Problem - Kevin Tafuro

If you are using a predefined padding method, you don’t have to worry about performing
any padding yourself. However, you do need to worry about message
length. If you try to encrypt a message that is too long, RSA_public_encrypt( ) will
return 0. Again, you should be expecting to encrypt messages of no more than 32
bytes, so this should not be a problem.
See Also
• PKCS #1 page: http://www.rsasecurity.com/rsalabs/pkcs/pkcs-1/
• Recipes 7.1, 7.2, 7.8, 10.1
7.11 Performing Raw Decryption Using an RSA
Private Key
Problem
You have a session key encrypted with an RSA public key (probably using a standard
padding algorithm), and you need to decrypt the value with the corresponding
RSA private key.
Solution
Your cryptographic library should have a straightforward API-to-RSA decryption
algorithm: you should be able to give it the public key, the data to decrypt, a buffer
for the results, and a specification as to what kind of padding was used for encryption
(EME-OAEP padding is recommended; see Recipe 7.10). The size of the input
message will always be equal to the bit length of RSA you’re using. The API function
should return the length of the result, and this length will usually be significantly
smaller than the input.
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
While RSA is believed to be secure if used properly, it is very easy to
use improperly. Be sure to read the Recipe on RSA encryption and the
general-purpose considerations for public key encryption in Recipe 7.1
and 7.2 in addition to this one.
336 | Chapter 7: Public Key Cryptography
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