when to do what?

calccrypto 13 Sep 2009 16:48

im trying to write the IDEA algorithm but im lost due to the wording here:

```
Those [values] that are added are replaced by their two's complement. Those that are multiplied in are replaced by their multiplicative inverse, modulo 65,537, in IDEA notation
when used to change blocks directly, but those used to calculate the cross-footed F-functions are not changed. Keys XORed in would not need to be changed, but there aren't
any such keys in IDEA.
```

however, the keys are already changed so that

```
1/K = multiplicative inverse mod 2^16+1
-K = 2's Complement
KD(1) = 1/K(49)
KD(2) = -K(50)
KD(3) = -K(51)
KD(4) = 1/K(52)
KD(5) = K(47)
KD(6) = K(48)
KD(7) = 1/K(43)
KD(8) = -K(45)
KD(9) = -K(44)
KD(10) = 1/K(46)
etc.
```

so how many times do i need to change the keys? when i add, do i just find the 2's complement for the data and then add the key, or something else? when i multiply?

and is 25's complement 2 or 281 or something else in this case?

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