Encrypt a message in C - c

Question

I have to write a function to encrypt a message given as a string input using the given encryption key. The function should output the encrypted message as a string to encryptedMessage in the argument list. The function prototype must be as follows: void encryptMessage(char *encryptedMessage, char *message, char *encryptionKey); The function must take the encryption key and convert each of its characters, which represent hexadecimal digits, to their equivalent decimal values as integers. I already wrote a function to convert hex2decimal. The message must then be encrypted by adding the first of these integer values to the ASCII value of the first character in the message, and the second of the integer values to the second character in the message, and so on, and start again with the first integer value after every 16. This will be necessary if the message is longer than the encryption key, which will usually be the case.
here is some of my code so far:
void encryptMessage(char *encryptedMessage, char *message, char *encryptionKey)
{
int *arr = malloc(sizeof(int)*getStringLength(encryptionKey));
int i;
for(i = 0;i < getStringLength(encryptionKey);i++){
arr[i] = hexDigit2Dec(encryptionKey[i]);
message[i] = message[i] + (char)arr[i];
if(getStringLength(message ) > getStringLength(encryptionKey)){
i = 0;
}
}
free(arr);
}
when i run it "project.exe has stopped" pops up. Please help me out, i am new at C and struggling a lot.

Answer 1

You need 2 counters: one for the encryption key position and one for the character being worked on.
You are using one and resetting it to zero whenever the message gets longer than the key causing an infinite loop
for ( keyPos = 0, msgPos = 0; msgPos < getStringLength(message ); ++keyPos, ++msgPos )
{
// calculate and append next char to encrypted message here
if ( keyPos >= getStringLength(encryptionKey)
{
keyPos = 0;
}
}

Answer 2

You'll have to learn to do some planning first and think about what you need to do ... I'll try to give kind of a step by step coding here so hopefully you will see where you went wrong.
But first, there's a standard C function for getting the length of a string: strlen(). I will use this. If you have certain requirements to use something different instead, oh well.
Let's start with the prototype you were given:
void encryptMessage(char *encryptedMessage, char *message, char *encryptionKey)
{
It's a bit ambiguous, but I guess encryptedMessage should be the output. For a real-world project, make it explicit by adding a const to the other pointers. But for now, let's ignore that. As encryptedMessage is a parameter here and not the return value, I assume it's the callers responsibility to provide storage. So, move on ...
You will need the length of your encryptionKey multiple times, let's put it in a variable:
size_t keyLen = strlen(encryptionKey);
And then you need the integer values of the key hex digits multiple times, too, so let's pre-calculate them:
char *keyDigits = malloc(keyLen); // we only need char-sized integers here
for (int i = 0; i < keyLen; ++i)
{
keyDigits[i] = (char)hexDigit2Dec(encryptionKey[i]);
}
Now it's time for the main loop ... you just need to take each character of message and add a value from keyDigits to it:
int keyPos = 0;
for (int i = 0; i < strlen(message); ++i)
{
encryptedMessage[i] = message[i] + keyDigits[keyPos];
if (++keyPos == keyLen) keyPos = 0;
}
And that's it ... free your temporary array and you're done.
free(keyDigits);
}