O'Reilly logo

Computer Security and Cryptography by Alan G. Konheim

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

12.3 THE RSA ENCIPHERMENT AND DECIPHERMENT OF ASCII-PLAINTEXT

RSA enciphers and deciphers integers in image. To encipher data, RSA needs to be extended to encipher n-grams of ASCII characters x = (x(0), x(1), …, x(n−1)). An Internet standard is described in [Linn, 1989]; I describe a slightly modified version, using 7 bits for each ASCII character rather than 8 bits.

The extension replaces ASCII text by a binary sequence, which is segmented into bit-vectors of length N2 − 1 where 2N2 − 1 − 1 ≤ N < 2N2. Each such bit-vector corresponds to an integer k in image to which RSA-exponentiation Ee can be applied. The resulting integer j = Ee(k) in general requires N2 bits in its base-2 representation so that there is an expansion under RSA encipherment.

12.3.1 RSA Encipherment of ASCII Plaintext

E1. Replace each character x(i) of ASCII plaintext x = (x(0), x(1), …, x(n−1)) by the 7-bit binary encoding of its ordinal value in the ASCII character code image (Table 12.1).

E2. Concatenate to form the (0,1)-plaintext z (x(0), x(1), …, x(n−1)) → z = (z0, z1, …, z7n−1).

E3. Assume integers in image require N2 bits in their ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required