Hash Function

What is Hash Function?

A cryptographic hash function is a mathematical algorithm that takes an input of any size — a document, an image, or any digital data — and produces a fixed-length output called a hash, digest, or fingerprint. The same input always produces the same hash, but even the tiniest change to the input produces a completely different hash. This property makes hash functions essential for document integrity verification.

In digital signatures, hash functions serve a dual purpose. First, they create a compact, fixed-length representation of the document that can be efficiently signed (encrypting the hash with the private key is much faster than encrypting the entire document). Second, they enable tamper detection — after signing, anyone can re-hash the document and compare the result to the hash embedded in the signature. If they match, the document is unaltered.

Good cryptographic hash functions have several important properties: they are deterministic (same input, same output), they are one-way (you cannot reconstruct the original input from the hash), they are collision-resistant (it is practically impossible to find two different inputs that produce the same hash), and they exhibit the avalanche effect (a small change in input produces a dramatically different hash).

Common hash algorithms used in electronic signatures include SHA-256 and SHA-384 (from the SHA-2 family). Older algorithms like MD5 and SHA-1 are considered cryptographically broken and should not be used for signatures. The choice of hash algorithm is important for long-term document validity — documents may need to remain verifiable for decades.