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Difference between Substitution Cipher Technique and Transposition Cipher Technique
Both Substitution Cipher and Transposition Cipher are traditional techniques used in cryptography to encrypt a plaintext into ciphertext in order to prevent it from getting tampered. Read through this article to find out more about Substitution cipher and Transposition cipher and how they are different from each other.
What is Substitution Cipher Technique?
In the Substitution Cipher technique, each character is substituted with other character/number or other symbol. This technique changes the identity of a character but not its position in the string.
A substitution cipher encrypts a text sequence by replacing letters or units of text with other letters or units of text. Substitution ciphers are an early form of cryptography that predates the development of computers and are now largely outdated.
Like B or C, a letter is converted into another letter in a substitution cipher, encrypting the sequence for a human reader. The problem is that basic substitution ciphers do not encrypt adequately in terms of computer assessment.
Substitution ciphers became relatively easy for computers to crack with the emergence of the personal computer. However, some of the substitution cipher's concepts have persisted, for example, some versions of current encryption may encrypt data using an extensive text set and a very clever substitution.
What is Transposition Cipher Technique?
In the Transposition Cipher Technique, each character's position is shifted to a different position. A transposition cipher is an encryption method in which plaintext units (usually letters or groups of characters) are shifted in a predictable way resulting in the ciphertext being a permutation of the plaintext. That is, the units' order is altered (the plaintext is reordered).
To encrypt, a bijective function is applied to the locations of the characters, and to decrypt, an inverse function is used.
The Rail Fence encryption is a type of transposition cipher named after how it is encoded. The plaintext is written on successive "rails" of an imaginary fence in the rail fence, downwards and diagonally, then moved up when we reach the bottom. The message is then read aloud in a series of rows.
The Rail Fence Cipher follows a scytale-like pattern, an old Greek mechanical device for constructing a transposition cipher. A cylinder and a ribbon wrapped around the cylinder comprised the system. The coiled ribbon was used to write the encrypted message. When the ribbon was uncoiled from the cylinder, the letters of the original message were rearranged. The message was decoded when the ribbon recoiled on a cylinder with the same diameter as the encrypting cylinder.
Difference between Substitution Cipher and Transposition Cipher
The following table highlights the important differences between Substitution Cipher and Transposition Cipher.
| Key | Substitution Cipher Technique | Transposition Cipher Technique |
|---|---|---|
| Algorithm | Each character is replaced with a different character, integer, or symbol. | Each character has been repositioned from its original place. |
| Forms | It comes in two forms: Mono-Alphabetic Substitution Cipher and Poly-Alphabetic Substitution Cipher. | It has two forms: Key-less transposition cipher and keyed transposition cipher. |
| Change | Character identity is changed but position remains same. | Character position is changed but identity remains same. |
| Detection | A letter less frequently used can be easily traced. | A letter near to original position can be easily traced. |
| Example | Caesar Cipher is an example of Substitution Cipher. | Reil Fence Cipher is an example of Transposition Cipher. |
Conclusion
Substitution cipher and Transposition cipher are traditional methods in cryptography, which are now outdated techniques. As their names imply, substitution ciphers substitute each character of a string with another character, number, or symbol; whereas transposition ciphers transpose each character of a string to a different position.
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