Merkle–Damgård construction
The Merkle–Damgård construction builds an arbitrary-length hash function by iteratively applying a fixed-size compression function to padded message blocks.
Hacking Cryptography
A hash function built by repeatedly applying a compression function to message blocks in the Merkle–Damgård construction.
- How it is used
- Explains why familiar hash functions process blocks iteratively and why that structure matters for authentication failures.
- How deep it goes
- Traces padding, chained compression, exposed final state, and a SHA-256 length-extension exploit without detailing compression-function internals.
- Examples
- Demonstrates chained message blocks and a forged bank transaction request that extends a SHA-256 secret-prefix MAC.
- What you can do after
- Evaluate/understand why a published Merkle–Damgård digest can enable extension of secret-prefix MAC inputs.
Read how Hacking Cryptography explains Merkle–Damgård construction →
Real-World Cryptography
The Merkle–Damgård construction builds an arbitrary-length hash function by iteratively applying a fixed-size compression function to padded message blocks.
- How it is used
- Explains how SHA-2 hashes arbitrary-length messages using repeated fixed-size compression calls.
- How deep it goes
- Defines padding, blocking, and chaining, but treats the compression function as a black box.
- Examples
- Uses SHA-256’s 512-bit blocks, block-processing diagrams, and OpenSSL digest commands for SHA-2 variants.
- What you can do after
- Evaluate/understand how fixed-size compression supports arbitrary-length hashing and where collision resistance comes from.
Read how Real-World Cryptography explains Merkle–Damgård construction →
Related concepts
- SHA-2includes · 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.
- md5implemented by · MD5 is a cryptographic hash algorithm that produces a 128-bit message digest and is considered insecure because practical collision attacks exist.
- SHA-1implemented by · SHA-1 is a cryptographic hash algorithm that produces a 160-bit digest and is no longer considered collision-resistant because practical collision attacks exist.
- 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.
- compression functionuses · A compression function is a function that transforms input information into a shorter or fixed-size representation.
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