The following errata report has been submitted for RFC8152,
"CBOR Object Signing and Encryption (COSE)".

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You may review the report below and at:
https://www.rfc-editor.org/errata/eid6597

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Type: Technical
Reported by: Anders Rundgren <[email protected]>

Section: 12.5.1.

Original Text
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The RFC is unclear to whether Concat KDF or HKDF is to be used:

In table 20 there is an entry:
ECDH-ES +  -31   | HKDF -  | yes        | A256KW | ECDH ES w/    |
   | A256KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | Wrap w/       |
   |           |       |         |            |        | 256-bit key  

That is, the table talks both about Concat and HKDF.

The IANA registry for this algorithm says Concat KDF

Jim's sample code for algorithm -31 says HKDF.

Corrected Text
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I have no corrected text to offer since I don't have the answer to the question 
raised.

Concat is referred to once and without any external references.  In JOSE, 
Concat denotes a NIST standard which is quite different to HKDF.

Notes
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It is pretty vital for interoperability knowing if Concat KDF or HKDF is to be 
used.

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RFC8152 (draft-ietf-cose-msg-24)
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Title               : CBOR Object Signing and Encryption (COSE)
Publication Date    : July 2017
Author(s)           : J. Schaad
Category            : PROPOSED STANDARD
Source              : CBOR Object Signing and Encryption
Area                : Security
Stream              : IETF
Verifying Party     : IESG

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