Cryptography, to most people, is concerned with keeping communications private. Indeed, the protection of sensitive communications has been the emphasis of

cryptography throughout much of its history. Encryption is the transformation of data into some unreadable form. Its purpose is to ensure privacy by keeping the information hidden from anyone for whom it is not intended, even those who can see the encrypted data. Decryption is the reverse of encryption; it is the transformation of encrypted data back into some intelligible form.

Encryption and decryption require the use of some secret information, usually referred to as a key. Depending on the encryption mechanism used, the same key might be used for both encryption and decryption, while for other mechanisms, the keys used for encryption and decryption might be different.

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** Example** Let the message be

PREPARE TO NEGOTIATE

and the encoding matrix be

_{}

We assign a number for each letter of the alphabet. For simplicity, let us associate each letter with its position in the alphabet: A is 1, B is 2, and so on. Also, we assign the number 27 (remember we have only 26 letters in the alphabet) to a space between two words. Thus the message becomes:

_{}

Since we are using a 3 by 3 matrix, we break the enumerated message above into a sequence of 3 by 1 vectors:

_{}

Note that it was necessary to add a space at the end of the message to complete the last vector. We now encode the message by multiplying each of the above vectors by the encoding matrix. This can be done by writing the above vectors as columns of a matrix and perform the matrix multiplication of that matrix with the encoding matrix as follows:

_{}

which gives the matrix

_{}

The columns of this matrix give the encoded message. The message is transmitted in the following linear form

_{}

To decode the message, the receiver writes this string as a sequence of 3 by 1 column matrices and repeats the technique using the inverse of the encoding matrix. The inverse of this encoding matrix, the decoding matrix, is:

_{}

(make sure that you compute it yourself). Thus, to decode the message, perform the matrix multiplication

_{}

and get the matrix

_{}

The columns of this matrix, written in linear form, give the original message:

_{}

For more information on cryptography, check

2. Elementary Cryptanalysis (a book on Cryptography)

3. The Cipher Exchange (a short introduction of number theory and cryptography)

5.
Index of Crypto
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