Code shared from the Rust Playground

playground.rs

#[allow(dead_code)]
fn encrypt(plaintext: &str, key: usize){
    let plaintext = &plaintext.to_uppercase();
    let alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    let mut cipher = String::from("");
    for each_char in plaintext.chars(){
        match alphabet.find(each_char) {
            Some(index) => {
                let char_index = (index + key) % 26;
                let char_place = alphabet.chars().nth(char_index).unwrap();
                cipher.push(char_place);
            },
            None => cipher.push(each_char)
        }
    }
    println!("Result {}", cipher);
}

fn decrypt(cipher: &str, key: usize){
    let cipher = &cipher.to_uppercase();
    let alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    let mut plaintext = String::from("");
    for each_char in cipher.chars(){
        match alphabet.find(each_char) {
            Some(index) => {
                let char_index = (key - 26) % index;
                let char_place = alphabet.chars().nth(char_index).unwrap();
                plaintext.push(char_place);
            },
            None => plaintext.push(each_char)
        }
    }
    println!("Result {}", plaintext);
}

fn main(){
    //encrypt("MORINGA", 4);
    decrypt("QSVMRKE", 4);
}