How likely are md5 false positive checksums?

MD5 is a 128 bit cryptographic hash function, so different messages should be distributed pretty well over the 128-bit space. That would mean that two files (excluding files specifically built to defeat MD5) should have a 1 in 2^128 chance of collision. In other words, if a pair of files is compared every nanosecond, it wouldn't have happened yet.


Sounds like a bug in their use of MD5 (maybe they are MD5-ing the wrong files), or a bug in the library that they're using. For example, an older MD5 program that I used once didn't handle files over 2GB.

This question suggests that, on average, you get a collision on average every 100 years if you were generating 6 billion files per second, so it's quite unlikely.


If a file is corrupted, then the probability that the corrupted file has the same md5 checksum as the uncorrupted file is 1:2^128. In other words, it will happen almost as "often" as never. It is astronomically more likely that your client is misreporting what really happened (like they are computing the wrong hash)

Tags:

Checksum

Md5