Internet DRAFT - draft-madden-jose-ecdh-1pu
draft-madden-jose-ecdh-1pu
Network Working Group N. Madden
Internet-Draft ForgeRock
Intended status: Standards Track May 6, 2021
Expires: November 7, 2021
Public Key Authenticated Encryption for JOSE: ECDH-1PU
draft-madden-jose-ecdh-1pu-04
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.
Status of This Memo
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include Simplified BSD License text as described in Section 4.e of
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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 One-Pass
Unified Model (ECDH-1PU) . . . . . . . . . . . . . . . . . . 4
2.1. Special Considerations for Key Agreement with Key
Wrapping mode . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Header Parameters used for ECDH Key Agreement . . . . . . 5
2.2.1. "skid" Header Parameter . . . . . . . . . . . . . . . 6
2.3. Key Derivation for ECDH-1PU Key Agreement . . . . . . . . 6
3. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8
3.1. JSON Web Signature and Encryption Algorithms Registration 8
3.1.1. ECDH-1PU . . . . . . . . . . . . . . . . . . . . . . 8
3.2. JSON Web Signature and Encryption Header Parameters
Registration . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. skid . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Normative References . . . . . . . . . . . . . . . . . . 10
5.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Example ECDH-1PU Key Agreement Computation with
A256GCM . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Example ECDH-1PU+A128KW Key Agreement computation
with A256CBC-HS256 . . . . . . . . . . . . . . . . . 14
B.1. JWE Protected Header . . . . . . . . . . . . . . . . . . 15
B.2. JWE Per-Recipient Unprotected Headers . . . . . . . . . . 15
B.3. JWE Shared Unprotected Header . . . . . . . . . . . . . . 15
B.4. Additional Authenticated Data . . . . . . . . . . . . . . 15
B.5. Content Encryption Key . . . . . . . . . . . . . . . . . 16
B.6. Initialization Vector . . . . . . . . . . . . . . . . . . 16
B.7. JWE Plaintext . . . . . . . . . . . . . . . . . . . . . . 16
B.8. Content Encryption . . . . . . . . . . . . . . . . . . . 16
B.9. Key Agreement for Bob . . . . . . . . . . . . . . . . . . 17
B.10. Key Agreement for Charlie . . . . . . . . . . . . . . . . 19
B.11. Complete JWE JSON Serialization Representation . . . . . 20
Appendix C. Document History . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21
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
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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. A 500-byte payload
when encrypted and authenticated with ECDH-1PU (with P-256 keys
and "A256GCM" Content Encryption Method) results in a 1087-byte
JWE in Compact Encoding. An equivalent nested signed-then-
encrypted JOSE message using the same keys and encryption method
is 1489 bytes (37% larger).
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.
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2. Key Agreement with Elliptic Curve Diffie-Hellman One-Pass Unified
Model (ECDH-1PU)
This section defines the specifics of key agreement with Elliptic
Curve Diffie-Hellman One-Pass Unified Model, 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 fresh ephemeral public key value MUST be generated for each
message. When encrypting the message to multiple recipients using
ECDH-1PU, the same ephemeral keys MAY be reused for multiple
recipients [MRES].
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
"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:
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+-----------------+-------------------------------------------------+
| "alg" Param | Key Management Algorithm |
| Value | |
+-----------------+-------------------------------------------------+
| ECDH-1PU+A128KW | ECDH-1PU using one-pass KDF and CEK wrapped |
| | with "A128KW" |
| ECDH-1PU+A192KW | ECDH-1PU using one-pass KDF and CEK wrapped |
| | with "A192KW" |
| ECDH-1PU+A256KW | ECDH-1PU using one-pass KDF and CEK wrapped |
| | with "A256KW" |
+-----------------+-------------------------------------------------+
2.1. Special Considerations for Key Agreement with Key Wrapping mode
In Key Agreement with Key Wrapping mode, the JWE Authentication Tag
is included in the input to the Key Derivation Function as described
in section Section 2.3. This ensures that the content of the JWE was
produced by the original sender and not by another recipient, as
described in section Section 4.
Key Agreement with Key Wrapping mode MUST only be used with content
encryption algorithms that are compactly committing AEADs as
described in [ccAEAD]. The AES_CBC_HMAC_SHA2 algorithms described in
section 5.2 of [RFC7518] are compactly committing and can be used
with ECDH-1PU in Key Agreement with Key Wrapping mode. Other content
encryption algorithms MUST be rejected. In Direct Key Agreement
mode, any JWE content encryption algorithm MAY be used.
The requirement to include the JWE Authentication Tag in the input to
the Key Derivation Function implies an adjustment to the order of
operations performed during JWE Message Encryption described in
section 5.1 of [RFC7516]. Steps 3-8 are deferred until after step
15, using the randomly generated CEK from step 2 for encryption of
the message content.
2.2. 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.
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2.2.1. "skid" Header Parameter
A new Header Parameter "skid" (Sender Key ID) is registered as a hint
as to which of the sender's keys was used to authenticate the JWE.
The structure of the "skid" value is unspecified. Its value MUST be
a case-sensitive string. Use of this Header Parameter is OPTIONAL.
When used with a JWK, the "skid" value is used to match a JWK "kid"
parameter value [RFC7517].
2.3. 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
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 (when sending) or the
recipient's static private key and the sender's ephemeral public
key (when receiving). 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 (when sending) or the
recipient's static private key and the sender's static public key
(when receiving).
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.
cctag In Direct Key Agreement mode this is set to an empty octet
string. In Key Agreement with Key Wrapping mode, this is set to a
value of the form Datalen || Data, where Data is the raw octets of
the JWE Authentication Tag, and Datalen is the big-endian 32-bit
length of the authentication tag (in octets).
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AlgorithmID The AlgorithmID 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. 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.
SuppPubInfo This is set to the keydatalen represented as a 32-bit
big-endian integer followed by the octets of the cctag.
SuppPrivInfo This is set to the empty octet sequence.
Applications need to specify 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 document, e.g., by using values that identify the producer and
consumer.
See Appendix A for an example key agreement computation using Direct
Key Agreement mode, and Appendix B for an example sending to multiple
recipients using Key Agreement with Key Wrapping mode.
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3. IANA considerations
This section registers identifiers under the IANA JSON Web Signature
and Encryption Algorithms Registry established by [RFC7518] and the
IANA JSON Web Signature and Encryption Header Parameters registry
established by [RFC7515].
3.1. JSON Web Signature and Encryption Algorithms Registration
This section registers JWE algorithms as per the registry established
in [RFC7518].
3.1.1. ECDH-1PU
Algorithm Name: "ECDH-1PU"
Algorithm Description: ECDH One-Pass Unified Model using one-pass
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]
Algorithm Name: "ECDH-1PU+A128KW"
Algorithm Description: ECDH One-Pass Unified Model using one-pass
KDF and "A128KW"
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]
Algorithm Name: "ECDH-1PU+A192KW"
Algorithm Description: ECDH One-Pass Unified Model using one-pass
KDF and "A192KW"
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]
Algorithm Name: "ECDH-1PU+A256KW"
Algorithm Description: ECDH One-Pass Unified Model using one-pass
KDF and "A256KW"
Algorithm Usage Location(s): "alg"
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JOSE Implementation Requirements: Optional
Change Controller: IESG
Specification Document(s): Section 2
Algorithm Analysis Document(s): [NIST.800-56A] (Section 7.3),
[PKAE]
3.2. JSON Web Signature and Encryption Header Parameters Registration
This section registers new Header Parameters as per the registry
established in [RFC7515].
3.2.1. skid
Header Parameter Name: "skid"
Header Parameter Description: Sender Key ID
Header Parameter Usage Location(s): JWE
Change Controller: IESG
Specification Document(s): Section 2.2.1
4. Security Considerations
The security considerations of [RFC7516] and [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. If resistance to KCI is
desired in a single message, then the sender SHOULD use a nested JWS
signature over the content.
When Key Agreement with Key Wrapping is used, the JWE Authentication
Tag is included in the input to the Key Derivation Function, as
described in section Section 2.3. Without this step, when the same
Content Encryption Key (CEK) is reused for multiple recipients, then
any of those recipients can produce a new message that appears to
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come from the original sender. If the MAC used by the content
encryption algorithm is not compactly committing ([ccAEAD]) then it
may be possible for a recipient to calculate an alternative message
that produces the same authentication tag. An alternative is to
encrypt the message separately to each recipient using Direct Key
Agreement, or to sign the message using a nested signed-then-
encrypted JOSE composition.
The security properties of the one-pass unified model are given in
Section 7.3 of [NIST.800-56A].
5. References
5.1. 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>.
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<https://www.rfc-editor.org/info/rfc7517>.
[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>.
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5.2. Informative References
[ccAEAD] Grubbs, P., Lu, J., and T. Ristenpart, "Message Franking
via Committing Authenticated Encryption", IACR ePrint
2017/664, 2017.
[MRES] Bellare, M., Boldyreva, A., Kurosawa, K., and J. Staddon,
"Multi-Recipient Encryption Schemes: Efficient
Constructions and their Security", IEEE Transactions on
Information Theory Vol. 53, Number 11, 2007.
[PKAE] An, J., "Authenticated Encryption in the Public-Key
Setting: Security Notions and Analyses", IACR ePrint
2001/079, 2001.
[RFC8037] Liusvaara, I., "CFRG Elliptic Curve Diffie-Hellman (ECDH)
and Signatures in JSON Object Signing and Encryption
(JOSE)", RFC 8037, DOI 10.17487/RFC8037, January 2017,
<https://www.rfc-editor.org/info/rfc8037>.
Appendix A. Example ECDH-1PU Key Agreement Computation with A256GCM
This example uses ECDH-1PU in Direct Key Agreement mode ("alg" value
"ECDH-1PU") to produce an agreed-upon key for AES GCM with a 256-bit
key ("enc" value "A256GCM"). The example re-uses the keys and
parameters of the example computation in Appendix C of [RFC7518],
with the addition of an extra static key-pair for Alice.
In this example, a producer Alice is encrypting content to a consumer
Bob. Alice's static key-pair (in JWK format) used for the key
agreement in this example (including the private part) is:
{"kty":"EC",
"crv":"P-256",
"x":"WKn-ZIGevcwGIyyrzFoZNBdaq9_TsqzGl96oc0CWuis",
"y":"y77t-RvAHRKTsSGdIYUfweuOvwrvDD-Q3Hv5J0fSKbE",
"d":"Hndv7ZZjs_ke8o9zXYo3iq-Yr8SewI5vrqd0pAvEPqg"}
Bob's static key-pair (in JWK format) is:
{"kty":"EC",
"crv":"P-256",
"x":"weNJy2HscCSM6AEDTDg04biOvhFhyyWvOHQfeF_PxMQ",
"y":"e8lnCO-AlStT-NJVX-crhB7QRYhiix03illJOVAOyck",
"d":"VEmDZpDXXK8p8N0Cndsxs924q6nS1RXFASRl6BfUqdw"}
The producer (Alice) generates an ephemeral key for the key agreement
computation. Alice's ephemeral key (in JWK format) is:
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{"kty":"EC",
"crv":"P-256",
"x":"gI0GAILBdu7T53akrFmMyGcsF3n5dO7MmwNBHKW5SV0",
"y":"SLW_xSffzlPWrHEVI30DHM_4egVwt3NQqeUD7nMFpps",
"d":"0_NxaRPUMQoAJt50Gz8YiTr8gRTwyEaCumd-MToTmIo"}
Header Parameter values used in this example are as follows. The
"apu" (agreement PartyUInfo) Header Parameter value is the base64url
encoding of the UTF-8 string "Alice" and the "apv" (agreement
PartyVInfo) Header Parameter value is the base64url encoding of the
UTF-8 string "Bob". The "epk" (ephemeral public key) Header
Parameter is used to communicate the producer's (Alice's) ephemeral
public key value to the consumer (Bob).
{"alg":"ECDH-1PU",
"enc":"A256GCM",
"apu":"QWxpY2U",
"apv":"Qm9i",
"epk":
{"kty":"EC",
"crv":"P-256",
"x":"gI0GAILBdu7T53akrFmMyGcsF3n5dO7MmwNBHKW5SV0",
"y":"SLW_xSffzlPWrHEVI30DHM_4egVwt3NQqeUD7nMFpps"
}
}
The resulting one-pass KDF [NIST.800-56A] parameter values are:
Ze This is set to the output of the ECDH key agreement between
Alice's ephemeral private key and Bob's static public key. In
this example, Ze is the following octet sequence (in hexadecimal
notation):
9e 56 d9 1d 81 71 35 d3 72 83 42 83 bf 84 26 9c
fb 31 6e a3 da 80 6a 48 f6 da a7 79 8c fe 90 c4
Zs This is set to the output of the ECDH key agreement between
Alice's static private key and Bob's static public key. In this
example, Zs is the following octet sequence (in hexadecimal
notation):
e3 ca 34 74 38 4c 9f 62 b3 0b fd 4c 68 8b 3e 7d
41 10 a1 b4 ba dc 3c c5 4e f7 b8 12 41 ef d5 0d
Z This is set to the concatenation of Ze followed by Zs. In this
example, Z is the following octet sequence (in hexadecimal
notation):
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9e 56 d9 1d 81 71 35 d3 72 83 42 83 bf 84 26 9c
fb 31 6e a3 da 80 6a 48 f6 da a7 79 8c fe 90 c4
e3 ca 34 74 38 4c 9f 62 b3 0b fd 4c 68 8b 3e 7d
41 10 a1 b4 ba dc 3c c5 4e f7 b8 12 41 ef d5 0d
keydatalen This value is 256 - the number of bits in the desired
output key (because "A256GCM" uses a 256-bit key).
cctag This value is the empty octet string.
AlgorithmID This is set to the octets representing the 32-bit big-
endian value 7 - 00 00 00 07 in hexadecimal notation - the number
of octets in the AlgorithmID content "A256GCM", followed by the
octets representing the ASCII string "A256GCM" - 41 32 35 36 47 43
4d (in hex). The complete value is therefore: 00 00 00 07 41 32
35 36 47 43 4d
PartyUInfo This is set to the octets representing the 32-bit big-
endian value 5, followed by the octets representing the UTF-8
string "Alice". In hexadecimal notation: 00 00 00 05 41 6c 69 63
65
PartyVInfo This is set to the octets representing the 32-bit big-
endian value 3, followed by the octets representing the UTF-8
string "Bob". In hexadecimal notation: 00 00 00 03 42 6f 62
SuppPubInfo This is set to the octets representing the 32-bit big-
endian value 256 - the keydatalen value. In hexadecimal notation:
00 00 01 00
SuppPrivInfo This is set to the empty octet sequence.
Concatenating the parameters AlgorithmID through SuppPrivInfo results
in a FixedInfo value in Concatenation Format (as per
Section 5.8.2.1.1 of [NIST.800-56A]) of (in hexidecimal notation):
00 00 00 07 41 32 35 36 47 43 4d 00 00 00 05 41
6c 69 63 65 00 00 00 03 42 6f 62 00 00 01 00
Concatenating the round number 1 (00 00 00 01), Z, and the FixedInfo
value results in a one-pass KDF round 1 hash input of (hexadecimal):
00 00 00 01 9e 56 d9 1d 81 71 35 d3 72 83 42 83
bf 84 26 9c fb 31 6e a3 da 80 6a 48 f6 da a7 79
8c fe 90 c4 e3 ca 34 74 38 4c 9f 62 b3 0b fd 4c
68 8b 3e 7d 41 10 a1 b4 ba dc 3c c5 4e f7 b8 12
41 ef d5 0d 00 00 00 07 41 32 35 36 47 43 4d 00
00 00 05 41 6c 69 63 65 00 00 00 03 42 6f 62 00
00 01 00
The resulting derived key, which is the full 256 bits of the round 1
hash output is:
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6c af 13 72 3d 14 85 0a d4 b4 2c d6 dd e9 35 bf
fd 2f ff 00 a9 ba 70 de 05 c2 03 a5 e1 72 2c a7
The base64url-encoded representation of this derived key is:
bK8Tcj0UhQrUtCzW3ek1v_0v_wCpunDeBcIDpeFyLKc
Appendix B. Example ECDH-1PU+A128KW Key Agreement computation with
A256CBC-HS256
This example uses ECDH-1PU in Key Agreement with Key Wrapping mode
("alg" value "ECDH-1PU+A128KW") to encrypt a JWE for multiple
recipients using the JWE JSON Serialization. The example uses X25519
key pairs, as described in [RFC8037]. Alice is sending an identical
message to Bob and Charlie. Because Bob and Charlie are using the
same curve (X25519), Alice reuses the same ephemeral key-pair for
both recipients and includes it in the JWE Protected Header. If this
was not the case, Alice should generate a separate ephemeral key-pair
for each recipient and include it in each per-recipient header
instead.
Alice's static key pair, represented as an OKP JWK (including the
private component) is:
{"kty": "OKP",
"crv": "X25519",
"x": "Knbm_BcdQr7WIoz-uqit9M0wbcfEr6y-9UfIZ8QnBD4",
"d": "i9KuFhSzEBsiv3PKVL5115OCdsqQai5nj_Flzfkw5jU"}
Bob's static key-pair (in JWK format) is:
{"kty": "OKP",
"crv": "X25519",
"x": "BT7aR0ItXfeDAldeeOlXL_wXqp-j5FltT0vRSG16kRw",
"d": "1gDirl_r_Y3-qUa3WXHgEXrrEHngWThU3c9zj9A2uBg"}
Charlie's static key-pair (in JWK format) is:
{"kty": "OKP",
"crv": "X25519",
"x": "q-LsvU772uV_2sPJhfAIq-3vnKNVefNoIlvyvg1hrnE",
"d": "Jcv8gklhMjC0b-lsk5onBbppWAx5ncNtbM63Jr9xBQE"}
Alice generates an ephemeral key-pair on the same curve. Alice's
ephemeral key-pair (in JWK format) is:
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{"kty": "OKP",
"crv": "X25519",
"x": "k9of_cpAajy0poW5gaixXGs9nHkwg1AFqUAFa39dyBc",
"d": "x8EVZH4Fwk673_mUujnliJoSrLz0zYzzCWp5GUX2fc8"}
B.1. JWE Protected Header
The JWE Protected Header is as follows. The "apu" (agreement
PartyUInfo) Header Parameter value is the base64url encoding of the
UTF-8 string "Alice" and the "apv" (agreement PartyVInfo) Header
Parameter value is the base64url encoding of the UTF-8 string "Bob
and Charlie". The "epk" (ephemeral public key) Header Parameter is
used to communicate the producer's (Alice's) ephemeral public key to
the consumers (Bob and Charlie).
{"alg":"ECDH-1PU+A128KW",
"enc":"A256CBC-HS512",
"apu":"QWxpY2U",
"apv":"Qm9iIGFuZCBDaGFybGll",
"epk":
{"kty":"OKP",
"crv":"X25519",
"x":"k9of_cpAajy0poW5gaixXGs9nHkwg1AFqUAFa39dyBc"}}
B.2. JWE Per-Recipient Unprotected Headers
The following JWE Per-Recipient Unprotected Header values are used
for Bob and Charlie respectively:
{"kid":"bob-key-2"}
{"kid":"2021-05-06"}
B.3. JWE Shared Unprotected Header
This JWE uses the "jku" Header Parameter to reference a JWK Set.
This is represented in the following JWE Shared Unprotected Header
value as:
{"jku":"https://alice.example.com/keys.jwks"}
B.4. Additional Authenticated Data
Let the Additional Authenticated Data encryption parameter be
ASCII(BASE64URL(UTF8(JWE Protected Header))). This value is:
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[123, 34, 97, 108, 103, 34, 58, 34, 69, 67, 68, 72, 45, 49, 80, 85,
43, 65, 49, 50, 56, 75, 87, 34, 44, 34, 101, 110, 99, 34, 58, 34,
65, 50, 53, 54, 67, 66, 67, 45, 72, 83, 53, 49, 50, 34, 44, 34, 97,
112, 117, 34, 58, 34, 81, 87, 120, 112, 89, 50, 85, 34, 44, 34, 97,
112, 118, 34, 58, 34, 81, 109, 57, 105, 73, 71, 70, 117, 90, 67, 66,
68, 97, 71, 70, 121, 98, 71, 108, 108, 34, 44, 34, 101, 112, 107,
34, 58, 123, 34, 107, 116, 121, 34, 58, 34, 79, 75, 80, 34, 44, 34,
99, 114, 118, 34, 58, 34, 88, 50, 53, 53, 49, 57, 34, 44, 34, 120,
34, 58, 34, 107, 57, 111, 102, 95, 99, 112, 65, 97, 106, 121, 48,
112, 111, 87, 53, 103, 97, 105, 120, 88, 71, 115, 57, 110, 72, 107,
119, 103, 49, 65, 70, 113, 85, 65, 70, 97, 51, 57, 100, 121, 66, 99,
34, 125, 125]
B.5. Content Encryption Key
Alice generates the following 512-bit Content Encryption Key (CEK)
for A256CBC-HS512 (shown in hexadecimal):
ff fe fd fc fb fa f9 f8 f7 f6 f5 f4 f3 f2 f1 f0
ef ee ed ec eb ea e9 e8 e7 e6 e5 e4 e3 e2 e1 e0
df de dd dc db da d9 d8 d7 d6 d5 d4 d3 d2 d1 d0
cf ce cd cc cb ca c9 c8 c7 c6 c5 c4 c3 c2 c1 c0
B.6. Initialization Vector
She then generates the following random JWE Initialization Vector
(IV):
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
B.7. JWE Plaintext
The plaintext of the message Alice sends to Bob and Charlie is the
UTF-8 bytes of the string "Three is a magic number." (without the
quotes). The octets of the plaintext are:
[84, 104, 114, 101, 101, 32, 105, 115, 32, 97, 32, 109, 97, 103, 105, 99,
32, 110, 117, 109, 98, 101, 114, 46]
B.8. Content Encryption
Alice performs authenticated encryption on the plaintext with the
AES_256_CBC_HMAC_SHA_512 algorithm using the CEK as the encryption
key, the JWE Initialization Vector, and the Additional Authenticated
Data value above. This algorithm is described in [RFC7518]. The
resulting ciphertext (in base64url encoding) is:
Az2IWsISEMDJvyc5XRL-3-d-RgNBOGolCsxFFoUXFYw
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The resulting JWE Authentication Tag is (in base64url encoding):
HLb4fTlm8spGmij3RyOs2gJ4DpHM4hhVRwdF_hGb3WQ
B.9. Key Agreement for Bob
The KDF input parameters for Bob are as follows:
Ze This is set to the ECDH key agreement output between Alice's
ephemeral private key and Bob's static public key. In this
example, Ze is the following octet sequence (in hexadecimal):
32 81 08 96 e0 fe 4d 57 0e d1 ac fc ed f6 71 17
dc 19 4e d5 da ac 21 d8 ff 7a f3 24 46 94 89 7f
Zs This is set to the ECDH key agreement output between Alice's
static private key and Bob's static public key. In this example,
Zs is the following octet sequence (in hexadecimal):
21 57 61 2c 90 48 ed fa e7 7c b2 e4 23 71 40 60
59 67 c0 5c 7f 77 a4 8e ea f2 cf 29 a5 73 7c 4a
Z Z is the concatenation of Ze followed by Zs. In this example, the
value of Z is:
32 81 08 96 e0 fe 4d 57 0e d1 ac fc ed f6 71 17
dc 19 4e d5 da ac 21 d8 ff 7a f3 24 46 94 89 7f
21 57 61 2c 90 48 ed fa e7 7c b2 e4 23 71 40 60
59 67 c0 5c 7f 77 a4 8e ea f2 cf 29 a5 73 7c 4a
keydatalen This value is 128 - the number of bits in the desired
output key (because "ECDH-1PU+A128KW" uses a 128-bit key-wrapping
key).
cctag This is set to the octets of the JWE Authentication Tag,
prefixed by the length of the authentication tag (number of
octets) as a big-endian 32-bit unsigned integer. For the
"A256CBC-HS512" algorithm used in this example, the tag is 32
octets in size (00 00 00 20 in hex). The complete value of the
cctag parameter for this example (in hex) is:
00 00 00 20 1c b6 f8 7d 39 66 f2 ca 46 9a 28 f7
47 23 ac da 02 78 0e 91 cc e2 18 55 47 07 45 fe
11 9b dd 64
AlgorithmID This is set to the octets representing the big-endian
value 15 - 00 00 00 0F in hexadecimal notation - the number of
octets in the ASCII encoding of "ECDH-1PU+A128KW", followed by the
octets representing that string - 45 43 44 48 2d 31 50 55 2b 41 31
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32 38 4b 57 (in hex). The complete value is therefore 00 00 00 0f
45 43 44 48 2d 31 50 55 2b 41 31 32 38 4b 57
PartyUInfo This is set to the octets representing the big-endian
value 5 followed by the octets of the UTF-8 encoding of "Alice":
00 00 00 05 41 6c 69 63 65 (in hex).
PartyVInfo This is set to the octets representing the big-endian
value 15 followed by the octets of the UTF-8 encoding of "Bob and
Charlie": 00 00 00 0f 42 6f 62 20 61 6e 64 20 43 68 61 72 6c 69 65
(in hex).
SuppPubInfo This is set to the octets representing the 32-bit big-
endian encoding of the keydatalen followed by the octets of the
cctag. The complete value is as follows (in hex):
00 00 00 80 00 00 00 20 1c b6 f8 7d 39 66 f2 ca
46 9a 28 f7 47 23 ac da 02 78 0e 91 cc e2 18 55
47 07 45 fe 11 9b dd 64
SuppPrivInfo This is set to the empty octet sequence.
Concatenating the parameters AlgorithmID through SuppPrivInfo results
in a FixedInfo value in Concatenation Format (as per
Section 5.8.2.1.1 of [NIST.800-56A] of (in hexadecimal notation):
00 00 00 0f 45 43 44 48 2d 31 50 55 2b 41 31 32
38 4b 57 00 00 00 05 41 6c 69 63 65 00 00 00 0f
42 6f 62 20 61 6e 64 20 43 68 61 72 6c 69 65 00
00 00 80 00 00 00 20 1c b6 f8 7d 39 66 f2 ca 46
9a 28 f7 47 23 ac da 02 78 0e 91 cc e2 18 55 47
07 45 fe 11 9b dd 64
Concatenating the round number 1 (00 00 00 01), Z, and the FixedInfo
value results in a one-pass KDF round 1 hash input of (hexadecimal):
00 00 00 01 32 81 08 96 e0 fe 4d 57 0e d1 ac fc
ed f6 71 17 dc 19 4e d5 da ac 21 d8 ff 7a f3 24
46 94 89 7f 21 57 61 2c 90 48 ed fa e7 7c b2 e4
23 71 40 60 59 67 c0 5c 7f 77 a4 8e ea f2 cf 29
a5 73 7c 4a 00 00 00 0f 45 43 44 48 2d 31 50 55
2b 41 31 32 38 4b 57 00 00 00 05 41 6c 69 63 65
00 00 00 0f 42 6f 62 20 61 6e 64 20 43 68 61 72
6c 69 65 00 00 00 80 00 00 00 20 1c b6 f8 7d 39
66 f2 ca 46 9a 28 f7 47 23 ac da 02 78 0e 91 cc
e2 18 55 47 07 45 fe 11 9b dd 64
The resulting derived key, which is the first 16 octets of the round
1 hash output is:
df 4c 37 a0 66 83 06 a1 1e 3d 6b 00 74 b5 d8 df
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The derived key is then used with the "A128KW" key-wrapping algorithm
described in [RFC7518] to encrypt the CEK, resulting the following
JWE Encrypted Key (in base64url encoding with line breaks for display
purposes only):
pOMVA9_PtoRe7xXW1139NzzN1UhiFoio8lGto9cf0t8PyU-sjNXH8-LIRLycq8CHJQ
bDwvQeU1cSl55cQ0hGezJu2N9IY0QN
B.10. Key Agreement for Charlie
The KDF input parameters for Charlie are as follows:
Ze This is set to the ECDH key agreement output between Alice's
ephemeral private key and Charlie's static public key. In this
example, Ze is the following octet sequence (in hexadecimal):
89 dc fe 4c 37 c1 dc 02 71 f3 46 b5 b3 b1 9c 3b
70 5c a2 a7 2f 9a 23 77 85 c3 44 06 fc b7 5f 10
Zs This is set to the ECDH key agreement output between Alice's
static private key and Charlie's static public key. In this
example, Zs is the following octet sequence (in hexadecimal):
78 fe 63 fc 66 1c f8 d1 8f 92 a8 42 2a 64 18 e4
ed 5e 20 a9 16 81 85 fd ee dc a1 c3 d8 e6 a6 1c
Z Z is the concatenation of Ze followed by Zs. In this example, the
value of Z is:
89 dc fe 4c 37 c1 dc 02 71 f3 46 b5 b3 b1 9c 3b
70 5c a2 a7 2f 9a 23 77 85 c3 44 06 fc b7 5f 10
78 fe 63 fc 66 1c f8 d1 8f 92 a8 42 2a 64 18 e4
ed 5e 20 a9 16 81 85 fd ee dc a1 c3 d8 e6 a6 1c
The FixedInfo value is identical to that computed for Bob.
Concatenating the round number 1 (00 00 00 01), Z, and the FixedInfo
value results in a one-pass KDF round 1 hash input of (hexadecimal):
00 00 00 01 89 dc fe 4c 37 c1 dc 02 71 f3 46 b5
b3 b1 9c 3b 70 5c a2 a7 2f 9a 23 77 85 c3 44 06
fc b7 5f 10 78 fe 63 fc 66 1c f8 d1 8f 92 a8 42
2a 64 18 e4 ed 5e 20 a9 16 81 85 fd ee dc a1 c3
d8 e6 a6 1c 00 00 00 0f 45 43 44 48 2d 31 50 55
2b 41 31 32 38 4b 57 00 00 00 05 41 6c 69 63 65
00 00 00 0f 42 6f 62 20 61 6e 64 20 43 68 61 72
6c 69 65 00 00 00 80 00 00 00 20 1c b6 f8 7d 39
66 f2 ca 46 9a 28 f7 47 23 ac da 02 78 0e 91 cc
e2 18 55 47 07 45 fe 11 9b dd 64
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The resulting derived key, which is the first 16 octets of the round
1 hash output is:
57 d8 12 6f 1b 7e c4 cc b0 58 4d ac 03 cb 27 cc
The derived key is then used with the "A128KW" key-wrapping algorithm
described in [RFC7518] to encrypt the CEK, resulting the following
JWE Encrypted Key (in base64url encoding with line breaks for display
purposes only):
56GVudgRLIMEElQ7DpXsijJVRSWUSDNdbWkdV3g0GUNq6hcT_GkxwnxlPIWrTXCqRp
VKQC8fe4z3PQ2YH2afvjQ28aiCTWFE
B.11. Complete JWE JSON Serialization Representation
The complete JWE JSON Serialization for these values is as follows
(with line breaks within values for display purposes only):
{
"protected":
"eyJhbGciOiJFQ0RILTFQVStBMTI4S1ciLCJlbmMiOiJBMjU2Q0JDLUhTNTEyIiwiYXB1Ijoi
UVd4cFkyVSIsImFwdiI6IlFtOWlJR0Z1WkNCRGFHRnliR2xsIiwiZXBrIjp7Imt0eSI6Ik9L
UCIsImNydiI6IlgyNTUxOSIsIngiOiJrOW9mX2NwQWFqeTBwb1c1Z2FpeFhHczluSGt3ZzFB
RnFVQUZhMzlkeUJjIn19",
"unprotected":
{"jku":"https://alice.example.com/keys.jwks"},
"recipients":[
{"header":
{"kid":"bob-key-2"},
"encrypted_key":
"pOMVA9_PtoRe7xXW1139NzzN1UhiFoio8lGto9cf0t8PyU-sjNXH8-LIRLycq8CHJQbDwvQ
eU1cSl55cQ0hGezJu2N9IY0QN"},
{"header":
{"kid":"2021-05-06"},
"encrypted_key":
"56GVudgRLIMEElQ7DpXsijJVRSWUSDNdbWkdV3g0GUNq6hcT_GkxwnxlPIWrTXCqRpVKQC8
fe4z3PQ2YH2afvjQ28aiCTWFE"}],
"iv":
"AAECAwQFBgcICQoLDA0ODw",
"ciphertext":
"Az2IWsISEMDJvyc5XRL-3-d-RgNBOGolCsxFFoUXFYw",
"tag":
"HLb4fTlm8spGmij3RyOs2gJ4DpHM4hhVRwdF_hGb3WQ"
}
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Appendix C. Document History
-04 Added requirement to include the JWE Authentication Tag in the
KDF input when using Key Agreement with Key Wrapping mode to
ensure security against insider threats when sending to multiple
recipients. Added worked example for a multi-recipient JWE in
Appendix B.
-03 Corrected typos and clarified wording. Removed unnecessary
references.
-02 Removed two-way interactive handshake protocol section and
example after discussion with Hannes Tschofenig.
-01 Added examples in Appendix A and a two-way handshake example.
Added "skid" Header Parameter and registration. Fleshed out
Security Considerations.
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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