PMAC

PMAC, or Parallelizable Message Authentication Code, is a family of MAC algorithms designed to produce a short authentication tag for a message using a fixed-key block cipher. Its defining feature is parallelizability: the message is processed in blocks with per-block masks or offsets derived from the key and block index, allowing multiple blocks to be processed simultaneously rather than strictly in sequence. The per-block results are combined to form the final tag.

In typical PMAC constructions, the message is divided into blocks, and each block is combined with a block-specific offset before the block cipher is applied. The outputs from the blocks are then aggregated to produce the authentication tag. The approach ensures that each block contributes uniquely to the final result and that processing can be distributed across available hardware, improving performance on parallel architectures. PMAC also includes a finalization step to correctly handle the last, possibly incomplete, block and the overall message length.

Security and correctness of PMAC rely on the security of the underlying block cipher. When the block cipher behaves as a pseudorandom permutation, PMAC provides standard MAC security guarantees: a successful forgery would imply breaking the cipher’s PRP security in the related construction. PMAC is related to other MAC families, such as CMAC and OMAC, but is designed to exploit parallel computation more effectively, making it attractive for implementations that benefit from parallel processing.

PMAC has been studied in cryptographic literature as a practical, provably secure MAC based on block ciphers like AES. It is used for data integrity and authentication in protocols and storage systems, alongside other MAC schemes. See also CMAC, OMAC, and GMAC.