Mastering Cryptanalysis of the Additive Cipher 🔐 | Lesson 7

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.


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