Affine Cipher
Introduction
The Affine cipher is a type of mono alphabetic substitution cipher, wherein each letter in an
alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function,
and converted back to a letter. The formula used means that each letter encrypts to one other
letter, and back again, meaning the cipher is essentially a standard substitution cipher with a
rule governing which letter goes to which.
The ‘key’ for the Affine cipher consists of 2 numbers, say K1 and
K2. K1 must be chosen such that K1 and 26 are coprime.
Encryption: C = (P × K1 + K2)mod26
Decryption: P = ((C - K2) × K1-1 )mod26
where, P → Plaintext
C → Ciphertext
K → Shift/Key
K1-1 is the multiplicative inverse of
K1
Security
The security of the Affine cipher is limited due to its small key space, susceptibility to frequency analysis, vulnerability to known plaintext attacks, ease of brute force attacks, and mathematical weaknesses in its encryption scheme. As a result, it is not considered secure for modern cryptographic needs and is primarily used for educational purposes or in situations where strong security is not a primary concern.
Advantages
- The Affine cipher is relatively easy to understand and implement, making it accessible for educational purposes and basic encryption needs.
- It can encrypt and decrypt messages quickly since it involves simple mathematical operations like multiplication and addition.
- The cipher allows for customization through the choice of keys K1 and K2, providing a degree of flexibility in the encryption process.
Disadvantages
- The key space of the Affine cipher is limited, making it vulnerable to brute force attacks and frequency analysis, especially for longer messages.
- If an attacker has access to known plaintext-ciphertext pairs, they can derive the key and decrypt other messages encrypted with the same key.
- The Affine cipher lacks mechanisms for authentication, making it susceptible to tampering or interception attacks in practical scenarios.