text: Al-Kindi is credited with developing a method whereby variations in the frequency of the occurrence of letters could be analyzed and exploited to break ciphers (i.e. cryptanalysis by frequency analysis). His book on this topic is Risāla fī Istikhrāj al-Kutub al-Muammāh  (رسالة في استخراج الكتب المعماة; literally: On Extracting Obscured Correspondence, more contemporarily: On Decrypting Encrypted Correspondence'). In his treatise on cryptanalysis, he wrote:
  Al-Kindi was influenced by the work of al-Khalil (717–786), who wrote the Book of Cryptographic Messages, which contains the first use of permutations and combinations to list all possible Arabic words with and without vowels.