Editing RC4 (section)

===Pseudo-random generation algorithm (PRGA)===
[[Image:RC4.svg|right|thumbnail|320px|The lookup stage of RC4. The output byte is selected by looking up the values of {{mono|S[i]}} and {{mono|S[j]}}, adding them together modulo 256, and then using the sum as an index into {{mono|S}}; {{mono|S(S[i] + S[j])}} is used as a byte of the key stream K.]]

For as many iterations as are needed, the PRGA modifies the state and outputs a byte of the keystream. In each iteration, the PRGA: 
* increments {{mono|''i''}};
* looks up the {{mono|''i''}}th element of {{mono|S}}, {{mono|S[''i'']}}, and adds that to {{mono|''j''}};
* exchanges the values of {{mono|S[''i'']}} and {{mono|S[''j'']}}, then uses the sum {{mono|S[''i''] + S[''j''] (modulo 256)}} as an index to fetch a third element of {{mono|S}} (the keystream value {{mono|K}} below);
* then bitwise exclusive ORed ([[Exclusive or|XOR]]ed) with the next byte of the message to produce the next byte of either ciphertext or plaintext.

Each element of S is swapped with another element at least once every 256 iterations.

 i := 0
 j := 0
 '''while''' GeneratingOutput:
     i := (i + 1) mod 256
     j := (j + S[i]) mod 256
     [[Swap (computer science)|swap values]] of S[i] and S[j]
     t := (S[i] + S[j]) mod 256
     K := S[t]
     output K
 '''endwhile'''

Thus, this produces a stream of {{mono|K[0], K[1], ...}} which are [[Exclusive or|XOR]]ed with the {{mono|''plaintext''}} to obtain the {{mono|''ciphertext''}}. So {{mono|ciphertext[''l''] {{=}} plaintext[''l''] ⊕ K[''l'']}}.