Back Door - an alternate, usually secret, method to decrypt an encrypted file or message. This method is usually only known to a few people, such as the author of the program. The presence of a back door greatly reduces the reliability of the implementation of an encryption algorithm. Cipher - a cryptographic algorithm used to encrypt and decrypt files and messages. Code - the usage of characters or words to represent words, sentences, or ideas.
Morse code is a common example, where combinations of dots and dashes represent letters and numbers.
Cryptanalysis - the art of breaking cryptosystems. The process of looking for errors or weaknesses in the implementation of an algorithm or of the algorithm itself. Cryptography - the art of creating and using cryptosystems. Cryptology - the study of both cryptography and cryptanalysis. Cryptosystem - the entire process of using cryptography.