My object in writing this book is to explain what cryptography is, and to recall what it has been from antiquity to the present day; in short, to relate my experiences as a decipherer. The first part of the volume contains a description of the principal systems of cryptography, together with a note on the rule played by cryptography in history. In the second part I relate how I succeeded in deciphering a dozen cryptograms of various kinds. In some chapters of this section I give the texts just as they came into ...
My object in writing this book is to explain what cryptography is, and to recall what it has been from antiquity to the present day; in short, to relate my experiences as a decipherer. The first part of the volume contains a description of the principal systems of cryptography, together with a note on the rule played by cryptography in history. In the second part I relate how I succeeded in deciphering a dozen cryptograms of various kinds. In some chapters of this section I give the texts just as they came into my hands; but in the majority of cases, though preserving the system of cryptography actually employed, I have, on grounds of expediency, substituted an approximate reading for the actual text, and have modified the plan, and even radical features of the narrative, in such a way as to render abortive any attempt at identification and localization. In the third part I give some advice in a general way on lines which have proved profitable to me, and, further, a certain number of tables and formulae; but while I recognize these to be very useful, too much reliance should not be placed on them, under penalty of striking the wrong track, as I shall have occasion to repeat farther on. In point of fact, as I have found by experience, in cryptography the exceptions are infinitely more frequent than the rule.
Until modern times cryptography referred almost exclusively to encryption, which is the process of converting ordinary information (called plaintext) into unintelligible gibberish (called ciphertext). Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A cipher (or cypher) is a pair of algorithms that create the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and in each instance by a "key". This is a secret parameter (ideally known only to the communicants) for a specific message exchange context. A "cryptosystem" is the ordered list of elements of finite possible plaintexts, finite possible cyphertexts, finite possible keys, and the encryption and decryption algorithms which correspond to each key. Keys are important, as ciphers without variable keys can be trivially broken with only the knowledge of the cipher used and are therefore useless (or even counter-productive) for most purposes. Historically, ciphers were often used directly for encryption or decryption without additional procedures such as authentication or integrity checks.
In colloquial use, the term "code" is often used to mean any method of encryption or concealment of meaning. However, in cryptography, code has a more specific meaning. It means the replacement of a unit of plaintext (i.e., a meaningful word or phrase) with a code word (for example, wallaby replaces attack at dawn). Codes are no longer used in serious cryptography—except incidentally for such things as unit designations (e.g., Bronco Flight or Operation Overlord)—since properly chosen ciphers are both more practical and more secure than even the best codes and also are better adapted to computers.
Cryptanalysis is the term used for the study of methods for obtaining the meaning of encrypted information without access to the key normally required to do so; i.e., it is the study of how to crack encryption algorithms or their implementations.
Some use the terms cryptography and cryptology interchangeably in English, while others (including US military practice generally) use cryptography to refer specifically to the use and practice of cryptographic techniques and cryptology to refer to the combined study of cryptography and cryptanalysis. English is more flexible than several other languages in which cryptology (done by cryptologists) is always used in the second sense above.