some essential definitions in the field of cryptography are:
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"Few false ideas have more firmly gripped the minds of so many intelligent men than the one that, if they just tried, they could invent a cipher that no one could break." (David Kahn)
The variants of encryption systems are endless.
For deciphering there exists always the same game of trial and error (first guessing the encryption method, then the code). A help to do so is pruning. Once, after a more or less long or short period a code/cipher breaks. Monoalphabetic ciphers can be broken easily and of course are no longer used today but for games.
for further information on codes and ciphers etc. see:
TEXTBLOCK 1/1 // URL: http://world-information.org/wio/infostructure/100437611776/100438659070
the word "cipher" comes from the Hebrew word "saphar", meaning "to number". Ciphers are mere substitutions. Each letter of the alphabet gets substituted; maybe by one letter or two or more.
an example:
PLAINTEXT a b c d e f g h i j k l m n o p q r s t u v w x y z
CIPHERTEXT D E F G H I J K L M N O P Q R S T U V W X Y Z A B C