Answer: MD5
What is MD5A 128-bit hash used for file checksums, but not safe where collision resistance matters
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The file stays on your device. Your browser makes a digital fingerprint of the file. TimeBinder records the fingerprint and the time, not the file.
Short answer
MD5 is a hash function that turns any file into a 128-bit value, usually shown as 32 hexadecimal characters. It is still seen in file checksums to catch accidental corruption. It is not suitable where collision resistance is needed, such as digital signatures, because researchers have shown ways to create two different inputs with the same MD5.
What MD5 does and why it is no longer trusted
MD5 was specified in 1992 in RFC 1321 and produces a 128-bit message digest. The IETF’s RFC 6151 says the published attacks against MD5 show it is not prudent to use it when collision resistance is required, and that it must not be used for digital signatures. The same document says it is not urgent to stop using MD5 in other ways, such as HMAC-MD5.
A CERT Coordination Center note from 2008 (last revised 2009) says MD5 should be considered cryptographically broken and unsuitable for further use, and records that practical collision attacks were demonstrated in 2004 (VU#836068). A collision means two different inputs that give the same hash; see what is a hash collision.
In practice: an MD5 value published next to a download can catch a corrupted file, but it does not stop someone who controls both the file and the value from swapping them. Use SHA-256 or stronger. See what is a checksum and how to check a SHA-256 checksum.
What TimeBinder uses
A TimeBind records a SHA-256 fingerprint of the exact file and a time, anchored to Bitcoin. The fingerprint is made in your browser and the file is not uploaded. It does not use MD5.
A fingerprint ties a time to the exact bytes of a file. A TimeBind shows a file existed by a recorded time and has not changed since. It does not register copyright, is not legal advice and does not by itself prove who made the file.
First Pooled TimeBind free, no card, then from US$5. An ASAP TimeBind is US$15.
Frequently asked questions
Is MD5 still safe to use?
For catching accidental corruption, it is commonly used. It is not suitable for security uses that depend on collision resistance, such as digital signatures.
What is the difference between MD5 and SHA-256?
MD5 produces a 128-bit value and has practical collision attacks. SHA-256 produces a 256-bit value. See the TimeBinder hash collision answer for the current position on SHA-256.
Is MD5 encryption?
No. It is a one-way hash, not encryption. You cannot decrypt it, but weak or common inputs can be guessed.
Can I use MD5 to prove a file is unchanged?
Only against accidental change. For evidence of deliberate tampering, use SHA-256 or stronger.
Sources
- IETF, RFC 1321: The MD5 Message-Digest Algorithm.
- IETF, RFC 6151: Updated security considerations for MD5.
- CERT/CC, Vulnerability Note VU#836068.
- TimeBinder, What is a hash collision?.
Related guides and tools
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