IV length vs. nonce in CCM mode

[Michaela Merz]

Hi again:

BouncyCastle uses the IV as nonce in CCM. This doesn't fly, as SJCL's IV length can be > 13 octets and re-calculates the size of the nonce. Would it be safe to assume, that SJCL always reduces larger IVs to 13 bytes of nonce? Would it be safe to simply reduce the IV to 13 octets?

Thanks.

Michaela

[Mike Hamburg]

Hi,

You're more or less correct.  SJCL truncates the IV down to the appropriate size, which is something like 15 - min(2, ceiling(log_256(1+length of message))) bytes.  So at most 13, but possibly as few as 11 bytes if the message is at least 16 MiB long.

-- Mike

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