Caesar cipher

In a Caesar cipher, all letters are shifted by a certain number of places. Then the characters are wrapped to fit within the alphabetical range.

In Swift 5.8, we can loop over integer values in a string with utf8. And with UnicodeScalar we can shift those characters into the desired range.

Example implementation

This code introduces the caesar() method. It accepts a String and a "shift" Int. And it returns a String.

Start We create an empty Character array. This is where we append() our shifted chars as we compute them.

Next We use an if-statement to test whether the value exceeds a lowercase "z." If so we shift it backwards.

Tip We use the UnicodeScalar type to convert an integer value (based on an arithmetic expression) back into a Character.

Finally We return a new string based on the characters we added in the Character array. This is the result of caesar().

func caesar(value: String, shift: Int) -> String {
    // Empty character array.
    var result = [Character]()

    // Loop over utf8 values.
    for u in value.utf8 {
        // Apply shift to UInt8.
        let s = Int(u) + shift

        // See if value exceeds Z.
        // ... The Z is 26 past "A" which is 97.
        // ... If greater than "Z," shift backwards 26.
        // ... If less than "A," shift forward 26.
        if s > 97 + 25 {
            result.append(Character(UnicodeScalar(s - 26)!))
        } else if s < 97 {
            result.append(Character(UnicodeScalar(s + 26)!))
        } else {
            result.append(Character(UnicodeScalar(s)!))
        }
    }
    // Return String from array.
    return String(result)
}

// Test Caesar cipher on this string.
let value1 = "test"
print(value1)

let value2 = caesar(value: value1, shift: 18)
let value3 = caesar(value: value2, shift: -18)
print("\(value2) \(value3)")

let value4 = caesar(value: value1, shift: 1)
let value5 = caesar(value: value4, shift: -1)
print("\(value4) \(value5)")

let value6 = "exxegoexsrgi"
let value7 = caesar(value: value6, shift: -4)
print("\(value6) \(value7)")test
lwkl test
uftu test
exxegoexsrgi attackatonce

Some notes

A Caesar cipher is a more general form of the ROT13 algorithm. In it we rotate characters any number of places, not just 13 places as in ROT13. These ciphers have limited use.

The Caesar cipher is not a good form of encryption. It only obfuscates text. But it can help us learn how to manipulate characters with their underlying integral values.