Al-Farahidi's "Book of Cryptographic Messages", was the first book on cryptography and cryptanalysis written by a linguist.[37][38] The lost work contains many "firsts", including the use of permutations and combinations to list all possible Arabic words with and without vowels.[39] Later Arab cryptographers explicitly resorted to al-Farahidi's phonological analysis for calculating letter frequency in their own works.[40] His work on cryptography influenced Al-Kindi (c. 801–873), who discovered the method of cryptanalysis by frequency analysis.[39]