[![enter image description here][1]][1] I learn from a minithesis to understand RC5. As we can see at the formula, the round 0 must be using A=A+S[0], because the loop is starting at r(round) 1. I tried addition and OR(bitwise) to get the answer DA679BFB but keep missing, Here is my result of calculating. A = A + S[0] = 4B726970 + BF0A8B1D = 10A7CF48D A = A OR S[0] = 4B726970 OR BF0A8B1D = FF7AEB7D The S Data is the result of 3rd phase key expanding on RC5. I really need help because I have been stuck here for 2 days and thank you in advance. [1]: https://i.sstatic.net/nbuYi.png