The important part about the length is how many letters appear on those 17 characters:
We have the plaintext of 12 of the 17 characters. This should probably let us recover 12 character of the key, and then guess the remaining characters based on context (key is probably made of words, not randomly).
So Vigenere is similar to caesar. So you start decrypting it like you do with caesar:
S - +2 --> U
O - +3 --> R
M - +5 --> R
E - +7 --> L
B - +11 -> M
O - +13 -> B
D - +17 -> U
Y - +19 -> R
So you may have noticed, that the text is decrypt via using the next bigger prime. Just subtract the primes from the decrypted text and you get ...
If I can prove there's no single key that can encrypt to both outputs I can prove with 100% certainty which message it is, but if not I can't.
"pqrs" or "jmlo"
You can identify which password it is by comparing each letter with the corresponding letter once encrypted. For example, compare $P-R$ with $J-L$ (treating each letter as a ...
frequency analysis works just fine even without a few common letters. In fact you don't even need to know the frequency in the underlying alphabet it is sufficient that it is far from uniform. You can rotate the columns so as to make the frequencies in each match.
Alternatively using an English bi-gram frequency table you can break this quickly even without ...