A processor includes an execution unit for executing a message padding instruction including an operand field indicating a register buffering a message block segment of a message block to be padded and a mode field indicating which hash functions is to be applied to the message block. The execution unit includes a padding circuit configured to receive a message block segment from a register indicated by the operand field, where the message block spans multiple registers in a register file. Based on which hash function is indicated by the mode field, the padding circuit selects a byte location in the message block segment at which to insert at least one padding byte and inserts the at least one padding byte at the byte location within the message block segment. The message block segment as padded by the at least one padding byte is written back to the register file.