Introduction to Cryptography with Open-Source Software

60 Cryptography with Open-Source Software

Cryptanalysis

With all these ciphers, cryptanalysis is very easy, and can be done speed-

ily using a ciphertext-only attack. Since in English text letters have known

frequencies

1

: the letter E is the most common, occurring somewhere between

11% and 12.7% of the time, followed by either E or T, with somewhere between

8% and 9%, all that’s needed is to make a list of all the frequencies of the

letters in the ciphertext, and compare that list with the standard frequencies .

Decryption is then immediate.

Historical note: Frequency analys is is believed to have been first considered

in the 9th century by the Arabic ...