Given a hash function (lets suppose 128bit output), there exists some collisions, at least if the input length is greater than the output.
Provided also that the hash function is meant to operate in full binary (block) mode.
Is the chance of a collision altered if we limit the input to be hashed to only alphanumeric characters (subset of the full binary range) ? Is that dependent on the hashing function algorithm ?
Context: A MD5 function is used to hash some text.
If an attacker wants to alter the text without being identified, he can only use text chatacters (lets suppose a-Z).
Is it any harder or easier (than full binary) to forge a different text which produces the same hash value ?
What if the input length of the original and forged messages cannot be more than a certain factor (so no unlimited length to forge a message) ?