Decryption

Decryption is the opposite of encryption - it’s where we take the ciphertext and use a reverse cipher - usually using an encryption key - to produce the plaintext.

The encryption key

We need a key. That’s because, to do the reverse of the encryption function, we need to know the ‘secret’ part of the function, which will allow only the person with the key to decode it.

Decryption functions

The decryption function is the reverse of the encryption function. For example, if the encryption function is:

E(x) = (x + 3) mod 26

(This effectively shifts the letters of the alphabet by 3 places, so A becomes D, B becomes E, etc. It’s known as a Caesar cipher - you’ll see it soon!)

Then the decryption function would be:

D(y) = (y - 3) mod 26

This means that if we have the ciphertext ‘D’, which corresponds to 3 (since A=0, B=1, C=2, D=3), we can apply the decryption function:

D(3) = (3 - 3) mod 26 = 0

The important thing to see is the 3. In an encryption scheme, this would be the key - it could change and be any number, and only someone with the key would be able to decrypt the message (in theory, this cipher is incredibly insecure).