SHA-2 HMAC support in linux kernel

Hi,

I believe that this is the right list for my question. I'm trying to get SHA-2 HMAC support working ipsec in linux kernel (I'm configuring via pfkey).

First, sha-384 and sha-512 as authentication algorithm always return function not support. But I noted that my linux kernel has a sha512 kernel module (with alias for sha384). Second, sha-256 uses a 12-byte hmac (96 bits).

Looking at the source http://lxr.linux.no/source/net/xfrm/xfrm_algo.c, it seems to confirm that this is true. In fact, sha-384 and sha-512 are not supported at this time and sha-256 is truncated to 96-bit.

However, the following ietf draft, which I believe is very closed to ratification (it has already been assigned iana numbers), specifies sha-256 to use 128-bits as hmac (page 18): http://www.ietf.org/internet-drafts/draft-kelly-ipsec-ciph-sha2-01.txt

sha-384 is 192 bits, and sha-512 is 256 bits.

1. Is adding sha-384 and sha-512 as simple as adding to the aalg_list structure? Can this be done for some subsequent kernel release in the future?
2. Can the sha-256 be changed to use 128 bits? Or in order to not break backward compatibility, another sha-256 hmac algorithm id be used for 128 bits?

Thanks,

Chinh

That's normal.
HMAC usage in IPsec specifies that we only use 96-bits of the result. This is a tradeoff in space in the packet vs absolute "security"

In addition should you be able to cause a collision in 96-bits by some
method other than brute force, you can not be sure if you guess the key properly.

Yes, but that's the key, not the result.
It is keyed with various sizes of bits, but the results are truncated.