Problem - Kevin Tafuro

little-endian machine and word-sized storage, each word will need to be byteswapped
before the value can be treated as a binary string.
Byte swapping can be done with the htonl( ) macro, which can be imported by
including arpa/inet.h on Unix or winsock.h on Windows.
7.10 Performing Raw Encryption with an RSA
Public Key
Problem
You want to encrypt a small message using an RSA public key so that only an entity
with the corresponding private key can decrypt the message.
Solution
Your cryptographic library should have a straightforward API to the RSA encryption
algorithm: you should be able to give it the public key, the data to encrypt, a buffer
for the results, an indication of the data’s length, and a specification as to what kind
of padding to use (EME-OAEP padding is recommended).
When using OpenSSL, this can be done with the RSA_public_encrypt( ) function,
defined in openssl/rsa.h.
If, for some reason, you need to implement RSA on your own (which we strongly
recommend against), refer to the Public Key Cryptography Standard (PKCS) #1,
Version 2.1 (the latest version).
Discussion
Be sure to read the generic considerations for public key cryptography
in Recipes 7.1 and 7.2.
Conceptually, RSA encryption is very simple. A message is translated into an integer
and encrypted with integer math. Given a message m written as an integer, if you
want to encrypt to a public key, you take the modulus n and the exponent e from
that public key. Then compute c = m e mod n, where c is the ciphertext, written as an
integer. Given the ciphertext, you must have the private key to recover m. The private
key consists of a single integer d, which can undo the encipherment with the
operation m = c d mod n.
This scheme is believed to be as “hard” as factoring a very large number. That’s
because n is the product of two secret primes, p and q. Given p and q, it is easy to
332 | Chapter 7: Public Key Cryptography
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