SHA-2 HMAC support in linux kernel

SHA-2 HMAC support in linux kernel

[Chinh Nguyen]

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

Re: SHA-2 HMAC support in linux kernel

[Michael Richardson]

Chinh Nguyen wrote:
> 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.

   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.

> 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

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