However, her final forged message must be different from any message she chose to query the signing algorithm before.

The MAC value protects both a messages data integrity as well as its authenticity, by allowing verifiers (who also possess the secret key) to detect any changes to the message content.

That is, return accepted when the message and tag are not tampered with or forged, and otherwise return rejected.

Clearly we require that any adversary cannot directly query the string x on S, since otherwise a valid tag can be easily obtained by that adversary.

To be considered secure, a MAC function must resist existential forgery under chosen-plaintext attacks.

This means that even if an attacker has access to an oracle which possesses the secret key and generates MACs for messages of the attackers choosing, the attacker cannot guess the MAC for other messages (which were not used to query the oracle) without performing infeasible amounts of computation.

This implies that the sender and receiver of a message must agree on the same key before initiating communications, as is the case with symmetric encryption.

For the same reason, MACs do not provide the property of non-repudiation offered by signatures specifically in the case of a network-wide shared secret key: any user who can verify a MAC is also capable of generating MACs for other messages.

Since this private key is only accessible to its holder, a digital signature proves that a document was signed by none other than that holder.

However, non-repudiation can be provided by systems that securely bind key usage information to the MAC key; the same key is in the possession of two people, but one has a copy of the key that can be used for MAC generation while the other has a copy of the key in a hardware security module that only permits MAC verification.

However, some authors 6 use MIC to refer to a message digest, which is different from a MAC a message digest does not use secret keys.

This lack of security means that any message digest intended for use gauging message integrity should be encrypted or otherwise be protected against tampering.

Message digest algorithms are created such that a given message will always produce the same message digest assuming the same algorithm is used to generate both.

Conversely, MAC algorithms are designed to produce matching MACs only if the same message, secret key and initialization vector are input to the same algorithm.

Message digests do not use secret keys and, when taken on their own, are therefore a much less reliable gauge of message integrity than MACs.

Because MACs use secret keys, they do not necessarily need to be encrypted to provide the same level of assurance.

However many of the fastest MAC algorithms like UMAC - VMAC and Poly1305-AES are constructed based on universal hashing.

For instance, in Transport Layer Security (TLS), the input data is split in halves that are each processed with a different hashing primitive ( SHA-1 and SHA-2 ) then XORed together to output the MAC.

These models and parameters allow more specific algorithms to be defined by nominating the parameters.

For example, the FIPS PUB 113 algorithm is functionally equivalent to ISOIEC 9797-1 MAC algorithm 1 with padding method 1 and a block cipher algorithm of DES.

The receiver in turn runs the message portion of the transmission through the same MAC algorithm using the same key, producing a second MAC data tag.

The receiver then compares the first MAC tag received in the transmission to the second generated MAC tag.

Otherwise an attacker could without even understanding its content record this message and play it back at a later time, producing the same result as the original sender.

Origin For Mac Access