After finding all the bigrams, I concluded the key length has to be either 5 or 10, In the case of 5, the index of coincidence is a 0, while for 10 it's 1/225.
Which 1 is more likely to be the key? And how do I decrypt the cipher after that?
2026-03-28 03:01:03.1774666863
Decrypting the Vigenere cipher ACTMEFPTQBFPLZRDPTQBFH
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You can see that PTQBF occurred twice and the two occurrences are separated by 10 elements, and assuming/using Kasiski examiantion we can conclude the length of the cipher is factor of 10. But 1 and 2 are too short, so lets settle with 5 or 10. Next step is easy.Remember you can not be sure if it's 5 or 10 until you find the plan text,(one thing to assume is the plain text has clear English meaning) so you have to look for key assuming both cases. Now we got the length, the problem becomes like Caesar's cipher and can be solved with frequency analysis as in here
Final solution gives key of SPAIN and plain text INTERNATIONALRELATIONS.