Building a Caesar cipher (a letter-shifting cipher)

This lesson covers implementing a Caesar cipher in Julia, 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 walks the text with for c in text, checks that the character is within the alphabet, calculates the shifted code with (Int(c) - Int('A') + shift) % 26 + Int('A'), and converts back with Char(). Note how Int() and Char() convert between characters and numbers.

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

function caesar_encrypt(text, shift)
    result = ""
    for c in text
        if c >= 'A' && c <= 'Z'
            code = (Int(c) - Int('A') + shift) % 26 + Int('A')
            result *= Char(code)
        else
            result *= c
        end
    end
    return result
end

println(caesar_encrypt("HELLO", 3))

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