A cryptographic hash function, such as SHA-1.
See NIST SP 800-135 Rev. 1 under HASH for more information.
A function which maps strings of bits to fixed-length strings of bits, satisfying the following two properties: it is computationally infeasible to find for a given output an input which maps to this output; and it is computationally infeasible to find for a given input a second input which maps to the same output.
See NIST SP 800-15 for more information.