Cipher plaintext shift

Plaintext Shift or Caesar Ciphers. The simplest ciphers are plaintext shift ciphers or Caesar Ciphers (so named because they were used by Julius Caesar [23]). This type of cipher is created by uniformly shifting the letters in the alphabet over a certain number of spaces. 

Substitution or Monoalphabetic Ciphers. A variation of the plaintext shift is the substitution or monoalphabetic cipher. This method involves permuting the letters of the alphabet. In other words, the letters are more than simply shifted they are jumbled up into an arbitrary order. For example, suppose that Alice wants to send Bob the message T/ie Berlin wall will fall tonight/ First, Alice and Bob must agree on the monoalphabetic cipher they will use. They agree to use the one presented in Figure 2.5. 

While these two ciphers work, they are not reliable as they can be broken very easily. This is where mathematics gets involved. Mathematics can be used to create sophisticated ciphers that are virtually impossible to break. To begin this discussion, we will first see how a cipher, very similar to a plaintext shift or Caesar cipher, can be obtained mathematically by using some of the number theoretic concepts introduced earlier in this chapter. 

Plaintext Shift or Caesar Ciphers Revisited. Recall the plaintext shift cipher presented earlier (see Figure 2.7). This cipher motivates the use of mathematics in cryptography. 

It will not be exactly the same, because this new cipher asssigns the space between words the nund>er 26. The space between words was not taken into account in the original plaintext shift cipher. 

The coded message YMJ HTU MFX GJJS GWTPJS has been intercepted. A plaintext shift (Caesar cipher) wa.s used to generate the message. Determine the specific plaintext shift used and what the plaintext message is,... 

Consider the nine-space plaintext shift cipher depicted in Figure 2.3. 

How many plaintext shift ciphers are there Explain how you arrived at your answer. 

Decryption reverses the process. The register initially contains the initialization vector. To decrypt m bits of ciphertext, the block cipher is used to encrypt the contents of the register, and the resulting leftmost m bits are xored with the m ciphertext bits to recover m plaintext bits. The m ciphertext bits are then shifted left into the register. 

Output feedback mode is similar to CFB mode, except that instead of the leftmost m bits of the ciphertext being shifted left into the register, the leftmost m bits of the output of the block cipher are used. As in CBC mode, encryption proceeds by encrypting the contents of the register using the block cipher and xoring the leftmost m bits of the result with the current m plaintext bits. However, OFB mode introduces insecurity unless m = n. As with CFB mode. The initialization vector can be made public and must be unique for each message encrypted with the same key. 

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