Length-extension attack
A length-extension attack exploits an iterative hash construction to compute a hash for an extended message from the original hash and message length.
Hacking Cryptography
A length-extension attack computes a valid hash for an extended message from a hash of the original message and its length when the hash construction exposes a reusable internal state.
- How it is used
- Demonstrates how naive API authentication with secret-prefix hashing can be forged.
- How deep it goes
- Works through padding generation, SHA-256 state replacement, and a forged HTTP request using OpenSSL.
- Examples
- Uses a SHA-256-signed bank transaction, extending amount=10 into a forged amount=424242 request.
- What you can do after
- Diagnose/debug with it by recognizing forgeable secret-prefix MAC authentication and selecting HMAC or sponge-based hashes.
Read how Hacking Cryptography explains length-extension attack →
Real-World Cryptography
A length-extension attack is an attack that exploits iterative hash constructions to compute a hash for an extended message when given a hash of an original message and its length.
- How it is used
- Explains why SHA-2 cannot safely authenticate secret-prefixed messages such as cookies.
- How deep it goes
- Works through digest continuation, padding, and appended data, but stops short of implementation details.
- Examples
- Uses a user=bob cookie extended with &admin=true and illustrates successful digest reuse.
- What you can do after
- Evaluate whether secret-prefixed SHA-2 authentication is vulnerable and avoid using it as a homemade MAC.
Read how Real-World Cryptography explains length-extension attack →
Related concepts
- secret-suffix MACaddressed by · A secret-suffix MAC is a message authentication construction that appends the secret key to the message before hashing.
- HMACcontrasts with · HMAC is a keyed message authentication code constructed from a cryptographic hash function and a secret key to authenticate data and verify its integrity.
- secret-prefix hashinguses · Secret-prefix hashing is an authentication construction that prepends a secret to a message before hashing the resulting data.
- SHA-2uses · SHA-2 is a standardized family of cryptographic hash algorithms that produces fixed-length digests, including SHA-224, SHA-256, SHA-384, and SHA-512.
- message authentication codeuses · A message authentication code is a keyed cryptographic value generated from a message to verify its integrity and authenticity.
- sponge constructioncontrasts with · A sponge construction absorbs input into a state through repeated permutation applications and squeezes output from that state, supporting outputs of arbitrary length.
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