RSA functions around:

(m**e)**d congruent m mod n

(m**d)**e congruent m mod n

0 <= m <= n

I think "a congruent b mod n" means that a - b is an integer multiple of n.

The public key is e and the private key is d.  So, m is the message and m**d is the encrypted data.

Code for this is in libcrypto.