I've been wondering, is there a term for a type of attack like this:

Given a message M, length function L(), and MD5 hash function H(); is there an attack which can generate message M', such that H(M)==H(M') _and_ L(M)==L(M')?

In other words: Two different messages, both of the same length, with the same hash?

It's almost like a chosen prefix collision attack, but with no prefix (so P is empty) and a given message (M is known, M' is up to the attacker).

I ask because I frequently use GridFTP for data transfer, and it uses both the file length and the MD5 has to verify that files were transferred correctly.