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Passphrase Based Encryption using Web Cryptography API

This post is a part 5 of Web Cryptography API Tutorial post series.

In this tutorial I will show you how create a symmetric key using a passphrase and then encrypt or decrypt data using that generated key.

Converting String to Array Buffer and Array Buffer to String

To encrypt text or binary data you first need to convert it to an array buffer type so that Web Cryptography API can encrypt it.

Here is code to convert a string to an array buffer

function convertStringToArrayBufferView(str)
{
    var bytes = new Uint8Array(str.length);
    for (var iii = 0; iii < str.length; iii++)
    {
        bytes[iii] = str.charCodeAt(iii);
    }

    return bytes;
}

After decrypting, Web Cryptography API returns original string as an array buffer. So we need to convert the array buffer to an string.

function convertArrayBufferViewtoString(buffer)
{
    var str = "";
    for (var iii = 0; iii < buffer.byteLength; iii++)
    {
        str += String.fromCharCode(buffer[iii]);
    }

    return str;
}

Generating Symmetric Public Key from Passphrase

Here is the code to generate key for encryption and decryption using a passphrase

var password = "password";

var key = null;

crypto.subtle.digest({name: "SHA-256"}, convertStringToArrayBufferView(password)).then(function(result){

    window.crypto.subtle.importKey("raw", result, {name: "AES-CBC"}, false, ["encrypt", "decrypt"]).then(function(e){
        key = e;
    }, function(e){
            console.log(e);
    });

});

Here I am first generating 256 bit hash of the passphrase because AES-CBC algorithm’s key size has to be 128 bit, 192 bit or 256 bit. This hash value is the symmetric encryption key now.

Then I am using importKey to create a key object from the hash value. importKey is actually meant to be used to import a key which was exported using exportKey. Here I am pretending the hash value was exported using exportKey

Encrypting String

Here is the code to encrypt an JavaScript string using the retrieved key

var vector = crypto.getRandomValues(new Uint8Array(16));  

var data  = "narayan prusty";

var encrypted_data = null;

function encrypt_data()
{
    crypto.subtle.encrypt({name: "AES-CBC", iv: vector}, key, convertStringToArrayBufferView(data)).then(
        function(result){
            encrypted_data = new Uint8Array(result);
        },
        function(e){
            console.log(e.message);
        }
    );
}

Decrypting String

Here is the code to decrypt an encrypt array buffer using the same key which was used for encryption

var decrypted_data = null;


function decrypt_data()
{
    crypto.subtle.decrypt({name: "AES-CBC", iv: vector}, key, encrypted_data).then(
        function(result){
            decrypted_data = new Uint8Array(result);
            console.log(convertArrayBufferViewtoString(decrypted_data));
        },
        function(e){
            console.log(e.message);
        }
    );
}

Complete Code

Here is complete code together for testing or to play with

function convertStringToArrayBufferView(str)
{
    var bytes = new Uint8Array(str.length);
    for (var iii = 0; iii < str.length; iii++)
    {
        bytes[iii] = str.charCodeAt(iii);
    }

    return bytes;
}

function convertArrayBufferViewtoString(buffer)
{
    var str = "";
    for (var iii = 0; iii < buffer.byteLength; iii++)
    {
        str += String.fromCharCode(buffer[iii]);
    }

    return str;
}

var password = "password";

var key = null;

crypto.subtle.digest({name: "SHA-256"}, convertStringToArrayBufferView(password)).then(function(result){

    window.crypto.subtle.importKey("raw", result, {name: "AES-CBC"}, false, ["encrypt", "decrypt"]).then(function(e){
        key = e;

        encrypt_data();
    }, function(e){
            console.log(e);
    });

});



var vector = crypto.getRandomValues(new Uint8Array(16));  

var data  = "narayan prusty";

var encrypted_data = null;

function encrypt_data()
{
    crypto.subtle.encrypt({name: "AES-CBC", iv: vector}, key, convertStringToArrayBufferView(data)).then(
        function(result){
            encrypted_data = new Uint8Array(result);
            decrypt_data();
        },
        function(e){
            console.log(e.message);
        }
    );
}      

var decrypted_data = null;


function decrypt_data()
{
    crypto.subtle.decrypt({name: "AES-CBC", iv: vector}, key, encrypted_data).then(
        function(result){
            decrypted_data = new Uint8Array(result);
            console.log(convertArrayBufferViewtoString(decrypted_data));
        },
        function(e){
            console.log(e.message);
        }
    );
}
Narayan Prusty
Track Adblock Users with Google AnalyticsJavaScript "0o" Literal
Comments: 6
  1. Manuel
    2 years ago

    The correct way to derivate a key from a password is using PBKDF2
    Unfortunately by now does not work in Safari
    The method you are showing here achieves very low security.

    ReplyCancel
  2. Massimo
    2 years ago

    Is there a way to save the passphrase key in local storage?

    ReplyCancel
  3. Massimo Cassandro
    2 years ago

    Very very interesting

    ReplyCancel
  4. 2 years ago

    Nice Blog!!.

    ReplyCancel
  5. 3 years ago

    *It?s hard to find knowledgeable people on this topic, but you sound like you know what you?re talking about! Thanks

    ReplyCancel
  6. sergeyt
    4 years ago

    convertStringToArrayBufferView() doesn’t support non-ascii characters.

    It is better to use TextEncoder:

    var buffer = new TextEncoder("utf-8").encode(str);

    ReplyCancel

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Narayan Prusty

I am a full-stack web developer. I specialize in Blockchain and JavaScript. This is my personal blog where I write about programming and technologies that I learn and feel interesting to share.

5 years ago 7 Comments Web Development
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