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With the Caesar cipher, encrypting a text multiple times provides no additional security. This is because two encryptions of, say, shift A and shift B, will be equivalent to a single encryption with shift A + B. In mathematical terms, the set of encryption operations under each possible key forms a group under composition. [26]
The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without knowledge of the key, it should be extremely difficult, if not impossible, to decrypt the resulting ciphertext into readable plaintext.
One brief comment in the text mentions, but does not mandate, the possibility of simply using the ASCII encoded value of a character string: "Finally, the key argument is a secret encryption key, which can be a user-chosen password of up to 56 bytes (including a terminating zero byte when the key is an ASCII string)." [1]
As above, let be the length of the string to be decrypted and let be the number of rails used during encryption. We will add two variables, x {\displaystyle x} and y {\displaystyle y} , where x + 1 {\displaystyle x+1} = the number of diagonals in the decrypted Rail Fence, and y {\displaystyle y} = the number of empty spaces in the last diagonal.
The ROT13 and ROT47 are fairly easy to implement using the Unix terminal application tr; to encrypt the string "Pack My Box With Five Dozen Liquor Jugs" in ROT13: $ # Map upper case A-Z to N-ZA-M and lower case a-z to n-za-m $ tr 'A-Za-z' 'N-ZA-Mn-za-m' <<< "Pack My Box With Five Dozen Liquor Jugs" Cnpx Zl Obk Jvgu Svir Qbmra Yvdhbe Whtf
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 carry out 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".
Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad using modular addition. [1] The resulting ciphertext will be impossible to decrypt or break if the following four conditions are met: [2] [3] The key must be at least as long as the plaintext. The key must be truly random.
With this logic, a string of text can be encrypted by applying the bitwise XOR operator to every character using a given key. To decrypt the output, merely reapplying the XOR function with the key will remove the cipher.