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Cryptographic Checksums

        Article #12345 on this BBS, which has checksum
        d3b07384d113edec49eaa6238ad5ff00, was really posted by me!

It's really important that it be difficult to generate a message with a given checksum
Otherwise, the BBS operator could make up a new message with the same checksum
And replace my real message with the new message

        I AM GAY! HEHEHEHEHE!              e5e8545a2466261c5c6fe5624f269b02 
        I AM GAY! HEHEHEHEHE!!             d32ec4e015eefec542b451d73a4b8b65 
        I AM GAY! HEHEHEHEHE!!!            9add7cfaebabda56e73fc17034ec25bd 
        I AM GAY!!   HEHE!!!               5e9befc4cfdbcbc762448970a41dade8 
        I AM GAY!!   HEHEHEHEHE!!!         99248cce20982c5d054671a4bbb68cf9 
        I AM GAY!! HEHEHEHEHE!             db52be257b750d581059be2e199f46de 
        I AM GAY!! HEHEHEHEHE!!            f243bd117dec0fbaf0b107f2f32520ea 
        I AM GAY!! HEHEHEHEHE!!!           6a9aa81f6e0e87e83f86eaa1430c5b05 
        I AM GAY!!!   HEHEHEHEHE!!!        357e6b7c31158f9fc355a96066117fb8 
        I AM GAY!!! HEHEHE!!!              d3b07384d113edec49eaa6238ad5ff00
        I AM GAY!!! HEHEHEHEHE!            34943e3c88b7966a69706be8ba6830e8 
        I AM GAY!!! HEHEHEHEHE!!           fa40b295047bf9a00f7c7b35ac1778c8 
        I AM GAY!!!!  HEHEHEHEHE!!!        3cdf061797a00ab6f7042983067c0b8b 
        I AM GAY!!!! HEHEHEHEHE!           0bb2b136fb1d03bd85bd578e83cfd6e3 
        I AM GAY!!!! HEHEHEHEHE!!          3a4b9f8c7853681f9161aab564f1ecf5 
        I AM GAY!!!! HEHEHEHEHE!!!         9e779160a8ebb249eab4fdb079ad8b93 
        I AM GAY!!!!! HEHEHEHEHE!!         d82d25b94cd01889e5efdc1f2ed73ba3 
        ...

As a special case, this also means it's hard to discover the original data that was checksummed
How to guarantee that it's hard to find a message with a given checksum?
Nobody knows
But we do have some methods that seem to work OK in practice