## The RSA cryptosystem

At first this may seem an impossible idea. For example, consider the cipher that consists of replacing each letter by the one that follows it. In this case, a cryptogram is decrypted by replacing each of its letters with the one that comes before it in the alphabet. Thus, for this cipher, knowledge of the encryption process immediately gives access to the decryption procedure. Unlike this cipher, the public key cryptosystems have a “trapdoor”, that is, an operation that is easy to perform, but difficult to undo. An example of a trapdoor is considered in the next section, where we give a rough description of the most popular of the public key cryptosystems, the RSA.