Suppose bob uses the rsa cryptosystem with a very large modulus n for which the factorization cannot be found in a reasonable amount of time. suppose alice sends a message to bob by representing each alphabetic character as an integer between 0 and 25 (a s 0, . . , z s 25), and then encrypting each number separately using rsa with large e and large n. is this method secure? if not, describe the most efficient attack against this encryption method.

The answer is this method is not secure but a frequency attack can be applied. Knowing the public key in this case is, nonetheless, adequate for an attacker to calculate a table, on behalf of one-to-one communication between the plaintext and the cipher text. Computed table allows him/her to decrypt any ciphertext, encoded using RSA Cryptosystem in such a means.

δabc ≅ δdcb by sas because

db ≅ ac . . given (side)

bc ≅ bc . . reflexive property (side)

∠dbc ≅ ∠ acb . . alternate interior angles where transversal bc crosses parallel lines bd and ac (included angle)