Frequency analysis
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Frequency analysis is the way to crack simple cryptosystems like the caesar cipher. It is based on the fact that certain letters occur in normal English sentences more often than others. The most common letters in normal English sentences are E, T, A, O and I; and the least common are Z, Q, X, J, and K. Therefore, if the letter P occurs most often in the ciphertext you are looking at, it is most likely that it is one of the first group of letters. Once a frequency table is built from the ciphertext, a cryptanalyst can begin guessing which letters match with which.