How do you offset a byte by an int in solidity? - ethereum

Question

I'm currently trying to offset bytes in solidity to implement a simple Caesar cipher decryption. However, I can't figure out how to do this. Here is my current code, which gives a few compiler errors in Remix:
function decrypt(bytes32 data, int key) public returns (bool) {
bytes32 decryptedData = data; // `data` here is the encrypted data
// decryption
for (int i = 0; i < decryptedData.length; i++) {
decryptedData[i] = (decryptedData[i] - key) % 256;
}
// returns if sha256(decryptedData) matches some value
}
However, this gives me the following errors:
TypeError: Expression has to be an lvalue.
decryptedData[i] = (decryptedData[i] - key) % 256;
^--------------^
TypeError: Operator - not compatible with types bytes1 and int256
decryptedData[i] = (decryptedData[i] - key) % 256;
^--------------------^
TypeError: Operator % not compatible with types bytes1 and int_const 256
decryptedData[i] = (decryptedData[i] - key) % 256;
^----------------------------^
Thanks!

Answer 1

Like Damian Green said, I'm a bit confused on the algorithm you're trying to write, but the contract below will decrypt a Caesar encrypted text. You should be able to modify it for your needs. (Please excuse the lazy hard-coding of ASCII values).
Note that you can't use bytes32 as that is treated as a special array in Solidity and is read-only. See "index access" section of http://solidity.readthedocs.io/en/develop/types.html#fixed-size-byte-arrays
pragma solidity ^0.4.17;
contract CaesarDecryption {
function decrypt(bytes data, int key) pure public returns (bytes) {
bytes memory decryptedData = data;
for (uint i = 0; i < decryptedData.length; i++) {
decryptedData[i] = decryptByte(decryptedData[i], key);
}
return decryptedData;
}
function decryptByte(byte b, int k) pure internal returns (byte) {
uint8 ascii = uint8(b);
uint8 asciiShift;
if (ascii >= 65 && ascii <= 90)
asciiShift = 65;
else if (ascii >= 97 && ascii <=122)
asciiShift = 97;
return byte(((ascii - asciiShift - k + 26) % 26) + asciiShift);
}
}