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).

flashcards

QuestionAnswer
DecryptionDecryption is the process of taking ciphertext and using a reverse cipher, usually with an encryption key, to produce plaintext.
Why do we need a key for decryption?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.
If the encryption function is E(x) = (x + 3) \mod 26, what is the decryption function?The decryption function is D(y) = (y - 3) \mod 26.
In the example with ciphertext ‘D’ (where A=0, B=1, C=2, D=3) and the decryption function D(y) = (y - 3) \mod 26, what is the decrypted value?D(3) = (3 - 3) \mod 26 = 0
What does the ‘3’ represent in the encryption function E(x) = (x + 3) \mod 26?The ‘3’ represents the key. It could change and be any number, and only someone with the key would be able to decrypt the message (in theory, though this cipher is incredibly insecure).
What is the relationship between the decryption function and the encryption function?The decryption function is the reverse of the encryption function.
What type of cipher is E(x) = (x + 3) \mod 26 an example of?It is an example of a Caesar cipher, which effectively shifts the letters of the alphabet by 3 places (A becomes D, B becomes E, etc.).