40hexcharacter

40hexcharacter refers to a string consisting of exactly 40 hexadecimal digits (0-9, a-f), case-insensitive, representing 160 bits. This length is most commonly associated with the hexadecimal encoding of a SHA-1 hash.

Common usages include the full-length identifiers seen in version control systems like Git, where object IDs are SHA-1 digests displayed as 40 hex characters. Other contexts use 40-hex strings as fingerprints for digital certificates, checksums, and data integrity verification.

Representation and validation: A canonical 40-hex string contains only the characters 0-9 and a-f (A-F are also allowed). A typical validation pattern is /^[0-9a-fA-F]{40}$/. Some formats may prefix the value with 0x or insert separators, but the core is 40 hex digits.

Security and limitations: The underlying SHA-1 digest is 160 bits and has known collision weaknesses. Consequently, 40-hex-character identifiers derived from SHA-1 are increasingly discouraged for security-critical purposes. Modern cryptographic practice favors longer hashes such as SHA-256 (64 hex digits) or SHA-3 variants, which provide stronger resistance to collisions.

Example: A full-length 40-hex string might be 0123456789abcdef0123456789abcdef01234567. In many systems, these identifiers are shortened for display (e.g., Git abbreviations of 7–12 characters) while remaining unique within a repository.