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Security Fundamentals & Crypto · Intermediate

Birthday Attack

Cryptographic attack exploiting the birthday paradox to find hash collisions in about the square root of the output space.

Definition

Because collisions among many random values occur much sooner than expected, an attacker needs roughly 2^(n/2) attempts to find a collision in an n-bit hash. This is why 128-bit hashes like MD5 provide only about 64 bits of collision resistance. Longer digests and stronger algorithms mitigate the attack.

Example

Sweet32 used the birthday bound on 64-bit block ciphers to recover data from long-lived 3DES sessions.

Exam tip

Birthday attacks target collisions, not preimages; doubling the hash length counters them.

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