How can we find the key used with a Vigenère cipher in small number of steps?
If we have text like:
Peit Wokm! Mhfa fepatgb ets bvvrvxmea ebte bae twkd qiqi
and we have key information like Key = ‘ ? a ? i ’, then how I can understand this and solve this to make the complete key?
You've been given the key length, plus half the characters in the key text. By the look of things, the ciphertext even contains word spaces and punctuation, which will help a lot.
Start by decrypting the text using the characters you already know:
> Peit Wokm! Mhfa fepatgb ets bvvrvxmea ebte bae twkd qiqi
.a.i .a.i .a.i .a.i.a. i.a .i.a.i.a. i.a. i.a .i.a .i.a
< .e.l .o.e! .h.s .e.s.g. w.s .n.r.p.e. w.t. t.e .o.d .a.i
Assuming the plaintext is English, it seems likely that the first character of the key is t, since this would decrypt the 5th and 8th words to was and the. Let's see how that changes things:
> Peit Wokm! Mhfa fepatgb ets bvvrvxmea ebte bae twkd qiqi
ta.i ta.i ta.i ta.ita. ita .ita.ita. ita. ita .ita .ita
< We.l Do.e! Th.s me.sag. was .ncr.pte. wit. the .ord .axi
Actually, you can stop here. It's already obvious what the message says.
