[Openid-specs-ab] [openid/connect] SHA-256 to get AES KeyWrap key? (issue #828)

Brian Campbell issues-reply at bitbucket.org
Thu Mar 28 13:50:47 UTC 2013


New issue 828: SHA-256 to get AES KeyWrap key?
https://bitbucket.org/openid/connect/issue/828/sha-256-to-get-aes-keywrap-key

Brian Campbell:

I know this is a PITA but the use of left truncated SHA-256 hash to get the symmetric encryption key isn't particularity cryptographically agile.   

It is sufficient for what's currently defined (A128KW & A256KW) but the text below couldn't accommodate algorithms needing keys larger that 256bits and seems like it precludes anything but AES based key wrapping.  I dunno, maybe that's okay. But I felt like I should raise it.

http://openid.net/specs/openid-connect-messages-1_0-16.html#enc
"Symmetric Encryption
    The symmetric encryption key is derived from the client_secret value by using a left truncated SHA-256 hash of the bytes of the UTF-8 representation of the client_secret. The SHA-256 value MUST be left truncated to the appropriate bit length for the AES KeyWrap algorithm used, for instance, to 128 bits for A128KW. "


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