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encryptionhmactrusted-vs-untrusted

Order of args to an HMAC?

If I send the following response more than once to an untrusted party:

{c, v, HMAC(c + v)}

where c is the same value from response to response.
where v varies from response to response.
where + is a concatenation.

Does having the constant as the start of the HMAC input "weaken" the HMAC?

Is it safer to compute the HMAC with the varying argument first:

{c, v, HMAC(v + c)}

or perhaps doesn't matter?

Solution

  • It doesn't matter. HMAC is designed specifically to address prefix attacks, among others.