Keyed Hash?

[Timo Teräs]

On 09/20/2010 01:23 AM, Denys Vlasenko wrote:
>>From a quick experiment it looks like w/o "-hmac KEY"
> part this command basically does sha1sum'ing.
> 
> What does -hmac KEY do? It seeds sha1 somehow? How exactly?

Yes, it sort of seeds it with password to allow message authentication.

   To compute HMAC over the data `text' we perform
         H(K XOR opad | H(K XOR ipad | text))

   where H = hash function used
         K = password
         ipad = 0x36, 0x36, ...
         opad = 0x5c, 0x5c, ...

   and | meaning concenation of binary data.

See also:
  http://en.wikipedia.org/wiki/HMAC
  http://tools.ietf.org/html/rfc2104

 - Timo