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hashmd5sha1collisiondigest

hash collision and appending data

Assume I have two strings (or byte arrays) A and B which both have the same hash (with hash I mean things like MD5 or SHA1). If I concatenate another string behind it, will A+C and B+C have the same hash H' as well? What happens to C+A and C+B?

I tested it with MD5 and in all my tests, appending something to the end made the hash the same, but appending at the beginning did not.

Is this always true (for all inputs)?

Is this true for all (well-known) hash functions? If no, is there a (well-known) hash function, where A+C and B+C will not collide (and C+A and C+B do not either)?

(besides from MD5(x + reverse(x)) and other constructed stuff I mean)

Solution

  • This depends entirely on the hash function. Also, the probability that you have those collisions is really small.