# What is Cryptographic Hashing?

> Cryptographic hashing is a one-way function that maps arbitrary input to a fixed-length digest, designed so the input cannot be practically recovered from the digest and two different inputs cannot practically produce the same digest.

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## Content

The three essential properties are preimage resistance (a digest reveals nothing about its input), second-preimage resistance (no different input with the same digest can be found for a given input), and collision resistance (no two inputs with the same digest can be found at all). Algorithms such as MD5 and SHA-1 have failed these properties and must not be used where they matter.

Hashing appears throughout security: integrity checks and tamper evidence, digital signatures that sign digests rather than whole messages, content addressing, and — with deliberate extra work — password storage. Passwords need salted, computationally expensive functions such as dedicated password-hashing algorithms; a fast general-purpose hash is the wrong tool for that specific job.

### Key points

- **Integrity use:** A matching digest confirms that content has not changed since the digest was computed — provided the digest itself was obtained through a trusted channel.
- **Password storage:** Store salted, slow-hash digests rather than fast hashes; the salt defeats precomputed lookups and the cost slows brute-force guessing at scale.
- **Algorithm selection:** Use current approved families (SHA-2, SHA-3) and treat MD5 and SHA-1 as broken for security purposes — they remain acceptable only for non-adversarial checksums.
- **Important limitation:** A hash proves only that two inputs matched under one function at one time. It provides no confidentiality — the digest of a guessable input is trivially recovered by trying candidates — and a hash computed over an untrusted channel can be swapped along with the content it vouches for.

### Related terms

[Cryptography](<https://yellowcube.eu/glossary/cryptography/>) · [Digital signature](<https://yellowcube.eu/glossary/digital-signature/>) · [Message authentication code (MAC)](<https://yellowcube.eu/glossary/message-authentication-code/>) · [Login credentials](<https://yellowcube.eu/glossary/login-credentials/>) · [Data integrity](<https://yellowcube.eu/glossary/data-integrity/>)

### Sources

[NIST FIPS 180-4: Secure Hash Standard](https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf) · [NIST glossary: Hash Function](https://csrc.nist.gov/glossary/term/hash_function) · [OWASP, Password Storage Cheat Sheet](https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html)

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