Question

. [9 marks] Assume that Alice has chosen a large RSA modulus n such that factorization is impossible with reasonable time and resources. She also then chooses a large random public exponent e < n for which the RSA problem is also not practical. However Bob decides to send a message to Alice by encrypting each alphabet character (represented by an integer between 0 and 25) separately using Alice’s public key < n, e >.

(a) Describe an efficient attack against this method.

(b) Suggest a countermeasure to this attack.

Answer 1

Solution is:

a)

Examination of encryption technique security:

The strategy  encryption by representing each alphabetic character  as a number somewhere in the range of 0and 25 and afterward apply RSA calculation for every whole number isn't an effective secure encryption technique.

Clarification:

1. consider that {A,B,C,....Z} is a lot of alphabetic characters.
2. Each alaphabetic charactor position is spoken by a number and it frames a lot of message block esteem SM={0,1,2,....25}.
3. The given technique shapes comparing figure text block esteem SC={0^e modN,1^e modN,.......25^e modN}.
4. It is posible for everybody to register the code text with information of BOB's public key.
5. If the public key is produced at that point there are a ton of posibilities to decript the code message and get a plain book .
6. The public key is effectively determined by everybody and security is lost .

Thus the encryption techinque is not secure.

b)

Most productive assault against the given the encription stratagy:

• First catch the messege by encoding with the capacity M^e modeN for all the concievable estimation of message M. It is the most effective assault against the plain in the given stratagy.
• Next make a look into table with figure text as a record and the comparing plain content as an incentive for suitable area in the table.