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Cleanup before -01 #23

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Cleanup before -01 #23

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175 changes: 100 additions & 75 deletions draft-ietf-cose-hash-envelope.md
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Expand Up @@ -37,9 +37,12 @@ author:
country: Germany

normative:
RFC9052: RFC9052
RFC9052: COSE
RFC8610: CDDL
I-D.draft-ietf-cbor-edn-literals: EDN

informative:
BCP205:

--- abstract

Expand All @@ -51,71 +54,41 @@ Additionally, hints of the detached payload's content format and availability ar

# Introduction

COSE defined detached payloads in Section 2 of {{-RFC9052}}, using `nil` as the payload.
COSE defined detached payloads in Section 2 of {{-COSE}}, using `nil` as the payload.
In order to verify a signature over a detached payload, the verifier must have access to the payload content.
Storing a hash of the content allows for small signature envelopes, that are easy to transport and verify independently.

Additional hints in the protected header ensure cryptographic agility for the hashing & signing algorithms, and discoverability for the original content which could be prohibitively large to move over a network.

## Attached Payload
When producing COSE_sign1 with remote signing services, such as a signing api exposed over HTTPS and backed by an HSM, the "ToBeSigned" bytes as described in {{Section 4.4 of RFC9052}} need to be transmitted to the HSM in order to be signed.

COSE_sign1 envelope with an attached payload, providing for signature validation.
Some signature algorithms such as ES256 or ES384 allow the "ToBeSigned" to be hashed on the client and sent to the server along with metadata in order to produce a signature.

~~~~ cbor-diag
18( / COSE Sign 1 /
[
h'a4013822...3a616263', / Protected /
{} / Unprotected /
h'317cedc7...c494e772', / Payload /
h'15280897...93ef39e5' / Signature /
]
)
~~~~
Other signature algorithms such as EdDSA with Ed25519, or ML-DSA do not expose such a capability.

## Detached Payload

COSE_sign1 envelope with a detached payload (`nil`), which is compact but the payload must be distributed out of band to validate the signature

~~~~ cbor-diag
18( / COSE Sign 1 /
[
h'a4013822...3a616263', / Protected /
{} / Unprotected /
nil, / Detached Payload /
h'15280897...93ef39e5' / Signature /
]
)
~~~~
By producing the "ToBeSigned" on the client, and ensuring that the payload is always a hashed value, the total size of the message to be sent to the servce for signing is constrained.

## Hashed Payload
It is still possible for the protected header to be large, but the payload will always be of a fixed size, associated with the hash function chosen.

A hashed payload functions equivalently to an attached payload, with the benefits of being compact in size and providing the ability to validate the signature.
# Terminology

~~~~ cbor-diag
18( / COSE Sign 1 /
[
h'a4013822...3a616263', / Protected /
{} / Unprotected /
h'935b5a91...e18a588a', / Payload /
h'15280897...93ef39e5' / Signature /
]
)
~~~~
{::boilerplate bcp14-tagged}

# Header Parameters
The terms COSE, CDDL, and EDN are defined in {{-COSE}}, {{-CDDL}}, {{-EDN}} respectively.

To represent a hash of a payload, the following headers are defined:
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TBD_1:
: the hash algorithm used to generate the hash of the payload
: the hash algorithm used to produce the payload.

TBD_2:
: the content type of the payload the hash represents
: the content type of the bytes that were hashed to produce the payload.

TBD_3:
: an identifier enabling a verifier to retrieve the full payload preimage.
: an identifier enabling a verifier to retrieve the bytes which were hashed to produce the payload.


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## Signed Hash Envelopes Example
# Hash Envelope CDDL

~~~ cddl
Hash_Envelope_Protected_Header = {
Expand All @@ -141,11 +114,11 @@ Hash_Envelope_Protected_Header = {
; storage.example/244f...9c19
? &(payload_location: TBD_3) => tstr

* int => any
* int / tstr => any
}

Hash_Envelope_Unprotected_Header = {
* int => any
* int / tstr => any
}

Hash_Envelope_as_COSE_Sign1 = [
Expand All @@ -158,30 +131,48 @@ Hash_Envelope_as_COSE_Sign1 = [
Hash_Envelope = #6.18(Hash_Envelope_as_COSE_Sign1)
~~~

## Protected Header

Label `16` (typ), label `TBD_1` (payload hash alg) and label `TBD_2` (content type of the preimage of the payload) MUST be present in the protected header and MUST NOT be present in the unprotected header.

Label `16` (typ) MAY be used to assign a content format or media type to the entire hash envelope.
Label `TBD_1` (payload hash alg) MUST be present in the protected header and MUST NOT be present in the unprotected header.
Label `TBD_2` (content type of the preimage of the payload) MAY be present in the protected header or unprotected header.
Label `TBD_3` (payload_location) MAY be added to the protected header and MUST NOT be presented in the unprotected header.
Label `3` (content_type) MUST NOT be present in the protected or unprotected headers.
Label `3` is easily confused with label `TBD_2` payload_preimage_content_type.
The difference between content_type (3) and payload_preimage_content_type (TBD2) is that content_type is used to identify the content format associated with payload, whereas payload_preimage_content_type is used to identify the content format of the bytes which are hashed to produce the payload.
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Label `3` (content_type) MUST NOT be used as it is easily confused with label `TBD_2` payload_preimage_content_type).
# Envelope EDN

For example:
A hashed payload functions equivalently to an attached payload, with the benefits of being compact in size and providing the ability to validate the signature.

~~~~ cbor-diag
{
/ alg : ES384 / 1: -35,
/ kid / 4: h'75726e3a...32636573',
/ typ / 16: application/hashed+cose
/ payload_hash_alg sha-256 / TBD_1: 1
/ payload_preimage_content_type / TBD_2: application/jwk+json
/ payload_location / TBD_3 : storage.example/244f...9c19
}
18( / COSE Sign 1 /
[
<<{
/ alg : ES384 / 1: -35,
/ kid / 4: h'75726e3a...32636573',
/ typ / 16: "application/example+cose"
/ payload_hash_alg /
TBD_1: -16 / sha-256 /
/ payload_preimage_content_type /
TBD_2: "application/example+json"
/ payload_location /
TBD_3 : "https://storage.example/244f" + ... "9c19"
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}>>
{} / Unprotected /
h'935b5a91...e18a588a', / Payload /
h'15280897...93ef39e5' / Signature /
]
)
~~~~

In this example, the sha256 hash algorithm (-16) is used to hash the payload, which is of content type "application/example+json".
The full payload is located at "https://storage.example/244f...9c19".
The COSE_sign1 is of type "application/example+cose".
The sha256 hash is signed with ES384 which starts by taking the sha384 hash of the payload (which is a sha256 hash).

# Encrypted Hashes

The cose headers defined in this document SHOULD NOT be used in unprotected or protected headers associated with COSE_Encrypt.
When present in COSE_Encrypt, the header parameters registered in this document leak information about the ciphertext.
These parameters SHOULD NOT be present in COSE_Encrypt headers unless this disclosure is acceptable.

# Security Considerations

Expand All @@ -194,34 +185,68 @@ For example, when signing with ECDSA using P-256 and SHA-256, use SHA-256 to has

# IANA Considerations

## Requirements Notation
## COSE Header Algorithm Parameters

{::boilerplate bcp14-tagged}
IANA is requested to add the following entries to the [COSE Header Algorithm Parameters Registry](https://www.iana.org/assignments/cose/cose.xhtml).

## COSE Header Algorithm Parameters
### Payload Hash Algorithm

- Name: payload hash algorithm
- Name: payload_hash_alg
- Label: TBD_1
- Value type: int
- Value registry: https://www.iana.org/assignments/named-information/named-information.xhtml
- Value registry: https://www.iana.org/assignments/cose/cose.xhtml#algorithms
- Description: Hash algorithm used to produce the payload.

## Named Information Hash Algorithm Registry
### Payload Pre-image Content Type

- Name: SHAKE256
- Name: payload_preimage_content_type
- Label: TBD_2
- Value type: int
- Value registry: https://www.iana.org/assignments/named-information/named-information.xhtml
- Description: SHAKE256 a described in https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.202.pdf
- Value registry: https://www.iana.org/assignments/core-parameters/core-parameters.xhtml#content-formats
- Description: The content format associated with the bytes that were hashed to produce the payload.

- Name: ASCON128
### Payload Location

- Name: payload_location
- Label: TBD_3
- Value type: int
- Value registry: https://www.iana.org/assignments/named-information/named-information.xhtml
- Description: ASCON128 a described in https://csrc.nist.gov/CSRC/media/Projects/lightweight-cryptography/documents/round-2/spec-doc-rnd2/ascon-spec-round2.pdf
- Value type: tstr
- Value registry: none
- Description: A string or URI as a hint for the location of the payload

--- back

# Implementation Status

Note to RFC Editor: Please remove this section as well as references to {{BCP205}} before AUTH48.

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in {{BCP205}}.
The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs.
Please note that the listing of any individual implementation here does not imply endorsement by the IETF.
Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors.
This is not intended as, and must not be construed to be, a catalog of available implementations or their features.
Readers are advised to note that other implementations may exist.

According to {{BCP205}}, "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as they see fit".

## Transmute Prototype

Organization: Transmute Industries Inc

Name: https://github.com/transmute-industries/transmute

Description: A command line tool and GitHub action for securing software artifacts in GitHub workflows.

Maturity: Prototype

Coverage: The current version ('main') implements this specification and demonstrates hash envelope signing with Azure Key Vault and Google Cloud KMS in addition to supporting local keys.

License: Apache-2.0

Implementation Experience: No interop testing has been done yet. The code works as proof of concept, but is not yet production ready.

Contact: Orie Steele ([email protected])

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# Acknowledgments
{:numbered="false"}

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