A Substitution cipher is an encoding technique where the alphabets/units of the plain-text are replaced with alphabets/units of the cipher text according to a well defined system. The decoding of the text by the receiver is done by performing an inverse substitution.
- Simple Substitution Cipher – where the cipher substitutes single units/alphabets.
- Polygraphic Substitution Cipher – where the cipher performs the substitution on larger groups of units/alphabets instead of single units. (Eg: Playfair Cipher, Hill Cipher)
- Mono-alphabetic Cipher – uses a fixed substitution alphabet over the entire message. (Eg: Caesar Cipher)
- Poly-alphabetic Cipher – uses multiple substitution cipher alphabets at different positions of the message. (Eg: Vigenere Cipher)
Simple Substitution Cipher
The simple substitution cipher is the most basic example of substitution ciphers, or in fact, one of the most basic amongst any cryptography algorithm.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 2 3 4 5 6 7 8 9 0
Q W E R T Y U I O P A S D F G H J K L Z X C V B N M 4 5 6 7 8 9 0 1 2 3
We encrypt the message by substituting each alphabet of the message with its equivalent alphabet of the cipher-text. For example,
Message = MY REAL NAME IS BOB 420
Encrypted message = DN KTQS YQDT OL WGW 753
An early historic example of this technique is the Caesar Cipher (Wikipedia) where each alphabet of the plain-text is right shifted by 3 alphabets to get the cipher text.
- The number of possible keys is 26! (for the english alphabet) which is very large.
- Despite the large number of possibility of keys, this system is very weak and can be easily broken using frequency analysis provided the message length is not too small.
- The cipher text alphabet is sometimes different from the plain text alphabet. (For example, see Wikipedia – Pigpen Cipher)
Head back to the << Study on Cryptography Series >> page for more.
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Mohonish ‘Xeno’ Chakraborty is a Computer Science and Engineering student from Kolkata, India. Apart from spending the entire day in front of his computer or tinkering with gadgets, he also likes to play a lot of guitar with his band, read a lot of classic fiction, and listen to a lot of heavy metal.