Subject: [Mip6] Question on HMAC-SHA-1 key length in 4285

From: Lakshminath Dondeti <[email-masked]>
Date: Mon, 20 Mar 2006 11:16:57 -0800

Hi all,

I was wondering if there is any reason behind using a short key 
(128-bit key) as the HMAC-SHA-1 key in 4285.

2104 says the following about key lengths:

"The key for HMAC can be of any length (keys longer than B bytes are
    first hashed using H).  However, less than L bytes is strongly
    discouraged as it would decrease the security strength of the
    function.  Keys longer than L bytes are acceptable but the extra
    length would not significantly increase the function strength. (A
    longer key may be advisable if the randomness of the key is
    considered weak.)"

L is the output length of the underlying hash function, and of course 
20B in case of SHA-1.

I think the key length MUST be 160 bits and I think it should be fixed in 4285.

regards,
Lakshminath

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