Building a Caesar cipher (a letter-shifting cipher)

This lesson covers implementing a Caesar cipher in Objective-C, with the aim of understanding how shifting letters encrypts text.

The Caesar cipher replaces each letter with another a fixed number of places along the alphabet. It comes from ancient Rome and is about as simple as a cipher gets.

The sample code assembles the result in an NSMutableString, taking each character with characterAtIndex: and shifting it while checking it is within the alphabet. Note the unichar type representing a character.

A common early stumble is the arithmetic on character codes. Cases where the shifted result runs past the end of the alphabet (A–Z) have to be handled correctly with % 26.

It is simple next to modern cryptography, but it makes a good introductory subject for practising arithmetic on character codes.

📖 Reference code

NSString *caesarEncrypt(NSString *text, NSInteger shift) {
    NSMutableString *result = [NSMutableString string];
    for (NSInteger i = 0; i < text.length; i++) {
        unichar c = [text characterAtIndex:i];
        if (c >= 'A' && c <= 'Z') {
            unichar code = (c - 'A' + shift) % 26 + 'A';
            [result appendFormat:@"%C", code];
        } else {
            [result appendFormat:@"%C", c];
        }
    }
    return result;
}

NSLog(@"%@", caesarEncrypt(@"HELLO", 3));

✍️ Your code

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🧪 This site can't compile or run Objective-C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).