A book on security1 has the following dedication: Si spy net work, big fedjaw io
ID: 3889287 • Letter: A
Question
A book on security1 has the following dedication: Si spy net work, big fedjaw iog link kyxogyDecrypt this dedication. Hints: Use a length-1 substitution cipher that preserves spaces and
punctuation, and is not case-sensitive. The plaintext is in English and the common English letter e
occurs three times. What word does a dedication usually start with? The second word is an article.
Two words end in s.
1)Make a decryption table for all the letters |s| i| s| p| y| n| . . . . . . Decript Using substitution cipher and the hints provided. A book on security1 has the following dedication: Si spy net work, big fedjaw iog link kyxogy
Decrypt this dedication. Hints: Use a length-1 substitution cipher that preserves spaces and
punctuation, and is not case-sensitive. The plaintext is in English and the common English letter e
occurs three times. What word does a dedication usually start with? The second word is an article.
Two words end in s.
1)Make a decryption table for all the letters |s| i| s| p| y| n| . . . . . . Decript Using substitution cipher and the hints provided. Decript Using substitution cipher and the hints provided.
Explanation / Answer
An important thing to remember with the Shift Cipher is that it works modulo 26 (which is the length of the standard alphabet). That is, a right shift of 1 is the same as a left shift of -1, which is just a shift of 25. Hence the key is unique modulo 26, and so a shift of 33 is actually the same as a shift of 33 mod 26 = 7.
Another interesting fact worth noting here is that composing multiple shifts (doing one shift followed by another shift) does not make the cipher any more secure. This is because a shift of a followed by a shift of b is the same as a shift of a + b (or in more concrete terms, a shift of 2 followed by a shift of 5 is identical to if we had just shifted the alphabet by 7 in the first place). If you can not see this readily, then have a go to check this result.
If it is known that a Shift Cipher has been used, but the key is unknown, then it is fairly simple to break the code by a simple brute force attack. This simply means using a trial and error approach to attack the cipher. The main weakness of the Shift cipher is the fact that there are only 26 keys, and hence ciphertext alphabets, one of which is the identity mapping that leaves the plaintext unaltered. For this reason, the Brute Force method of attack is very effective on the Shift cipher. In it's most bare form, this entails going through each key, and working out what the plaintext would be if that key had been used.
That is, given the intercepted ciphertext "RFWHZX HWFXXZX", where we do not know what key has been used, but we do know that a Shift Cipher has been implemented, we must first try a key of 1, then a key of 2, then a key of 3 and so on, until a plaintext that makes sense is returned. For this ciphertext we would get:
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