If you were to encrypt a file using the cipher of Exercise P9.2, the letters will mixed up, and will appear as that you cannot decrypt it without the keyword. Also, it would seem illogical to try and guess the keyword, but someone who is trained in decryption would have no trouble breaking the cipher. The average letter frequencies of English letters are common knowledge. The most common letter E, occurs about 13% of the time.
- See Exhibit B.
- Write a program that reads as an input file and shows the letter frequencies in the file. This tool is invaluable to a code breaker. If the most frequent letters are H and K there is a high probability that they are the encryptions of E and T.