Contents
With an HMAC, the verification key is a shared secret, and only a holder of the shared secret can verify the message. Because a private key is used to generate the signature, only the sender (the holder of the private key) could have generated the signature, and the recipient can prove it to a third party.
Does HMAC use a secret key?
HMAC uses two passes of hash computation. The secret key is first used to derive two keys – inner and outer. The first pass of the algorithm produces an internal hash derived from the message and the inner key. Thus the algorithm provides better immunity against length extension attacks.
How many keys does HMAC use?
An encryption algorithm alters data, and a recipient needs a specific code (or key) to make it readable once more. HMAC relies on two sets of keys.
How is the HMAC key exchanged between client and server?
Using the server’s public key, more asymmetric cryptography occurs (this time a key exchange, possible based on asymmetric encryption, not signatures), which ends up with a nice result: client and server now know a shared secret value, and nobody spying on the line would be able to rebuild that value.
How is the HMAC key exchanged in SSL / TLS?
In SSL/TLS, the usual MAC algorithm is HMAC. The complete picture is that PKI is used to distribute public keys, and these public keys are then used with asymmetric cryptography to dynamically create a shared secret, and that shared secret is then used with symmetric algorithms such as HMAC.
Which is SHA256 hash algorithm does HMAC use?
HMAC user-input keys that are longer than the specific hash algorithms blocksize are first shortened. (By running the long keys through the hash. And then using that hash as the actual key.) SHA256 outputs 256 bit hashes.
What are the requirements for HMAC secret key generation?
(A longer key may be advisable if the randomness of the key is considered weak.) Keys need to be chosen at random (or using a cryptographically strong pseudo-random generator seeded with a random seed), and periodically refreshed.