Output: a random byte (to be XORed with plaintext)

Initialization: Let K₀, K₁, K₂, ... K₂₅₅ be the bytes of the key, repeating the key as necessary
j = 0
for i = 0 to 255
    S_i = i
for i = 0 to 255
    j = (j + S_i + K_i) mod 256
    swap S_i and S_j
i = 0
j = 0

Algorithm:
i = ( i + 1 ) mod 256
j = ( j + S_i ) mod 256
swap S_i and S_j
t = (S_i + S_j ) mod 256
output S_t

Reference: