Lesson 1
Introduction:
Cryptography involves taking the message you want to send (the plaintext) and turning it into what looks more like gibberish (the ciphertext) to keep the message secret and hidden. In this class we will be focusing on classical cryptography - i.e. ciphers that can be done with pen/paper (or via e-mail) and that do not require hours of computer power to solve. The ciphers explored will be symmetric - i.e. the same "key" is used for both encrypting (making the ciphertext) and decryptic (turning the ciphertext back into plaintext). Throughout this class we'll mostly be focusing on using the codes as opposed to their use in history etc. We'll be starting with substitution ciphers where you substitute one letter for another letter (or a number etc). We'll later look at transposition ciphers where the letters remain the same but are just jumbled up.
Pigpen cipher:
Also known as the Masonic Cipher and is based on symbols rather than letters. It's very easy to do with pen & paper and can be done easily enough using Paint. Each letter of the alphabet is made to correspond with a symbol and the symbols are effectively formed by placing the letters of the alphabet in a 3×3 grid (and using dots to distinguish between letters when you run out of space). A typical layout is given below along with an example of how to understand the grid.
Cryptography involves taking the message you want to send (the plaintext) and turning it into what looks more like gibberish (the ciphertext) to keep the message secret and hidden. In this class we will be focusing on classical cryptography - i.e. ciphers that can be done with pen/paper (or via e-mail) and that do not require hours of computer power to solve. The ciphers explored will be symmetric - i.e. the same "key" is used for both encrypting (making the ciphertext) and decryptic (turning the ciphertext back into plaintext). Throughout this class we'll mostly be focusing on using the codes as opposed to their use in history etc. We'll be starting with substitution ciphers where you substitute one letter for another letter (or a number etc). We'll later look at transposition ciphers where the letters remain the same but are just jumbled up.
Pigpen cipher:
Also known as the Masonic Cipher and is based on symbols rather than letters. It's very easy to do with pen & paper and can be done easily enough using Paint. Each letter of the alphabet is made to correspond with a symbol and the symbols are effectively formed by placing the letters of the alphabet in a 3×3 grid (and using dots to distinguish between letters when you run out of space). A typical layout is given below along with an example of how to understand the grid.
Book cipher:
For this cipher, both the sender and recipient of the message are required to have exactly the same piece of text. Each letter (or word) of the plaintext is replaced by a code indicating the position of the letter/word in the chosen text. This text used can be a book, a webpage or simply a sentence (for the purposes of this class, paragraphs of text or webpages will be used).
For example if the following is the text you are using:
For this cipher, both the sender and recipient of the message are required to have exactly the same piece of text. Each letter (or word) of the plaintext is replaced by a code indicating the position of the letter/word in the chosen text. This text used can be a book, a webpage or simply a sentence (for the purposes of this class, paragraphs of text or webpages will be used).
For example if the following is the text you are using:
Whenever I dress cool,
My parents put up a fight
And if I’m a hotshot,
Mom will cut hair at night
In the morning I’m sure of my identity
I scream Mom and Dad
Why can't I be who I wanna be?
I just wanna be myself,
And I want you to love me for who I am
I just wanna be myself,
And I want you to know, I am my Hair
I’ve had enough
This is my prayer
That I’ll die living just as free as my hair
(Lyrics: Lady Gaga, Hair)
My parents put up a fight
And if I’m a hotshot,
Mom will cut hair at night
In the morning I’m sure of my identity
I scream Mom and Dad
Why can't I be who I wanna be?
I just wanna be myself,
And I want you to love me for who I am
I just wanna be myself,
And I want you to know, I am my Hair
I’ve had enough
This is my prayer
That I’ll die living just as free as my hair
(Lyrics: Lady Gaga, Hair)
And your plaintext is "I LIKE CAKE" - you need to find each of these letters in the chosen text and then choose how to identify their location. In this case the letter will be identified by LINE#-WORD#-LETTER#. Taking the "I" from the first line we would reference it as: L1W2L1 or as 1-2-1. I've encoded the entire plaintext below if you want to check you understand it; letters are separated by spaces :)
CIPHERTEXT: 1-2-1 10-5-5 3-2-1 11-6-1 5-8-3 6-2-2 13-4-3 11-6-1 1-1-3
Atbash cipher:
Traditionally used with the Hebrew alphabet this cipher involves reversing the alphabet so that a plaintext A becomes a ciphertext Z, and a plaintext Z becomes a ciphertext A. A chart showing the plaintext –> ciphertext conversion for the full alphabet is given below and a larger version can be found here!
For example the plaintext "CAKE" (cake is GOOD, k?) would become: XZPV
CIPHERTEXT: 1-2-1 10-5-5 3-2-1 11-6-1 5-8-3 6-2-2 13-4-3 11-6-1 1-1-3
Atbash cipher:
Traditionally used with the Hebrew alphabet this cipher involves reversing the alphabet so that a plaintext A becomes a ciphertext Z, and a plaintext Z becomes a ciphertext A. A chart showing the plaintext –> ciphertext conversion for the full alphabet is given below and a larger version can be found here!
For example the plaintext "CAKE" (cake is GOOD, k?) would become: XZPV
The Atbash cipher can be identified easily - just remember a few common words, e.g. "the" will always be encrypted to GSV and "and" will always be ZMW.
Caesar cipher:
Also known as the Shift Cipher and Caesar’s Code and named after Julius Caesar. Each letter of the plain text is replaced by a letter a fixed number of positions down the alphabet, hence the alternate name "Shift Cipher” as the entire alphabet is simply shifted along.
For example, if you shift every letter of the alphabet along 3 spaces to the left, you get the following:
Plaintext -> Ciphertext
A -> D
B -> E
C -> F
We'll look into how to break this kind of cipher in the next lesson.
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Of course simple substitution ciphers where one letter of the alphabet is replaced by another random letter (i.e. with there being pattern in the arrangement of ciphertext letters) can also be used. Any, that's it for this month, onto your ASSIGNMENT.