In this video, three classical cipher techniques are explained, Ceaser Cipher, Mono-alphabetic Cipher, and Poly-alphabetic (vigenere) Cipher.
I first talked about Caesar Cipher where each alphabet (or letter) {A, …, Z} is shifted by 3rd position alphabet (or letter); this cipher is a particular case of Shift Cipher where we shift each alphabet {A, …, Z} by k position. Here k is the key and it can take value 0 to 25 as there are 26 total possible alphabets. So, Shift cipher has 25 possibilities (excluding k=0 which will keep the plain text as plain text only.)
For example, A can shift to B or C or D or …..or Z. If A shift to D, then k=3; and so the other letter follows the same shift. B will shift to F, C will shift to F, and so on.
We can use the following Encryption process:
Cipher Text { \congruent to} Plain Text+k(mod 26)
And for Decryption process:
Plain Text { \congruent to} Cipher Text-k (mod 26)
Breaking of Shift Cipher: This can be break easily as, even if the key is unknown, there are only 25 non-trivial possibilities for k to get a meaningful text.
The second Cipher, that I discussed is the Monoalphabetic Cipher: In this cipher, each alphabet is replaced by another alphabet randomly but with a fixed choice only.
• For the plain-text A we can use the ciphertext letter A, B, C, …, Y or Z. (26 possible letters)
• For the plain text B we can use the ciphertext letter A, B, C, …, Y or Z; but not the letter we used for A. (25 possible letters)
• For the plain text C we can use the ciphertext letter A, B, C, …, Y or Z; but not the letter we used for A & B. (24 possible letters)
• …
• …
• For the plain text Z, we now left with only one letter choice. (1 possible letter)
The possible keys are 26!.
Breaking of Shift Cipher: This can be break using frequency analysis. For this an example is discussed in another video, check playlist of classical Cryptography: [ Ссылка ]
The third Cipher, that I discussed is Polyalphabetic Cipher. In this Cipher, each alphabet (or letter) is replaced by more than one alphabet in the cipher text. Vigenere Cipher is one such approach. In this, a sequence of n-letters with numerical equivalents will serve as the keyword. The plain text message is expressed as successive blocks. An example is discussed in the video where “Ready’ is used as key with numerical Equivalents as “17 04 00 03 24 ”.
Breaking of Vigenere Cipher: This can be break using frequency analysis once key length is known which is a weakness of this cipher. For example, if the key was ‘READY’, it has length 5, and so after 5 length same distance or position will repeat, so each the cipher text block of length 5 can be deciphered using Shift Cipher & Frequency Analysis. An example of this is covered in another video. Check playlist of classical Cryptography: [ Ссылка ]
A variant of Vigenere Cipher: Use running key, a random letter, called Seed Letter, is used as a prefix to plain text, key length is the same as of plain text. The key has the same length as the plain text we truncate the last letter in plain text. This is considered safer than the previous version of Vigenere’s Cipher.
