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Someone pointed out recently to me that a cryptographic hash function " is not designed as a bijective mapping from N bit input to N bit output".
So if I feed an N-bit cryptographic hash function with N bits of random input, there's a loss of entropy between the input and output of the hash function.
Considering the md5 hash function, is there a way to estimate that loss of entropy? And is this loss cumulative so I could say, if I apply the hash function enough times, I end up with a 50% loss of entropy?