Hashvergleichen

Hashvergleichen, or hash comparison, is the practice of determining whether two data items are equal by comparing their hash digests. Hashing applies a deterministic function to input data to produce a fixed-size digest. If the inputs are identical, the digests should be identical; if the inputs differ, the digests are overwhelmingly likely to differ. However, hash functions can in principle produce collisions, where different inputs yield the same digest.

Common use cases include verifying data integrity for downloaded files or backups by comparing the computed hash of the received data with a known good value; ensuring software distributions have not been tampered with; and password authentication, where a system stores the hash of a password and compares it with the hash of the user-provided password during login.

Choosing the right hashes is important: cryptographic hash functions such as SHA-256, SHA-3, or BLAKE2 are designed to be deterministic and collision resistant, whereas non-cryptographic checksums are not suitable for security purposes. For password storage, it is essential to use salted and preferably memory-hard hashing algorithms (Argon2, bcrypt, scrypt) to slow off-line attacks. When comparing hashes, a constant-time comparison should be used to mitigate timing attacks.

Hash comparison has limitations: it does not provide data provenance or authenticity by itself and is vulnerable to certain attacks if weak algorithms are used. It should be complemented by digital signatures, HMACs, or secure transport and, in the case of password verification, by proper password storage practices.