguish an activation request for the base installation and the add-on while still being able to correlate the requests due to different SKU fields in the request. If this is a base installation or no add-ons exist at all, the client add-on ID MUST be set to the nil UUID (all bits are zero). | For example, take an on-premise version control system hosting | platform. The base installation needs to be activated for the | product to work at all. Layered on top, a second activation is | needed for an add-on, such as single-sign on support. If there | were only the SKU and client ID, it would not be possible to | correlate the two installations on the server, especially when | considering NAT, except through use of such information in the | client seed field. The client base ID is the same for both the | base installation and the add-on SKU, whereas the SKU and | client add-on ID vary between two activation requests. The client-side unique, hard-to-predict information (client seed) is typically supplied and generated at installation time. It could, for example, consist of a unique license key concatenated with hardware information. No format is specified; the actual value of the client seed is out of scope, but it MUST be a byte string consisting of one or more bytes; it is RECOMMENDED to include at least 16 bytes of cryptographically secure random data. Additionally, the client must be aware of its own stock-keeping unit (SKU) identifier. This is a UUID[RFC4122]. The server information is typically hard-coded at compile time. It consists of an X25519 public key[RFC7748] and an Ed25519 signing key[RFC8032]. | In particular, this means that the server information could | possibly be obtained dynamically during installation time, for | example validated through some kind of public key signature | system with only, say, a root certificate built-in at compile | time. Similarly, it is possible to provision _all_ client | information at compile time; this implies that every | installation package shipped is unique, but has the upside that | all identifiers are known ahead of time and can be checked | against a central database. Lorlacks Expires 16 March 2022 [Page 4] Internet-Draft License Activation Protocol September 2021 3.2. Service Discovery The license activation server has a DNS SRV record[RFC2782] for the service name "lap", e.g. _lap._udp.licensing.example.com. The record describes the UDP port to use for the protocol. No fixed UDP port is assigned; the UDP port for an individual deployment may therefore be chosen in accordance with IANA policy and the constraints of the network(s) involved. The license activation server MAY be auto-discovered using DNS-based service discovery (DNS-SD)[RFC6763]. The associated TXT record is empty. Individual clients MUST provide a documented mechanism to manually override an auto-discovered server address. 3.3. Request Once the client has discovered the license activation server to use (hostname, UDP port), it sends the request. The request has this structure: Lorlacks Expires 16 March 2022 [Page 5] Internet-Draft License Activation Protocol September 2021 +============+==================+===================================+ | Size | Name | Description | | (bytes) | | | +============+==================+===================================+ | 1 | Version | Protocol version, currently | | | | always 2 | +------------+------------------+-----------------------------------+ | 2 | Size | Size of the request in bytes in | | | | little-endian byte order | +------------+------------------+-----------------------------------+ | 5 | ClientTime | The client's idea of the current | | | | time in Seconds Since the Epoch | +------------+------------------+-----------------------------------+ | 16 | ClientBaseId | Client-generated UUID uniquely | | | | identifying the base | | | | installation | +------------+------------------+-----------------------------------+ | 16 | ClientAddOnId | Client-generated UUID uniquely | | | | identifying the installation of | | | | the add-on | +------------+------------------+-----------------------------------+ | 16 | SKUId | Stock-keeping unit (SKU) UUID | | | | that identifies the product of | | | | the client | +------------+------------------+-----------------------------------+ | 16 | CurrentLicenseId | The license UUID issued by the | | | | server for this installation | +------------+------------------+-----------------------------------+ | 16 | | Reserved, all-zero | +------------+------------------+-----------------------------------+ | (variable) | ClientSeed | The client seed value | +------------+------------------+-----------------------------------+ Table 1: Client request packet structure There MUST NOT be any padding between the fields. The Size field is computed starting at the Version field. Since the client seed (and thus the ClientSeed field) is not permitted to be empty (see Section 3.1, Paragraph 6), the minimum value for the Size field is 89. The CurrentLicenseId field indicates to the server the LicenseID of the last response from the server. If this is the first activation request ever, the CurrentLicenseId is the nil UUID. Note that the server MAY issue a LicenseID that differs from the request even if CurrentLicenseId is not the nil UUID. Lorlacks Expires 16 March 2022 [Page 6] Internet-Draft License Activation Protocol September 2021 The term "Seconds Since the Epoch" is defined in section 4.16 of the Base Definitions volume of [POSIX.1-2017]; the "Epoch" itself is defined in section 3.150 thereof. "Seconds Since the Epoch" is commonly known as "UNIX time". This field is 5 bytes in length to ensure this protocol working at least until the year 10,000. This integer value is transmitted in little-endian byte order and does not include fractions of a second. The client then generates an ephemeral X25519[RFC7748] key pair. Therefore, the client MUST have access to a cryptographically secure random number generator. It performs X25519 with its ephemeral key and the server's public key that was obtained at provisioning time. The nonce is set to 0. The key is the result of applying HKDF[RFC5869] as follows, where | signifies concatenation: 1. Hash = SHA-512[SHS] (thus HashLen = 64) 2. PRK = HKDF-Extract(salt=none, IKM=client's ephemeral X25519 public key | server's X25519 public key | server's Ed25519 public key | the X25519 shared secret) 3. OKM = HKDF-Expand(PRK, info=[ASCII representation of "56065c4d- d2e0-4ba9-bf9f-76f9159e2987-LAP-V02"], L=64) 4. the symmetric key for the client-to-server packet is now the first 32 bytes of OKM; 5. the symmetric key for the server-to-client packet is now the second 32 bytes of OKM. | Since this use of HKDF is trivial, determining PRK and OKM in | practice simplifies to: | | 1. PRK = HMAC-SHA512(key=[64 all-zero bytes], msg=client's | ephemeral X25519 public key | server's X25519 public | key | server's Ed25519 public key | the X25519 shared | secret) | | 2. OKM = HMAC-SHA512(key=PRK, msg=[ASCII representation of | "56065c4d-d2e0-4ba9-bf9f-76f9159e2987-LAP-V02"] | 0x01) The encryption key is used with ChaCha20 and Poly1305 (AEAD_CHACHA20_POLY1305)[RFC8439] to encrypt the request. There is no additional authenticated data. The client then transmits: 1. its 32-byte ephemeral X25519 public key; 2. the encrypted request; Lorlacks Expires 16 March 2022 [Page 7] Internet-Draft License Activation Protocol September 2021 3. the 16-byte Poly1305 authentication tag. The client MUST transmit this information in a single UDP packet, in that particular order and without padding inbetween to the server. This implies that a packet may not exceed the size of what can be transmitted in a single UDP packet. 3.4. Response Before issuing a response, the server validates the client's request. If the result of the validation process is negative, the server does not respond to the request and drops the UDP packet. Clients MUST NOT probe whether servers drop invalid requests. In particular, the server MUST validate that: 1. the result of the X25519 function is not all-zero; 2. decryption succeeded (Poly1305 tag is valid); 3. the Version field is 1; 4. the Size field is equal to or greater than 89; 5. the ClientTime fields match the server's idea of the current time with at most 30 seconds of difference; 6. the SKU ID is known to the server; 7. the SKU ID may be for a base installation if the ClientAddOnId is the nil UUID or that the SKU ID may be used for an add-on installation if the ClientAddOnId is not the nil UUID; 8. the ClientSeed field is valid according to the rules known to the server; 9. the client is licensed according to the rules known to the server. Additionally, the server MAY check whether the UUIDs match the generation procedures outlined in [RFC4122] and follow the expected UUID generation algorithm for that particular SKU. | There is no impetus to validate the UUID format of the UUIDs | sent by the client closely. For one, it is possible that new | UUID formats will be released and used, otherwise causing churn | on the server-side if the teams working on the clients and the | server work separately. On the other hand, if the type of UUID | is known ahead of time, it may be an extra vector of Lorlacks Expires 16 March 2022 [Page 8] Internet-Draft License Activation Protocol September 2021 | verification; however, it is questionable if much beyond UUID | variant and version can even be verified. Time-based UUIDs | cannot be checked for accuracy because at installation time, | the system may not yet have had access to a network- | synchronized clock. The node ID does not have to be an | IEEE 802 address (and, for privacy reasons, shouldn't be one in | the first place), so there is not much to verify there. Name- | based UUIDs would, however, lend themselves to extra | validation. Choosing an appropriate name space and data is out | of scope for this document. It remains questionable if this | has any tangible benefit. If the validation passes, the server sends the following response structure: +============+============+==================================+ | Size | Name | Description | | (bytes) | | | +============+============+==================================+ | 1 | Version | Protocol version, currently | | | | always 2 | +------------+------------+----------------------------------+ | 2 | Size | Size of the response in bytes in | | | | little-endian byte order | +------------+------------+----------------------------------+ | 5 | ServerTime | The server's idea of the current | | | | time in Seconds Since the Epoch | | | | in little-endian byte order, see | | | | Section 3.3, Paragraph 6 | +------------+------------+----------------------------------+ | 16 | ClientId | The client-generated UUID echoed | | | | back | +------------+------------+----------------------------------+ | 16 | SKUId | The SKU ID echoed back | +------------+------------+----------------------------------+ | 16 | LicenseId | A UUID that identifies the | | | | license; in case of volume | | | | licensing, multiple clients MAY | | | | share the same value | +------------+------------+----------------------------------+ | (variable) | ServerData | Application-specific data for | | | | the client, which MAY be empty | +------------+------------+----------------------------------+ Table 2: Server response packet structure There MUST NOT be any padding between the fields. Lorlacks Expires 16 March 2022 [Page 9] Internet-Draft License Activation Protocol September 2021 The Size field is computed starting at the Version field, i.e. the very beginning of the message. Since the server data is permitted to be empty, the minimum value for the Size field is 56. The ClientId field refers to the client-generated ClientBaseId if ClientAddOnId was the nil UUID and to the ClientAddOnId otherwise. The ServerData field may be used, for example, to transmit a date/ time by which the client must consider itself de-activated and needs to re-authenticate or to transmit feature flags that are supposed to be enabled. The server encryption key that was derived in Section 3.3, Paragraph 7 is then used to encrypt the response. The nonce is set to 0. There is no additional authenticated data. The server then transmits: 1. its 64-byte Ed25519 signature over the rest; 2. the encrypted response; 3. the 16-byte Poly1305 authentication tag. The server MUST transmit this information in a single UDP packet, in that particular order and without padding inbetween to the client. This implies that a packet may not exceed the size of what can be transmitted in a single UDP packet. The client finally validates the server response according to the criteria it deems fit, but it MUST at least verify the Ed25519 signature. The client considers itself activated if the response validated successfully. 4. Security Considerations The client SHOULD NOT be considered trusted. Tampering with DRM is a notorious issue. In particular, as noted in the previous sections, an implementation MUST verify all inputs rigorously. Conversely, the client SHOULD NOT rely on the server's response being well-formed in principle; practical considerations (such as with embedded microprocessors) may necessitate this nonetheless. All routines and secrets pertaining to DRM SHOULD be protected by hardware-based mechanisms such as trusted platform modules (TPMs), hardware security modules (HSMs), and trusted execution environment technologies. Obfuscation can also aid longevity of DRM by deterring insufficiently motivated attackers. Lorlacks Expires 16 March 2022 [Page 10] Internet-Draft License Activation Protocol September 2021 The server information secrets SHOULD be protected by hardware security modules. Cloud-based hardware security modules MAY be chosen for this task. It is RECOMMENDED that accesses to these keys is monitored; for example, an automated system could cross-reference accesses to the secrets with timing of incoming requests. While the contents of activation responses in the ServerData field are unspecified, because activation procedures are often time- limited, the accuracy of the client's clock is important. Otherwise, the server may issue responses that are too far into the future or already in the past for the client, bypassing temporal licensing limitations. It is therefore RECOMMENDED that clients synchronize their time over the network, for example using NTP[RFC5905]. | While an accurate clock on the host system should by now have | become the norm, it is still not possible to rely on the | existence of one. SNTP, which is essentially a stripped-down | version of NTP for clients and also described in [RFC5905], is | relatively easy to implement. It is therefore considered no | issue to obtain an accurate timestamp when one is required. In considering an implementation, care should be taken to avoid network amplification attacks. Notably, the server response packet SHOULD NOT exceed the size of a client packet under any circumstances. In particular, this means that the length of ClientSeed should be equal or greater than the length of ServerData. | Larger server response packets may be an option in highly | trusted setups or on a delegated setup with an intermediate | server on the local network, where the software vendor does not | bear the cost of the network traffic. However, this is rare. | It is possible that the language may change from a | recommendation to a hard requirement in the future. Using a fixed nonce for the encryption of the request and response is unproblematic because there is a new AEAD_CHACHA20_POLY1305 key for every request. Nonces only need to be unique per key. Access to a cryptographically secure random number generator is required. It is therefore no issue to fix the nonce since keys are guaranteed to be unique. 5. IANA Considerations The discovery mechanism described in section Section 3.2 requires a service name. The Service Name and Transport Protocol Port Number Registry therefore needs to be updated accordingly. In accordance with [BCP165], it is hereby requested that IANA create a new entry in the Service Name and Transport Protocol Port Number Registry reading: Lorlacks Expires 16 March 2022 [Page 11] Internet-Draft License Activation Protocol September 2021 +---------------------+----------------------------+ | Service Name | lap | +---------------------+----------------------------+ | Transport Protocols | udp | +---------------------+----------------------------+ | Assignee | Maximilian Lorlacks | | | | +---------------------+----------------------------+ | Contact | Maximilian Lorlacks | | | | +---------------------+----------------------------+ | Description | License Activation | | | Protocol | +---------------------+----------------------------+ | Reference | [this document] | +---------------------+----------------------------+ | Port Number | | +---------------------+----------------------------+ | Service Code | | +---------------------+----------------------------+ | Known Unauthorized | | | Uses | | +---------------------+----------------------------+ | Assignment Notes | Defined TXT keys: None. | +---------------------+----------------------------+ Table 3 Document History [RFC Editor: Please remove this section] Note to the RFC Editor: Please remove this section before publication. draft-lorlacks-license-activation-protocol-04 * Added the CurrentLicenseId to the client request field. This makes it possible to differentiate on the server whether an activation request is new or just a routine check-in for the same installation (with possibly differing client seed information). * Reserved an additional 16 bytes in the request structure so that an implementation needn't account for the cryptographic overhead and just make sure their custom-sized structs are of equal length or the server data is shorter. This simplifies thinking about the implementation. * Bumped the protocol version accordingly. Lorlacks Expires 16 March 2022 [Page 12] Internet-Draft License Activation Protocol September 2021 * Added an appendix with implementation notes. draft-lorlacks-license-activation-protocol-03 * A pedantic note regarding the definition of a "byte" has been added. * Drop the requirement to validate UUIDs per se. It seems exceedingly unrealistic for an attacker to be able to desync the UUID in a meaningful way. Assume network interference: The Poly1305 tag identifies this. Assume unauthorized code modification: A reverse engineer would not blindly flip bytes in the code; if anything, the UUID generation code would be identified, labeled and then mostly ignored because it's not very interesting. The SKU IDs are known to the server ahead of time, so validation is pointless. The request IDs are client-generated and thus mostly meaningless. The only exception to this are UUIDv1 and v2, which contain actual time information, which an attacker may genuinely forget about and thus give away tampering easily. * Remove the request ID from the Request packet. A client-generated request ID may be helpful, but is untrusted. An attacker may intentionally modify request IDs to impersonate another client's request ID, so request IDs may not be relied upon for anything in the first place. In its place, the client ID was split into the client installation ID and client product ID. * Bump the version field from 0 to 1 in light of the aforementioned change. * Do not assume version 1 UUIDs are the only kind of UUID with time information. Version 2 UUIDs also contain time informations (though RFC 4122 does not explicitly detail them), and looking at the work in draft-peabody-dispatch-new-uuid-format, it is quite possible that new UUID formats with time information may follow on the future. Not accounting for this would just cause unnecessary churn in this document immediately thereafter. Not only that, but the client IDs are generated at _installation_ time, so by the time they're sent over the wire, their timestamps have become completely meaningless. * Clarify that the server validation of the client packet is a requirement. * Fix one more instance of grammatical fall-out from the removal of the ClientDate/ClientTime split. Lorlacks Expires 16 March 2022 [Page 13] Internet-Draft License Activation Protocol September 2021 * Add various