Encrypt-Me

Abbr:

PT - Plain Text
CT - Cipher Text
Ek - Encryption Function
Dk - Decryption Function
k - Key

Encryption and Decryption

CT = Ek(PT)
PT = Dk(CT)

1. Mono Alphabetic Cipher -

A monoalphabetic cipher is any cipher in which the letters of the plain text are mapped to cipher text letters based on a single alphabetic key. One to One Mapping

• Additional Cipher

src/additional-cipher.js

Functions:

encrypt(plain,key):cipher
CT = (PT + k) mod 26
decrypt(cipher,key):plain
PT = (CT - k) mod 26

Requirements:

Key must have additional inverse

Key Domain : [0-25] ie 26

k - 26 keys only
including zero as key in that case we will have
same plain and cipher text

• Multiplication Cipher

src/multiplication-cipher.js

Functions:

encrypt(plain,key):cipher
CT = (PT x k) mod 26
decrypt(cipher,key):plain
PT = (CT x inv(k)) mod 26

Requirements:

k x inv(k) mod 26 = 1
Key inverse must exists

Key Domain : 12

k - 12 Keys only with Key inverse
including one as key in that case we will have
same plain and cipher text

3. Affine Cipher

src/multiplication-cipher.js

Functions:

encrypt(plain, key: {kadd, kmul}):cipher
CT = ((PT x kmul) + kadd) mod 26

decrypt(cipher, key: {kadd, kmul}):plain
PT = ((CT - kadd) x inv(kmul)) mod 26

Requirements:

kadd - Additional Cipher Key
kmul - Multiplication Cipher Key
kmul x inv(kmul) mod 26 = 1
Key inverse must exists

Key Domain : 26 x 12 = 312

kadd - 26 values
kmul - 12 values

4. Monoalphabetic Cipher

src/monoalphabetic-cipher.js

Functions:

encrypt(plain, key):cipher
CT = key[PT]

decrypt(cipher, key):plain
PT = inv(key)[CT]

Requirements:

key - Key of length 26 containing all the alphabetic only once
ie It is mapping of between plain characters and cipher characters

Key Domain : 26!

Quite large key domain, so brute force attack is not feasible.

5. Autokey Cipher

src/autokey-cipher.js

Functions:

encrypt(plain, primer): cipher
CT(i) = (PT(i) + key(i)) mod 26

decrypt(cipher, primer): plain
PT(i) = (CT(i) - key(i)) mod 26

Requirements:

key - Key is any alphabetic character.
Encryption - New Key is generated by primer at its initial and merging the plain text stream. 
Decryption - New Key is generated by primer at its initial and from tabula rectat stream continues. 

Key Domain : Depends upon primer size n ^ size of primer

Since only initial primer is to guessed.

5. Playfair Cipher

src/playfair-cipher.js

Functions:

encrypt(plain, primer): cipher
CT(i) = (PT(i) + key(i)) mod 26

decrypt(cipher, primer): plain
PT(i) = (CT(i) - key(i)) mod 26

Requirements:

key - Key is any alphabetic character.
Encryption - New Key is generated by primer at its initial and merging the plain text stream. 
Decryption - New Key is generated by primer at its initial and from tabula rectat stream continues. 

Key Domain : Depends upon primer size n ^ size of primer

Since only initial primer is to guessed.