--- 1/draft-ietf-cose-hpke-00.txt 2022-03-07 03:14:01.254937701 -0800 +++ 2/draft-ietf-cose-hpke-01.txt 2022-03-07 03:14:01.282938405 -0800 @@ -1,44 +1,52 @@ COSE H. Tschofenig Internet-Draft Arm Limited Intended status: Standards Track R. Housley -Expires: 14 July 2022 Vigil Security +Expires: 8 September 2022 Vigil Security B. Moran Arm Limited - 10 January 2022 + 7 March 2022 Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and Encryption (COSE) - draft-ietf-cose-hpke-00 + draft-ietf-cose-hpke-01 Abstract This specification defines hybrid public-key encryption (HPKE) for - use with CBOR Object Signing and Encryption (COSE). + use with CBOR Object Signing and Encryption (COSE). HPKE offers a + variant of public-key encryption of arbitrary-sized plaintexts for a + recipient public key. + + HPKE works for any combination of an asymmetric key encapsulation + mechanism (KEM), key derivation function (KDF), and authenticated + encryption with additional data (AEAD) encryption function. + Authentication for HPKE in COSE is provided by COSE-native security + mechanisms. 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 14 July 2022. + This Internet-Draft will expire on 8 September 2022. Copyright Notice Copyright (c) 2022 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 @@ -54,161 +62,172 @@ modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents - 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3 3. HPKE for COSE . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3 3.2. HPKE Encryption with SealBase . . . . . . . . . . . . . . 4 - 3.3. HPKE Decryption with Open . . . . . . . . . . . . . . . . 5 + 3.3. HPKE Decryption with OpenBase . . . . . . . . . . . . . . 5 3.4. Info Structure . . . . . . . . . . . . . . . . . . . . . 5 4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 - 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 + 6.1. HPKE/P-256+HKDF-256 and AES-128-GCM . . . . . . . . . . . 8 + 6.2. HPKE/P-512+HKDF-512 and AES-256-GCM . . . . . . . . . . . 8 + 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 7.1. Normative References . . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . 9 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction Hybrid public-key encryption (HPKE) [I-D.irtf-cfrg-hpke] is a scheme that provides public key encryption of arbitrary-sized plaintexts given a recipient's public key. HPKE utilizes a non-interactive ephemeral-static Diffie-Hellman exchange to establish a shared - secret, which is then used to encrypt plaintext. + secret. The motivation for standardizing a public key encryption + scheme is explained in the introduction of [I-D.irtf-cfrg-hpke]. The HPKE specification defines several features for use with public key encryption and a subset of those features is applied to COSE - [RFC8152]. Since COSE provides constructs for authenticcation, those + [RFC8152]. Since COSE provides constructs for authentication, those are not re-used from the HPKE specification. This specification uses - the "base" mode (as it is called in HPKE specification language). + the "base" mode, as it is called in HPKE specification language. 2. Conventions and Terminology 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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. This specification uses the following abbreviations and terms: - - Content-encryption key (CEK), a term defined in RFC 2630 [RFC2630]. - + Content-encryption key (CEK), a term defined in CMS [RFC2630]. - Hybrid Public Key Encryption (HPKE) is defined in [I-D.irtf-cfrg-hpke]. - pkR is the public key of the recipient, as defined in [I-D.irtf-cfrg-hpke]. - skR is the private key of the recipient, as defined in [I-D.irtf-cfrg-hpke]. 3. HPKE for COSE 3.1. Overview The CDDL for the COSE_Encrypt structure, as used with this - specification, is shown in Figure 1. The structures referenced below - are found in the CDDL. + specification, is shown in Figure 1. - HPKE, when used with COSE, follows a three layer structure: + HPKE, when used with COSE, follows a two layer structure: * Layer 0 (corresponding to the COSE_Encrypt structure) contains - content encrypted with the CEK. This ciphertext may be detached. - If not detached, then it is included in the COSE_Encrypt - structure. - - * Layer 1 (see COSE_recipient_outer structure) includes the - encrypted CEK. + content (plaintext) encrypted with the CEK. This ciphertext may + be detached. If not detached, then it is included in the + COSE_Encrypt structure. - * Layer 2 (in the COSE_recipient_inner structure) contains + * Layer 1 (corresponding to a recipient structure) contains parameters needed for HPKE to generate a shared secret used to - encrypt the CEK from layer 1. + encrypt the CEK. This layer includes the encrypted CEK. + + This two-layer structure is used to encrypt content that can also be + shared with multiple parties at the expense of a single additional + encryption operation. As stated above, the specification uses a CEK + to encrypt the content at layer 0. + + For example, the content encrypted at layer 0 is a firmware image. + The same ciphertext firmware image is processed by all of the + recipients; however, each recipient uses their own private key to + obtain the CEK. COSE_Encrypt_Tagged = #6.96(COSE_Encrypt) - SUIT_Encryption_Info = COSE_Encrypt_Tagged + HPKE_Encryption_Info = COSE_Encrypt_Tagged ; Layer 0 COSE_Encrypt = [ Headers, ciphertext : bstr / nil, - recipients : [+COSE_recipient_outer] + recipients : + COSE_recipient ] ; Layer 1 - COSE_recipient_outer = [ - protected : bstr .size 0, - unprotected : header_map, ; must contain alg - encCEK : bstr, ; CEK encrypted with HPKE-derived shared secret - recipients : [ + COSE_recipient_inner ] - ] - - ; Layer 2 - COSE_recipient_inner = [ - protected : bstr .cbor header_map, ; must contain HPKE alg + COSE_recipient = [ + protected : bstr .cbor header_map, ; must contain alg parameter unprotected : header_map, ; must contain kid and ephemeral public key - empty : null, - empty : null + encCEK : bstr, ; CEK encrypted with HPKE-derived shared secret ] header_map = { Generic_Headers, * label =values, } Figure 1: CDDL for HPKE-based COSE_Encrypt Structure - The COSE_recipient_outer structure shown in Figure 1 includes the - encrypted CEK (in the encCEK structure) and the COSE_recipient_inner - structure, also shown in Figure 1, contains the ephemeral public key - (in the unprotected structure). + The COSE_recipient structure shown in Figure 1 is repeated for each + recipient, and it includes the encrypted CEK as well as the sender- + generated ephemeral public key in the unprotected header structure. 3.2. HPKE Encryption with SealBase The SealBase(pkR, info, aad, pt) function is used to encrypt a - plaintext pt to a recipient's public key (pkR). For use in this - specification, the plaintext "pt" passed into the SealBase is the - CEK. The CEK is a random byte sequence of length appropriate for the - encryption algorithm selected in layer 0. For example, AES-128-GCM - requires a 16 byte key and the CEK would therefore be 16 bytes long. + plaintext pt to a recipient's public key (pkR). + + IMPORTANT: For use in this specification, the plaintext "pt" passed + into the SealBase is the CEK. The CEK is a random byte sequence of + length appropriate for the encryption algorithm selected in layer 0. + For example, AES-128-GCM requires a 16 byte key and the CEK would + therefore be 16 bytes long. The "info" parameter can be used to influence the generation of keys and the "aad" parameter provides additional authenticated data to the - AEAD algorithm in use. If successful, SealBase() will output a - ciphertext "ct" and an encapsulated key "enc". The content of enc is - the ephemeral public key. + AEAD algorithm in use. This specification does not mandate the use + of the info and the aad parameters. + + If SealBase() is successful, it will output a ciphertext "ct" and an + encapsulated key "enc". The content of enc is the ephemeral public + key. The content of the info parameter is based on the 'COSE_KDF_Context' structure, which is detailed in Figure 2. -3.3. HPKE Decryption with Open +3.3. HPKE Decryption with OpenBase The recipient will use the OpenBase(enc, skR, info, aad, ct) function with the enc and ct parameters received from the sender. The "aad" and the "info" parameters are obtained via the context of the usage. The OpenBase function will, if successful, decrypt "ct". When decrypted, the result will be the CEK. The CK is the symmetric key - used to decrypt the ciphertext in the COSE_Encrypt structure. + used to decrypt the ciphertext in layer 0 of the COSE_Encrypt + structure. 3.4. Info Structure + This section provides a suggestion for constructing the info + structure, when used with SealBase() and OpenBase(). Note that the + use of the aad and the info structures for these two functions is + optional. Profiles of this specification may require their use and + may define different info structure. + This specification re-uses the context information structure defined - in [RFC8152] for use with the HPKE algorithm. This payload becomes - the content of the info parameter for the HPKE functions. For better - readability of this specification the COSE_KDF_Context structure is - repeated in Figure 2. + in [RFC8152] as a foundation for the info structure. This payload + becomes the content of the info parameter for the HPKE functions, + when utilized. For better readability of this specification the + COSE_KDF_Context structure is repeated in Figure 2. PartyInfo = ( identity : bstr / nil, nonce : bstr / int / nil, other : bstr / nil ) COSE_KDF_Context = [ AlgorithmID : int / tstr, PartyUInfo : [ PartyInfo ], @@ -216,155 +235,144 @@ SuppPubInfo : [ keyDataLength : uint, protected : empty_or_serialized_map, ? other : bstr ], ? SuppPrivInfo : bstr ] Figure 2: COSE_KDF_Context Data Structure for info parameter - Since this specification may be used in a number of different - deployment environments flexibility for populating the fields in the - COSE_KDF_Context structure is provided. - - For better interoperability, the following recommended settings are - provided: + The fields in Figure 2 are populated as follows: * PartyUInfo.identity corresponds to the kid found in the COSE_Sign_Tagged or COSE_Sign1_Tagged structure (when a digital signature is used). When utilizing a MAC, then the kid is found in the COSE_Mac_Tagged or COSE_Mac0_Tagged structure. * PartyVInfo.identity corresponds to the kid used for the respective recipient from the inner-most recipients array. * The value in the AlgorithmID field corresponds to the alg - parameter in the protected structure in the inner-most recipients - array. + parameter in the unprotected header structure of the recipient + structure. * keyDataLength is set to the number of bits of the desired output value. * protected refers to the protected structure of the inner-most array. 4. Example An example of the COSE_Encrypt structure using the HPKE scheme is - shown in Figure 3. It uses the following algorithm combination: + shown in Figure 3. Line breaks and comments have been inserted for + better readability. It uses the following algorithm combination: - * AES-GCM-128 for encryption of detached ciphertext. + * AES-GCM-128 for encryption of detached ciphertext in layer 0. - * AES-GCM-128 for encryption of the CEK. + * AES-GCM-128 for encryption of the CEK in layer 1 as well as ECDH + with NIST P-256 and HKDF-SHA256 as a Key Encapsulation Mechanism + (KEM). - * Key Encapsulation Mechanism (KEM): NIST P-256 + The algorithm selection is based on the registry of the values + offered by the alg parameters. - * Key Derivation Function (KDF): HKDF-SHA256 - 96( - [ - // protected field with alg=AES-GCM-128 - h'A10101', - { // unprotected field with iv - 5: h'26682306D4FB28CA01B43B80' - }, - // null because of detached ciphertext + 96_0([ + / protected header with alg=AES-GCM-128 / + h'a10101', + / unprotected header with nonce / + {5: h'938b528516193cc7123ff037809f4c2a'}, + / detached ciphertext / null, - [ // COSE_recipient_outer - h'', // empty protected field - { // unprotected field with ... - 1: 1 // alg=A128GCM - }, - // Encrypted CEK - h'FA55A50CF110908DA6443149F2C2062011A7D8333A72721A', - / recipients / [ // COSE_recipient_inner + / recipient structure / [ - / protected / h'a1013818' / { - \ alg \ 1:TBD1 \ HPKE/P-256+HKDF-256 \ - } / , - / unprotected / { - // HPKE encapsulated key - / ephemeral / -1:{ - / kty / 1:2, - / crv / -1:1, - / x / -2:h'98f50a4ff6c05861c8...90bbf91d6280', - / y / -3:true - }, - // kid for recipient static ECDH public key - / kid / 4:'meriadoc.brandybuck@buckland.example' + / protected field with alg for HPKE / + h'a1013863', + / unprotected header / + { + / ephemeral public key with x / y coodinate / + -1: h'a401022001215820a596f2ca8d159c04942308ca90 + cfbfca65b108ca127df8fe191a063d00d7c5172258 + 20aef47a45d6d6c572e7bd1b9f3e69b50ad3875c68 + f6da0caaa90c675df4162c39', + / kid for recipient static ECDH public key / + 4: h'6b69642d32', }, - // empty ciphertext - / ciphertext / h'' - ] - ] - ] - ] - ) + / encrypted CEK / + h'9aba6fa44e9b2cef9d646614dcda670dbdb31a3b9d37c7a + 65b099a8152533062', + ], + ]) Figure 3: COSE_Encrypt Example for HPKE + Note that the COSE_Sign1 wrapper outside the COSE_Encrypt structure + is not shown in the example above. + 5. Security Considerations This specification is based on HPKE and the security considerations of HPKE [I-D.irtf-cfrg-hpke] are therefore applicable also to this specification. - HPKE assumes that the sender is in possession of the public key of - the recipient. A system using HPKE COSE has to assume the same - assumptions and public key distribution mechanism is assumed to - exist. + HPKE assumes the sender is in possession of the public key of the + recipient and HPKE COSE makes the same assumptions. Some form of + public key distribution mechanism is assumed to exist. Since the CEK is randomly generated it must be ensured that the guidelines for random number generations are followed, see [RFC8937]. - The SUIT_Encryption_Info structure shown in this document does not - provide authentication. Hence, the SUIT_Encryption_Info structure - has to be used in combination with other COSE constructs, such as the - COSE_Sign or COSE_Sign1. + The COSE_Encrypt structure must be authenticated using COSE + constructs like COSE_Sign, or COSE_Sign1. 6. IANA Considerations - This document requests IANA to create new entries in the COSE - Algorithms registry established with [RFC8152]. + This document requests IANA to add new values to the COSE Algorithms + registry defined in [RFC8152] (in the Standards Action With Expert + Review category): - +-------------+-------+---------+------------+--------+---------------+ - | Name | Value | KDF | Ephemeral- | Key | Description | - | | | | Static | Wrap | | - +-------------+-------+---------+------------+--------+---------------+ - | HPKE/P-256+ | TBD1 | HKDF - | yes | none | HPKE with | - | HKDF-256 | | SHA-256 | | | ECDH-ES | - | | | | | | (P-256) + | - | | | | | | HKDF-256 | - +-------------+-------+---------+------------+--------+---------------+ - | HPKE/P-384+ | TBD2 | HKDF - | yes | none | HPKE with | - | HKDF-SHA384 | | SHA-384 | | | ECDH-ES | - | | | | | | (P-384) + | - | | | | | | HKDF-384 | - +-------------+-------+---------+------------+--------+---------------+ - | HPKE/P-521+ | TBD3 | HKDF - | yes | none | HPKE with | - | HKDF-SHA521 | | SHA-521 | | | ECDH-ES | - | | | | | | (P-521) + | - | | | | | | HKDF-521 | - +-------------+-------+---------+------------+--------+---------------+ - | HPKE | TBD4 | HKDF - | yes | none | HPKE with | - | X25519 + | | SHA-256 | | | ECDH-ES | - | HKDF-SHA256 | | | | | (X25519) + | - | | | | | | HKDF-256 | - +-------------+-------+---------+------------+--------+---------------+ - | HPKE | TBD4 | HKDF - | yes | none | HPKE with | - | X448 + | | SHA-512 | | | ECDH-ES | - | HKDF-SHA512 | | | | | (X448) + | - | | | | | | HKDF-512 | - +-------------+-------+---------+------------+--------+---------------+ +6.1. HPKE/P-256+HKDF-256 and AES-128-GCM -7. References + * Name: HPKE_P256_HKDF256_AES128_GCM + + * Value: TBD1 + + * Description: HPKE/P-256+HKDF-256 and AES-128-GCM + + * Capabilities: [kty] + + * Change Controller: IESG + + * Reference: [[TBD: This RFC]] + + * Recommended: Yes + +6.2. HPKE/P-512+HKDF-512 and AES-256-GCM + + * Name: HPKE_P521_HKDF512_AES256_GCM + + * Value: TBD2 + + * Description: HPKE/P-512+HKDF-512 and AES-256-GCM + * Capabilities: [kty] + + * Change Controller: IESG + + * Reference: [[TBD: This RFC]] + + * Recommended: Yes + + TBD: More values to be added. + +7. References 7.1. Normative References [I-D.irtf-cfrg-hpke] Barnes, R. L., Bhargavan, K., Lipp, B., and C. A. Wood, "Hybrid Public Key Encryption", Work in Progress, Internet-Draft, draft-irtf-cfrg-hpke-12, 2 September 2021, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate @@ -386,27 +394,25 @@ DOI 10.17487/RFC2630, June 1999, . [RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N., and C. Wood, "Randomness Improvements for Security Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020, . Appendix A. Acknowledgements - TBD: Add your name here. + We would like to thank Goeran Selander, John Mattsson and Ilari + Liusvaara for their review feedback. Authors' Addresses Hannes Tschofenig Arm Limited - Email: hannes.tschofenig@arm.com + Russ Housley Vigil Security, LLC - Email: housley@vigilsec.com - Brendan Moran Arm Limited - Email: Brendan.Moran@arm.com