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In a symmetric-key system, Bob knows Alice's encryption key. Once the message is encrypted, Alice can safely transmit it to Bob (assuming no one else knows the key). In order to read Alice's message, Bob must decrypt the ciphertext using which is known as the decryption cipher, :
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]
Pad C n with the extracted ciphertext in the tail end of D n (placed there in step 3 of the ECB encryption process). P n = Head (D n, M). Select the first M bits of D n to create P n. As described in step 3 of the ECB encryption process, the first M bits of D n contain P n. We queue this last (possibly partial) block for eventual output. P n− ...
Despite its problems, the one-time-pad retains some practical interest. In some hypothetical espionage situations, the one-time pad might be useful because encryption and decryption can be computed by hand with only pencil and paper. Nearly all other high quality ciphers are entirely impractical without computers.
In this decryption example, the ciphertext that will be decrypted is the ciphertext from the encryption example. The corresponding decryption function is D(y) = 21(y − b) mod 26, where a −1 is calculated to be 21, and b is 8. To begin, write the numeric equivalents to each letter in the ciphertext, as shown in the table below.
The key is generated from the message in some automated fashion, sometimes by selecting certain letters from the text or, more commonly, by adding a short primer key to the front of the message. There are two forms of autokey cipher: key-autokey and text-autokey ciphers.
The key text was the Harry Potter books, but the messages were sent via a The Lord of the Rings forum to make the key text harder to identify. In Lost: Mystery of the Island, a series of four jigsaw puzzles released in 2007, a book cipher was used on each puzzle's box to hide spoilers and reveal information about the show to the fans.
The encryption input also includes a public nonce N, the output - authentication tag T, size of the ciphertext C is the same as that of P. The decryption uses N, A, C, and T as inputs and produces either P or signals verification failure if the message has been altered. Nonce and tag have the same size as the key K (k bits). [6]