Understanding hash, digest, checksum, and fingerprint

While "hash," "digest," "checksum," and "fingerprint" are related concepts and sometimes used interchangeably, their usage depends on context and technical precision. Here’s a breakdown:

1. Hash

• A hash is the general term for the output of a hashing algorithm.

• It is a fixed-size value derived from input data of arbitrary size.

• Used in many applications, including data structures (hash tables), cryptography, and integrity verification.

• Example: SHA-256("hello") → "2cf24dba5..."

2. Digest (Message Digest)

• A digest is a cryptographic hash, often used specifically for verifying data integrity.

• Historically tied to algorithms like MD5 (Message Digest 5) or SHA-1.

• Emphasizes the idea of "summarizing" data into a fixed-size representation.

• Example: MD5("hello") → "5d41402a..."

3. Checksum

• A checksum is a simpler form of hash, typically used for error detection (not cryptographic security).

• Often smaller in size (e.g., CRC32, Adler-32).

• Designed to detect accidental changes (e.g., file corruption, network errors).

• Example: CRC32("hello") → 3610a686

4. Fingerprint

• A fingerprint is a hash used to uniquely identify data (like a human fingerprint).

• Often applied to public keys (e.g., SSH key fingerprints) or file identities.

• May be a shortened or formatted version of a hash for readability.

• Example: SHA-256 fingerprint of an SSH key → "SHA256:AbCdE..."

Key Differences:

Term	Typical Use Case	Security Strength	Example Algorithms
Hash	General-purpose, data structures, cryptography	Varies (weak to strong)	SHA-256, MurmurHash
Digest	Cryptographic integrity checks	Strong (usually)	MD5, SHA-1, SHA-256
Checksum	Error detection (non-crypto)	Weak	CRC32, Adler-32
Fingerprint	Unique identification	Strong (usually)	Truncated SHA-256

When They Overlap:

• A cryptographic hash (e.g., SHA-256) can be called a digest, hash, or fingerprint depending on context.

• A checksum is technically a hash but is rarely cryptographic.

• Fingerprint implies human-readable identification, often derived from a hash.

Summary:

• Hash = Broad term for any fixed-size output from a hashing function.

• Digest = Cryptographic hash used for integrity (common in older algorithms like MD5).

• Checksum = Simple hash for error detection (not secure against tampering).

• Fingerprint = Human-friendly hash for identification (e.g., SSH keys).