prehashing

Prehashing is the practice of applying a cryptographic hash function to input data before performing another cryptographic operation, such as signing or encryption. The hash output, a fixed-size digest, represents the original data and is used in place of the full input for subsequent processing.

One primary motivation for prehashing is efficiency. Hashing can compress large or streaming data into a compact form, enabling operations that require fixed-size input or that benefit from reduced I/O and memory usage. Prehashing also facilitates handling very large messages or data streams where the complete content may not be available at once, or where the underlying primitive works on fixed-length inputs.

In digital signatures and related protocols, prehashing allows the signer to operate on a digest rather than the entire message. This can improve performance for large messages and enables standardization across varying message formats. When prehashing is used, the scheme must specify domain separation or other measures to ensure that the prehashed input cannot be confused with direct-input inputs and to prevent certain collision or length-extension risks.

Security considerations are important. The chosen hash function must be appropriate for the intended security level, and domain separation must be clearly defined to avoid ambiguities between different inputs or applications. Improper prehashing can introduce vulnerabilities such as collisions or preimage attacks if the hash function is weak or misapplied. Proper parameter choices and adherence to the scheme’s specifications are essential.

See also: hash function, digital signature, message authentication code, domain separation.