Bug 27403 - Should PBKDF2 be allowed on non-multiple of 8 lengths for consistency?

[mwatson2]

Bug 27403 from Bugzilla:

PBKDF2 throws an OperationError if length is not a multiple of 8 bits.

This seems inconsistent with deriveBits() for other algorithms, which simply truncate to the specified bit length (for instance ECDH and DH).

[jimsch]

Looking at RFC 2898 - the answer is no. PBKDF2 is defined as taking as input dkLen - intended length in octets of the derived key. This means that the algorithm itself says that odd numbers of bits are not supported.

[mwatson2]

@jimsch It would still be possible for us to call the RFC2898 algorithm with a dkLen = ceil( length / 8 ) and then truncate. Wouldn't this be more consistent with ECDH ?

[jimsch]

I would prefer to make this match the algorithm definition rather than try and make this consistent. If people need this they can do the truncation themselves. I am more than willing to have the inconsistency.

[mwatson2]

If there are no other comments, then I'll close this with no changes.