Cipher A Message, And Then Decode It in Objective-C - objective-c

Question

I can't think of a better way to describe it, so here it is.
I'm working on a program, very simply, that you input a message you want encoded, it takes it and makes it into a coded message, such as taking 'easy' and, for example, translating it into 'gcua' (achieved by moving each letter forward two in the alphabet)
Then, the person on the receiving end could stick it into the program and it would bring it back to the original text. I'm obviously thinking something a little more complicated than moving the letters up two or something, but, I digress.
I have some really.. REALLY basic code for this. The problem I'm having is breaking each letter of the message up to apply the encoding, then putting it back together.
I'm not asking for the program, I'm more so asking of how to get up to the point of being able to encode the message. (ex. putting the string into a format possible to encode). It isn't intended to be anything elaborate, just very simple, straight to the point.
Here's a basic outline (Thanks to those who helped troubleshoot):
int main (int argc, const char * argv[]) {
#autoreleasepool {
char choice, yes = 'y', no = 'n', buf[1024];
NSString *plainText, *encrypted, *decoded;
NSLog(#"Encode (Y) or Decode (N)");
scanf("%c", &choice);
choice = tolower(choice);
if (choice == yes) {
scanf("%1024s", buf);
plainText = [NSString stringWithCString:buf encoding:NSUTF8StringEncoding];
} else if (choice == no) {
}
}
return 0;
}

Answer 1

%c implies a char* argument, but you are passing char**.
You should make your variables char, not a char*:
char choice, yes = 'y', no = 'n';
It would also make sense to convert choice to lower case before comparing to y and n, because you ask end-users to enter uppercase Y or N
scanf("%c", &choice);
choice = tolower(choice);
EDIT in response to the edit of the question: You cannot use %s to read a value into NSString. You should use char buf[MAX], and limit the length in the scanf:
char buf[1024];
scanf("%1024s", buf);
plainText = [NSString stringWithCString:buf encoding:NSUTF8StringEncoding];

Answer 2

The %s specification to scanf tells it to read into an array of char that you have already allocated. But you are passing a nil pointer to NSString instead.
Try something like this:
NSData *data = [[NSFileHandle fileHandleWithStandardInput] readDataToEndOfFile];
plainText = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];