test.js

function encrypt_wrapper() {
    let i = document.getElementById('input').value;
    let k = Number(document.getElementById('key').value);
    let output = document.getElementById('outputStr');
    typewriter(encrypt(i, k), output, 0.3);
}

function decrypt_wrapper() {
    let i = document.getElementById('inputDecrypt').value;
    let k = Number(document.getElementById('keyDecrypt').value);
    let output = document.getElementById('outputStr');
    typewriter(decrypt(i, k), output, 0.3);
}

how about splitting the common parts out into a new function?

this is how you split out i into a parameter:

function encrypt_wrapper(input_id) {
    let i = document.getElementById(input_id).value;
    let k = Number(document.getElementById('key').value);
    let output = document.getElementById('outputStr');
    typewriter(encrypt(i, k), output, 0.3);
}

function decrypt_wrapper(input_id) {
    let i = document.getElementById(input_id).value;
    let k = Number(document.getElementById('keyDecrypt').value);
    let output = document.getElementById('outputStr');
    typewriter(decrypt(i, k), output, 0.3);
}