Suppose I have two messages encrypted by RSA with the same public key (N, e), where:
M1 = "Hello " + name1 + message
M2 = "Hello " + name2 + message
The names and the message is not known, but we know the length of both names is 5, and the goal is to find out what the message is.
My idea is to use the Franklin Reiter related message attack. By definition, the Franklin Reiter related message attack works in a scenario where two messages differ only by a fixed known difference.
I am aware that we need (N, e, C1, C2, f) for a Franklin Reiter related message attack, and N, e, C1, C2 are given in this question, but what is f in this case? How can we represent such two messages in a ax+b form?