<Greys>
if you have two encrypted things, can you determine without decrypting them if they were encrypted with the same key
<Iskierka>
pretty sure that can only be determined by knowing the key
<Iskierka>
example: even with trivial vigenere cipher, the most you can determine from a simple analysis that stops short of the actual key is the periodicity, ie; the length of the key
<Iskierka>
to know more than that you need to work out what the key is
<Iskierka>
technically perhaps you could stop short at knowing the letter offset of each letter in the key, but at that point you pretty much know the key and just need to do a little dictionary matching
<Greys>
that all makes sense but what if there's an alternate analysis vector that can establish similarity in isolation
<Iskierka>
I don't think it should be possible as there's too many definitions of similarity; above all equal-length keys are similar, the encrypted source may be similar, etc
<Iskierka>
but poke egg for confirmation
<Greys>
eeeeeeeeeeegg
<UmbralRaptor>
I'd lean on metadata. eg: Alice uses PGP, so we can assume that the messages Bob and Carol send her are encrypted using her public key.
<Iskierka>
that would be a probabilistic way to guess if they used extant keys but you can't determine for two arbitrary messages
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<G-Mobile>
would using the wrong key give similar gibrish if they shared a key