How to find just the first character or value of a MD5 hash string

Question

I want to know how to find just the first character or digit in first part of a 64 length string from a MD5 hash. For example, we have a hash:

f95debd50b95304af9de1975393069143b7fb7df10ea00d189657d55b89c555c

and MD5 is:

be254c231d70885f6db16c0399be3066

In this case, what is the solution to find "f" that is the first part of string from the MD5?

Answer 1

The MD5 pre-image resistance theoretically is broken bu not practically. Instead of the generic preimage attack with 2¹²⁸ complexity, there is an attack that requires 2^123.4 complexity. MD5, however, is not practically broken. You cannot reach the 123-bit search space in a meaningful time. Consider the collective power of BitCoin miners; in 2020/1/17 they reached 126.1314 Exahashes per second, that makes

• 2⁶⁷ SHA256 double hashes in a second, and
• 2⁹² SHA256 double hashes in a year. So you need 2²¹ years with that power.

What you asking is against the design principle of the hash functions; the avalanche criteria. That is changing one bit of input will flip approximately half of the output bits, randomly.

If you can find even a single bit better than the generic pre-image attack, then you have a theoretical weakness. you can use this attack like divide and conquer technique to reduce the overall pre-image attack complexity.