Editing RC5 (section)

===Encryption===

Encryption involved several rounds of a simple function, with 12 or 20 rounds seemingly recommended, depending on security needs and time considerations. Beyond the variables used above, the following variables are used in this algorithm:

* A, B - The two words composing the block of [[plaintext]] to be encrypted.

<syntaxhighlight lang="python">
A = A + S[0]
B = B + S[1]
for i = 1 to r do:
    A = ((A ^ B) <<< B) + S[2 * i]
    B = ((B ^ A) <<< A) + S[2 * i + 1]

# The ciphertext block consists of the two-word wide block composed of A and B, in that order.
return A, B
</syntaxhighlight>

The example C code given by Rivest is this.

<syntaxhighlight lang="c">
void RC5_ENCRYPT(WORD *pt, WORD *ct)
{
   WORD i, A = pt[0] + S[0], B = pt[1] + S[1];
   
   for (i = 1; i <= r; i++)
   {
      A = ROTL(A ^ B, B) + S[2*i];
      B = ROTL(B ^ A, A) + S[2*i + 1];
   }
   ct[0] = A; ct[1] = B;
}
</syntaxhighlight>