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I want to be able to uniquely identify a file by its contents. I read the file a

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Question: I want to be able to uniquely identify a file in Java program by its contents. I read the file and use java.utils.zip.CRC32 to compute the checksum value.

Does exist possibility to have two files with different contents that return the same checksum?

Answer: Java uses CRC32 which generates a 32-bit checksum. There are only 2^32 possible values. So when you have more than 2^32 files you're guaranteed to have a collision. The possible solution to increase security could be to use Message Digest - technology based on MD5 or SHA.

Message digests are secure one-way hash functions that take arbitrary-sized data and output a fixed-length hash value.

The size of Message digests is always the same, independent of the size or content of the message from which it was created - 1024 bits.

Of course, exist possibility that two files can have the same Message Digests as well.

But the probability that two different files have the same checksums with CRC32 and the same message digests is negligibly low and probably (who can disprove?) equals almost zero.

"Bulletproof" approach should be:
1. Check for CRC32 checksum for two files
2. If they are the same check the message digests...


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