To find another string that maps to the same digest (i.e. a collision in the MD5-space) should theoretically take 2^128 steps.

Actually that is right and wrong. Hash collisions conform to the Birthday Paradox which says that if you have 23 people in a room there is a 50% probability that 2 people will share a birthday (have a collision). This 'paradox' applies to hashes as well. For example you have a 50% probability of a collision in a 32 bit hash after a mere 77163 tries (roughly 2^16 ie nothing like 2^32)

Update

Corrected technical inexactitude. The birthday paradox is the correct answer if you simply want a collision. To get a specific collision with a given digest it is not.

cheers

tachyon

In reply to Re: On showing the weakness in the MD5 digest function and getting bitten by scalar context by tachyon
in thread On showing the weakness in the MD5 digest function and getting bitten by scalar context by grinder

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