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Security Fundamentals & Crypto · Foundational

Hash Function

One-way function that maps input of any size to a fixed-size digest, used to verify integrity.

Definition

Cryptographic hash functions must be preimage resistant, second-preimage resistant, and collision resistant. The same input always produces the same output, and small changes produce very different digests. Hashes are used for integrity checks, digital signatures, password storage (with salts and stretching), and data deduplication.

Example

sha256sum ubuntu.iso produces a 64-hex-character digest to compare with the published value.

Exam tip

Hashing is one-way and not encryption; it cannot be 'decrypted'.

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