All the examples of MD5 hash collisions I've come across comprise two different messages (inputs) of the same length. The first and second messages have different values for a number of bits and the resulting MD5's are equal.
Does the theory of finding an MD5 hash collision (exploiting the weakness in the algorithm) prescribe that the lengths of the messages be the same? In other words, would the theory be unusable to find a collision of two messages of different lengths? Has such a collision ever been found?