I'm working on breaking a base64 Vigenere cipher. My guess is that the plaintext has been first base64-encoded, and then a variation of Vigenere with a 64*64 alphabet table used, the alphabet being composed of abc..xyzABC..XYZ012..789/+"
The cipher is quite short, and I have pratically no repetitions so Kasiski's test does not apply. I was thinking about using a kind of base64-adapted index of coincidence to guess the key length.
Do you have any idea on interesting strategies?