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SESK: Stuff Everyone Should Know · sesk.info/secplus-md5-collision

Published attacks show MD5 is not prudent to use when you need which property?

Collision resistance, MD5 can be made to give two inputs the same hash.

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  1. Fast speed
  2. Fixed-length output
  3. Collision resistance ✔

Why: A collision is two different inputs with the same hash. If attackers can create collisions, they can swap a malicious file or certificate for a good one.

Huh, didn't know that: MD5 produces a 128-bit message digest, as described in RFC 6151.

The dad joke
MD5 and its twin walked in with the same hash. Nobody could tell them apart, which was the problem.
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References

Answer: Collision resistance. A collision is two different inputs with the same hash. If attackers can create collisions, they can swap a malicious file or certificate for a good one.

  1. RFC 6151, Updated Security Considerations for MD5 www.rfc-editor.org/rfc/rfc6151
    The published attacks against MD5 show that it is not prudent to use MD5 when collision resistance is required.
    Checked 2026-10-10.
  2. RFC 6151, Updated Security Considerations for MD5 www.rfc-editor.org/rfc/rfc6151
    produces as output a 128-bit "fingerprint" or "message digest" of the input
    Checked 2026-10-10.

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Security & PrivacyA file's hash value no longer matches after someone edits the file. Which CIA property was affected?Security & PrivacyHow long is a SHA-256 hash output, called the message digest?Security & PrivacyWhat cryptographically links each block in a blockchain to the one before it?

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