Hot answers tagged cipher
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 ...
Here $f_i$ is simply the number of times the character $i$ appears in the ciphertext of length $N$ and where $Z$ is the alphabet size. If you had ciphertext ADCXU ZMDYZ DXZUM and which was derived from English plaintext then $N=15$ and $f_A=1,f_B=0,f_C=1, f_D=3,\ldots, f_Z=2.$
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