Use the Vigenere cipher with keyword GOLF to decrypt the ciphertext JFTAKTZWYVZBVIEYLCCIUIRM

Name: Problem 9, Chapter 4: Applied Algebra: Codes, Ciphers and Discrete Algorithms
Uploaded: 2020-10-07T03:27:19-08:00
Duration: 10 min 35 s
Channel: Victoria Dollar
Description: Use the Vigenere cipher with keyword GOLF to decrypt the ciphertext JFTAKTZWYVZBVIEYLCCIUIRM

Use the Vigenere cipher with keyword GOLF to decrypt the...

Key Concepts

Vigenere Cipher

The Vigenere cipher is a method of encrypting alphabetic text by using a simple form of polyalphabetic substitution. It employs a series of different Caesar ciphers based on the letters of a keyword, thereby making the ciphertext less susceptible to frequency analysis compared to a simple shift cipher.

Keyword Usage

A keyword is central to the Vigenere cipher; it determines the sequence of shifts applied to the plaintext. The keyword is repeated or truncated as necessary to match the length of the text, and each letter of the keyword represents a specific shift value in the encryption or decryption process.

Decryption Process

Decryption in the Vigenere cipher involves reversing the encryption process. Each letter of the ciphertext is shifted backward by the value associated with the corresponding letter from the keyword. This process requires modular arithmetic to ensure the shifts wrap around the alphabet correctly, effectively recovering the original plaintext.

Transcript

00:01 Okay, so for this particular problem, we have, of course, i have my letters and what they're associated with.

00:07 And then, of course, we have the original cryptogram.

00:10 And then i have, i've already assigned the letters based on what they actually are.

00:15 And we're going to use the inverse.

00:17 So we're going to use p minus k, mod 26.

00:21 So p is whatever the letter value is that's been assigned in the cryptogram.

00:25 And then k is the key and what letter it stands for.

00:29 And then mod 26 is, you know, every 26 letters we have to subtract from 26 or add 26 or what have you to get the appropriate letter.

00:38 So essentially the idea behind it is the values are going to equal zero to 25.

00:43 So we don't want, if we have 26, we would subtract 26 from it and come back to zero.

00:48 So that's kind of the idea behind it.

00:50 So now what we need to do is we need to figure out our k value.

00:54 So usually the rule of thumb that i follow when i do these is i look through and see which letter do i use.

00:59 Usually see the most common.

01:01 And in this case, it's v.

01:04 So i have, i think, a total of one, two, three, four, five, six, seven vs in here.

01:11 So out of all the letters, that's quite a few, the most common letter that's used in the english language is the letter e.

01:18 So then at that point that i, if e doesn't happen to fit, then i'll try other letters a, r s, usually s, and t, and go from there until i get the right one.

01:28 So what i want to do is i want to do is i want to to look at the letter v.

01:32 So v equals 21 in this one.

01:36 E equals four.

01:38 So if we take 21 and subtract four, that gives us 17.

01:42 So k is 17.

01:45 So what i want to do is i want to go through and i want to just subtract 17 from each of these.

01:52 So 3 minus 17 is going to be negative 14, 21 minus 17, is 4.

02:07 And then 4 minus 17 is going to be negative.

02:22 So 26, i'll see, so 4 minus 17 would be negative 13.

02:28 So next, so 2 minus 17 would be negative 15.

02:35 And then 5 minus 17 would be negative 12.

02:45 12 minus 17 is negative 5.