Affine Cipher Encoder Decoder
Choose a and b, then encode or decode text locally. The multiplier a must be coprime with 26.
A QUICK WALKTHROUGH
How to use this tool
- Enter text and choose Encode or Decode.
- Set a to a value coprime with 26 and b from 0 to 25.
- Run the tool and review or copy the result.
Reversible keys
The multiplier a must satisfy gcd(a, 26) = 1. Valid a values are 1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, and 25. Decode uses the modular inverse of a and subtracts b.
Local and limited by design
Only ASCII A–Z and a–z are transformed in this browser. Case is preserved; spaces, punctuation, digits, accented letters, emoji, and other non-ASCII characters pass through unchanged. This classical cipher is not secure encryption.
GOOD TO KNOW
Common questions
Which a values are valid?
Any value from 0 to 25 whose greatest common divisor with 26 is 1: 1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, or 25.
What does b do?
b is the additive shift and may be any integer from 0 to 25.
Is this secure?
No. Affine cipher is a classical learning and puzzle tool, not protection for sensitive data.