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Versions: 00 01

Network Working Group                                          N. Madden
Internet-Draft                                                 ForgeRock
Intended status: Standards Track                             May 8, 2019
Expires: November 9, 2019


         Public Key Authenticated Encryption for JOSE: ECDH-1PU
                     draft-madden-jose-ecdh-1pu-00

Abstract

   This document describes the ECDH-1PU public key authenticated
   encryption algorithm for JWE.  The algorithm is similar to the
   existing ECDH-ES encryption algorithm, but adds an additional ECDH
   key agreement between static keys of the sender and recipient.  This
   additional step allows the recipient to be assured of sender
   authenticity without requiring a nested signed-then-encrypted message
   structure.  The mode is also a useful building block for constructing
   interactive handshake protocols on top of JOSE.

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
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   This Internet-Draft will expire on November 9, 2019.

Copyright Notice

   Copyright (c) 2019 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
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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
   2.  Key Agreement with Elliptic Curve Diffie-Hellman Ephemeral-
       Static Static-Static (ECDH-1PU) . . . . . . . . . . . . . . .   3
     2.1.  Header Parameters used for ECDH Key Agreement . . . . . .   4
     2.2.  Key Derivation for ECDH-1PU Key Agreement . . . . . . . .   4
   3.  Two-way interactive handshake . . . . . . . . . . . . . . . .   6
   4.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  ECDH-1PU  . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   JSON Object Signing and Encryption (JOSE) defines a number of
   encryption (JWE) [RFC7516] and digital signature (JWS) [RFC7515]
   algorithms.  When symmetric cryptography is used, JWE provides
   authenticated encryption that ensures both confidentiality and sender
   authentication.  However, for public key cryptography the existing
   JWE encryption algorithms provide only confidentiality and some level
   of ciphertext integrity.  When sender authentication is required,
   users must resort to nested signed-then-encrypted structures, which
   increases the overhead and size of resulting messages.  This document
   describes an alternative encryption algorithm called ECDH-1PU that
   provides public key authenticated encryption, allowing the benefits
   of authenticated encryption to be enjoyed for public key JWE as it
   currently is for symmetric cryptography.

   ECDH-1PU is based on the One-Pass Unified Model for Elliptic Curve
   Diffie-Hellman key agreement described in [NIST.800-56A].

   The advantages of public key authenticated encryption with ECDH-1PU
   compared to using nested signed-then-encrypted documents include the
   following:

   o  The resulting message size is more compact as an additional layer
      of headers and base64url-encoding is avoided.





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   o  The same primitives are used for both confidentiality and
      authenticity, providing savings in code size for constrained
      environments.

   o  The generic composition of signatures and public key encryption
      involves a number of subtle details that are essential to security
      [PKAE].  Providing a dedicated algorithm for public key
      authenticated encryption reduces complexity for users of JOSE
      libraries.

   o  ECDH-1PU provides only authenticity and not the stronger security
      properties of non-repudiation or third-party verifiability.  This
      can be an advantage in applications where privacy, anonymity, or
      plausible deniability are goals.

1.1.  Requirements 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 [RFC8174] when, and only when, they appear in all capitals, as
   shown here.

2.  Key Agreement with Elliptic Curve Diffie-Hellman Ephemeral-Static
    Static-Static (ECDH-1PU)

   This section defines the specifics of key agreement with Elliptic
   Curve Diffie-Hellman Ephemeral-Static Static-Static, in combination
   with the one-step KDF, as defined in Section 5.8.2.1 of
   [NIST.800-56A] using the Concatenation Format of Section 5.8.2.1.1.
   This is identical to the ConcatKDF function used by the existing JWE
   ECDH-ES algorithm defined in Section 4.6 of [RFC7518].  As for ECDH-
   ES, the key agreement result can be used in one of two ways:

   1.  directly as the Content Encryption Key (CEK) for the "enc"
       algorithm, in the Direct Key Agreement mode, or

   2.  as a symmetric key used to wrap the CEK with the "A128KW",
       "A192KW", or "A256KW" algorithms, in the Key Agreement with Key
       Wrapping mode.

   A new ephemeral public key value MUST be generated for each key
   agreement operation.

   In Direct Key Agreement mode, the output of the KDF MUST be a key of
   the same length as that used by the "enc" algorithm.  In this case,
   the empty octet sequence is used as the JWE Encrypted Key value.  The




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   "alg" (algorithm) Header Parameter value "ECDH-1PU" is used in Direct
   Key Agreement mode.

   In Key Agreement with Key Wrapping mode, the output of the KDF MUST
   be a key of the length needed for the specified key wrapping
   algorithm.  In this case, the JWE Encrypted Key is the CEK wrapped
   with the agreed-upon key.

   The following "alg" (algorithm) Header Parameter values are used to
   indicate the JWE Encrypted Key is the result of encrypting the CEK
   using the result of the key agreement algorithm as the key encryption
   key for the corresponding key wrapping algorithm:

   +-----------------+-------------------------------------------------+
   | "alg" Param     | Key Management Algorithm                        |
   | Value           |                                                 |
   +-----------------+-------------------------------------------------+
   | ECDH-1PU+A128KW | ECDH-1PU using Concat KDF and CEK wrapped with  |
   |                 | "A128KW"                                        |
   | ECDH-1PU+A192KW | ECDH-1PU using Concat KDF and CEK wrapped with  |
   |                 | "A192KW"                                        |
   | ECDH-1PU+A256KW | ECDH-1PU using Concat KDF and CEK wrapped with  |
   |                 | "A256KW"                                        |
   +-----------------+-------------------------------------------------+

2.1.  Header Parameters used for ECDH Key Agreement

   The "epk" (ephemeral public key), "apu" (Agreement PartyUInfo), and
   "apv" (Agreement PartyVInfo) header parameters are used in ECDH-1PU
   exactly as defined in Section 4.6.1 of [RFC7518].

   When no other values are supplied, it is RECOMMENDED that the
   producer software initializes the "apu" header to the base64url-
   encoding of the SHA-256 hash of the concatenation of the sender's
   static public key and the ephemeral public key, and the "apv" header
   to the base64url-encoding of the SHA-256 hash of the recipient's
   static public key.  This ensures that all keys involved in the key
   agreement are cryptographically bound to the derived keys.

2.2.  Key Derivation for ECDH-1PU Key Agreement

   The key derivation process derives the agreed-upon key from the
   shared secret Z established through the ECDH algorithm, per
   Section 6.2.1.2 of [NIST.800-56A].  For the NIST prime order curves
   "P-256", "P-384", and "P-521", the ECC CDH primitive for cofactor
   Diffie-Hellman defined in Section 5.7.1.2 of [NIST.800-56A] is used
   (taking note that the cofactor for all these curves is 1).  For




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   curves "X25519" and "X448" the appropriate ECDH primitive from
   Section 5 of [RFC7748] is used.

   Key derivation is performed using the one-step KDF, as defined in
   Section 5.8.1 and Section 5.8.2.1 of [NIST.800-56A] using the
   Concatenation Format of Section 5.8.2.1.1, where the Auxilary
   Function H is SHA-256.  The KDF parameters are set as follows:

   Z  This is set to the representation of the shared secret Z as an
      octet sequence.  As per Section 6.2.1.2 of [NIST.800-56A] Z is the
      concatenation of Ze and Zs, where Ze is the shared secret derived
      from applying the ECDH primitive to the sender's ephemeral private
      key and the recipient's static public key.  Zs is the shared
      secret derived from applying the ECDH primitive to the sender's
      static private key and the recipient's static public key.

   keydatalen  This is set to the number of bits in the desired output
      key.  For "ECDH-1PU", this is the length of the key used by the
      "enc" algorithm.  For "ECDH-1PU+A128KW", "ECDH-1PU+A192KW", and
      "ECDH-1PU+A256KW", this is 128, 192, and 256, respectively.

   AlgorithmID  The AlgorithmID values is of the form Datalen || Data,
      where Data is a variable-length string of zero or more octets, and
      Datalen is a fixed-length, big-endian 32-bit counter that
      indicates the length (in octets) of Data.  In the Direct Key
      Agreement case, Data is set to the octets of the ASCII
      representation of the "enc" Header Parameter value.  In the Key
      Agreement with Key Wrapping case, Data is set to the octets of the
      ASCII representation of the "alg" (algorithm) Header Parameter
      value.

   PartyUInfo  The PartyUInfo value is of the form Datalen || Data,
      where Data is a variable-length string of zero or more octets, and
      Datalen is a fixed-length, big-endian 32-bit counter that
      indicates the length (in octets) of Data.  If an "apu" (agreement
      PartyUInfo) Header Parameter is present, Data is set to the result
      of base64url decoding the "apu" value and Datalen is set to the
      number of octets in Data.  Otherwise, Datalen is set to 0 and Data
      is set to the empty octet sequence.

   PartyVInfo  The PartyVInfo value is of the form Datalen || Data,
      where Data is a variable-length string of zero or more octets, and
      Datalen is a fixed-length, big-endian 32-bit counter that
      indicates the length (in octets) of Data.  If an "apv" (agreement
      PartyVInfo) Header Parameter is present, Data is set to the result
      of base64url decoding the "apv" value and Datalen is set to the
      number of octets in Data.  Otherwise, Datalen is set to 0 and Data
      is set to the empty octet sequence.



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   SuppPubInfo  This is set to the keydatalen represented as a 32-bit
      big-endian integer.

   SuppPrivInfo  This is set to the empty octet sequence.

   Applications need to specifiy how the "apu" and "apv" Header
   Parameters are used for that application.  The "apu" and "apv" values
   MUST be distinct, when used.  Applications wishing to conform to
   [NIST.800-56A] need to provide values that meet the requirements of
   that doucument, e.g., by using values that identify the producer and
   consumer.

3.  Two-way interactive handshake

   A party that has received a JWE encrypted with ECDH-1PU MAY reply to
   that message by creating a new JWE using ECDH-1PU, but using the
   ephemeral public key ("epk") from the first message as if it was the
   originating party's static public key.  In this case, key agreement
   proceeds exactly as for Section 2, but with the originator's
   ephemeral public key used as the recipient (Party V) static public
   key.  The "alg" (algorithm) Header Parameter in the response MUST be
   identical to the "alg" Header Parameter of the original message.

   The value of the "apu" (Agreement PartyUInfo) Header Parameter value
   from the original message SHOULD be reflected as the "apv" (Agreement
   PartyVInfo) Header Parameter value in the new message.  Applications
   need to specify how the new "apu" Header Parameter should be
   constructed.

   If a "kid" claim was included in the ephemeral public key of the
   original message, then a "kid" Header Parameter with the same value
   MUST be included in the reply JWE.

   After the initial message and a reply have been exchanged, the two
   parties may communicate using the derived key from the second message
   as the encryption key for any number of additional messages.  When
   ECDH-1PU is used in Direct Key Agreement mode, then subsequent
   messages using the derived key MUST be encrypted using the "dir"
   (Direct) JWE algorithm.  When used in Key Agreement with Key Wrapping
   mode, subsequent messages using the derived key MUST be encrypted
   using the associated Key Wrapping algorithm, as shown in the
   following table:









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       +----------------------------+------------------------------+
       | ECDH-1PU "alg" Param Value | Subsequent "alg" Param Value |
       +----------------------------+------------------------------+
       | ECDH-1PU+A128KW            | A128KW                       |
       | ECDH-1PU+A192KW            | A192KW                       |
       | ECDH-1PU+A256KW            | A256KW                       |
       +----------------------------+------------------------------+

4.  IANA considerations

   This section registers JWE algorithms as per the registry established
   in [RFC7518].

4.1.  ECDH-1PU

      Algorithm Name: "ECDH-1PU"
      Algorithm Description: ECDH One-Pass Unified Model using Concat
      KDF
      Algorithm Usage Location(s): "alg"
      JOSE Implementation Requirements: Optional
      Change Controller: IESG
      Specification Document(s): Section 2
      Algorithm Analysis Document(s): [NIST.800-56A] (Section 7.3),
      [PKAE]

5.  Security Considerations

   The security considerations of [RFC7518] relevant to ECDH-ES also
   apply to this specification.

   The security considerations of [NIST.800-56A] apply here.

   When performing an ECDH key agreement between a static private key
   and any untrusted public key, care should be taken to ensure that the
   public key is a valid point on the same curve as the private key.
   Failure to do so may result in compromise of the static private key.
   For the NIST curves P-256, P-384, and P-521, appropriate validation
   routines are given in Section 5.6.2.3.3 of [NIST.800-56A].  For the
   curves used by X25519 and X448, consult the security considerations
   of [RFC7748].

   The ECDH-1PU algorithm is vulnerable to Key Compromise Impersonation
   (KCI) attacks.  If the long-term static private key of a party is
   compromised, then the attacker can not only impersonate that party to
   other parties, but also impersonate any other party when
   communicating with the compromised party.  The second and any
   subsequent messages in the two-way interactive handshake described in
   Section 3 are not vulnerable to KCI.  If resistance to KCI is desired



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   in a single message, then it is RECOMMENDED to use a nested JWS
   signature over the content.

   When Key Agreement with Key Wrapping is used, with the same Content
   Encryption Key (CEK) reused for multiple recipients, any of those
   recipients can produce a new message that appears to come from the
   original sender and will be trusted by any of the other recipients.
   It is RECOMMENDED that a unique CEK is used for each recipient.

6.  References

6.1.  Normative References

   [NIST.800-56A]
              Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
              Davis, "Recommendation for Pair-Wise Key Establishment
              Using Discrete Logarithm Cryptography Revision 3.", NIST
              Special Publication 800-56A, April 2018.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <https://www.rfc-editor.org/info/rfc7516>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <https://www.rfc-editor.org/info/rfc7518>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

6.2.  Informative References

   [PKAE]     An, J., "Authenticated Encryption in the Public-Key
              Setting: Security Notions and Analyses", IACR ePrint
              2001/079, 2001.







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Author's Address

   Neil Madden
   ForgeRock
   Broad Quay House
   Prince Street
   Bristol  BS1 4DJ
   United Kingdom

   Email: neil.madden@forgerock.com









































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