Cryptanalysis for Additive Cipher

In this class, We discuss Cryptanalysis for Additive Cipher.

The reader should have prior knowledge of crypt analysis attacks. Click Here.

Brute force method:

We need to try all the possible keys in the brute force method.

The total possible keys are from 0 to 25.

The key domain is very small, so the brute force method is possible in additive cipher.

Example:

Identified cipher text: uvaclyfzljbyl

Take Key = 1

The first character is “u.”

The value of “u” is 20.

(20 – 1) mod 26 = 19 = t.

Like the above, convert all the characters.

The plain text for key = 1 is “tuzbkxeykiaxk”

Similarly, do this for all the keys.

At key = 7, we got the plain text P: not very secure

The key value is 7.

Statistical Attack:

Identified Cipher text C = “xlilsywimwrsajsvwepijsvjsyvqmppmsrsppevwmxmwasvxlqsvilyvvcfijsvixliwippivvigimziwqsvisjjivw”

The above cipher text i repeated 14 times.

The highest frequency character is i.

We assume i as e

To convert i to e, we need key value 4.

i = 9, so (9 – 4) mod 26 = 5 = e.

By using the key value 4 we get the below plain text.

The house is now for sale for four million dollars. It is worth more hurry before the seller receives more offers.