2 Use in Cryptanalysis key concept-monoalphabetic substitution ciphers do not change relative letter frequencies discovered by Arabian scientists in gtn century calculate letter frequencies for ciphertext compare counts/plots against known values if Caesar cipher look for common peaks/troughs peaks at: A-E-I triple, No pair, RST triple troughs at: JK, X-Z for monoalphabetic must identify each letter tables of common double/triple letters help 復大辱软件学院 LiJTLiJT 9 Use in Cryptanalysis • key concept - monoalphabetic substitution ciphers do not change relative letter frequencies • discovered by Arabian scientists in 9th century • calculate letter frequencies for ciphertext • compare counts/plots against known values • if Caesar cipher look for common peaks/troughs – peaks at: A-E-I triple, NO pair, RST triple – troughs at: JK, X-Z • for monoalphabetic must identify each letter – tables of common double/triple letters help