Encrypt in Javascript, decrypt in PHP, using public-key cryptography

Be careful with implementing RSA. In fact, you probably shouldn't use RSA at all. (Use libsodium instead!)

Even if you're using a library (e.g. PHP's OpenSSL extension directly or, until recently, Zend\Crypt), there's still plenty that can go wrong. In particular:

PKCS1v1.5 padding, which is the default (and in many cases the only supported padding mode), is vulnerable to a class of chosen-ciphertext attacks called a padding oracle. This was first discovered by Daniel Bleichenbacher. In 1998.
RSA is not suitable for encrypting large messages, so what implementors often do is take a long message, break it up into fixed-size blocks, and encrypt each block separately. Not only is this slow, it's analogous to the dreaded ECB mode for symmetric-key cryptography.

The Best Thing to Do, with Libsodium

You might want to read JavaScript Cryptography Considered Harmful a few times before going down this route. But that said...

Use TLSv1.2 with HSTS and HPKP, preferably with ChaCha20-Poly1305 and/or AES-GCM and an ECDSA-P256 certificate (important: when the IETF christens Curve25519 and Ed25519, switch to that instead).
Add libsodium.js to your project.
Use crypto_box_seal() with a public key to encrypt your messages, client-side.
In PHP, use \Sodium\crypto_box_seal_open() with the corresponding secret key for the public key to decrypt the message.

I need to use RSA to solve this problem.

Please don't. Elliptic curve cryptography is faster, simpler, and far easier to implement without side-channels. Most libraries do this for you already. (Libsodium!)

But I really want to use RSA!

Fine, follow these recommendations to the letter and don't come crying to StackOverflow when you make a mistake (like SaltStack did) that renders your cryptography useless.

One option (which does not come with a complementary JavaScript implementation, and please don't ask for one) that aims to provide simple and easy RSA encryption is paragonie/easyrsa.

It avoids the padding oracles by using RSA-OAEP with MGF1+SHA256 instead of PKCS1v1.5.
It avoids the ECB mode by clever protocol design:

The EasyRSA Encryption Protocol

EasyRSA generates a random 128-bit key for symmetric key cryptography (via AES).
Your plaintext message is encrypted with defuse/php-encryption.
Your AES key is encrypted with RSA, provided by phpseclib, using the correct mode (mentioned above).
This information is packed together as a simple string (with a checksum).

But, really, if you find a valid use case for public key cryptography, you want libsodium instead.

Bonus: Encryption with JavaScript, Decryption with PHP

We're going to use sodium-plus to accomplish this goal. (Adopted from this post.)

const publicKey = X25519PublicKey.from('fb1a219011c1e0d17699900ef22723e8a2b6e3b52ddbc268d763df4b0c002e73', 'hex');

async function sendEncryptedMessage() {
    let key = await getExampleKey();
    let message = $("#user-input").val();
    let encrypted = await sodium.crypto_box_seal(message, publicKey);
    $.post("/send-message", {"message": encrypted.toString('hex')}, function (response) {
        console.log(response);
        $("#output").append("<li><pre>" + response.message + "</pre></li>");
    });
}

And then the congruent PHP code:

<?php
declare(strict_types=1);
require 'vendor/autoload.php'; // Composer
header('Content-Type: application/json');
$keypair = sodium_hex2bin(
    '0202040a9fbf98e1e712b0be8f4e46e73e4f72e25edb72e0cdec026b370f4787' .
    'fb1a219011c1e0d17699900ef22723e8a2b6e3b52ddbc268d763df4b0c002e73'
);

$encrypted = $_POST['message'] ?? null;
if (!$encrypted) {
    echo json_encode(
        ['message' => null, 'error' => 'no message provided'],
        JSON_PRETTY_PRINT
    );
    exit(1);
}
$plaintext = sodium_crypto_box_seal_open(sodium_hex2bin($encrypted), $keypair);

echo json_encode(
    ['message' => $plaintext, 'original' => $encrypted],
    JSON_PRETTY_PRINT
);

I've used something similar for my login page; it encrypts login credentials using the given public key information (N, e) which can be decrypted in PHP.

It uses the following files that are part of JSBN:

jsbn.js - to work with big integers
rsa.js - for RSA encryption only (uses jsbn.js)
rng.js - basic entropy collector
prng4.js - ARC4 RNG backend

To encrypt data:

$pk = '-----BEGIN RSA PRIVATE KEY-----
...
-----END RSA PRIVATE KEY-----';
$kh = openssl_pkey_get_private($pk);
$details = openssl_pkey_get_details($kh);

function to_hex($data)
{
    return strtoupper(bin2hex($data));
}

?>
<script>
var rsa = new RSAKey();
rsa.setPublic('<?php echo to_hex($details['rsa']['n']) ?>', '<?php echo to_hex($details['rsa']['e']) ?>');

// encrypt using RSA
var data = rsa.encrypt('hello world');
</script>

This is how you would decode the sent data:

$kh = openssl_pkey_get_private($pk);
$details = openssl_pkey_get_details($kh);
// convert data from hexadecimal notation
$data = pack('H*', $data);
if (openssl_private_decrypt($data, $r, $kh)) {
   echo $r;
}

RSA example usage for pidCrypt (js) and phpseclib (php).

Do not reuse the private key in this working example.

pidCrypt encryption

//From the pidCrypt example sandbox
function certParser(cert) {
    var lines = cert.split('\n');
    var read = false;
    var b64 = false;
    var end = false;
    var flag = '';
    var retObj = {
    };
    retObj.info = '';
    retObj.salt = '';
    retObj.iv;
    retObj.b64 = '';
    retObj.aes = false;
    retObj.mode = '';
    retObj.bits = 0;
    for (var i = 0; i < lines.length; i++) {
        flag = lines[i].substr(0, 9);
        if (i == 1 && flag != 'Proc-Type' && flag.indexOf('M') == 0)//unencrypted cert?
        b64 = true;
        switch (flag) {
            case '-----BEGI':
                read = true;
                break;
            case 'Proc-Type':
                if (read)retObj.info = lines[i];
                break;
            case 'DEK-Info:':
                if (read) {
                    var tmp = lines[i].split(',');
                    var dek = tmp[0].split(': ');
                    var aes = dek[1].split('-');
                    retObj.aes = (aes[0] == 'AES') ? true : false;
                    retObj.mode = aes[2];
                    retObj.bits = parseInt(aes[1]);
                    retObj.salt = tmp[1].substr(0, 16);
                    retObj.iv = tmp[1];
                }
                break;
            case '':
                if (read)b64 = true;
                break;
            case '-----END ':
                if (read) {
                    b64 = false;
                    read = false;
                }
                break;
                default : if (read && b64)retObj.b64 += pidCryptUtil.stripLineFeeds(lines[i]);
        }
    }
    return retObj;
}

var strCreditCardPublicKey="-----BEGIN PUBLIC KEY-----\nMIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC\/tI7cw+gnUPK2LqWp50XboJ1i\njrLDn+4\/gPOe+pB5kz4VJX2KWwg9iYMG9UJ1M+AeN33qT7xt9ob2dxgtTh7Mug2S\nn1TLz4donuIzxCmW+SZdU1Y+WNDINds194hWsAVhMC1ClMQTfldUGzQnI5sXvZTF\nJWp\/9jheCNLDRIkAnQIDAQAB\n-----END PUBLIC KEY-----\n";

var objParams=certParser(strCreditCardPublicKey);
var binaryPrivateKey=pidCryptUtil.decodeBase64(objParams.b64);

var rsa=new pidCrypt.RSA();

var asn=pidCrypt.ASN1.decode(pidCryptUtil.toByteArray(key));
var tree=asn.toHexTree();
rsa.setPublicKeyFromASN(tree);

var strHexSensitiveDataEncrypted=rsa.encrypt("4111111111111111");

var strBase64SensitiveDataEncrypted=pidCryptUtil.fragment(pidCryptUtil.encodeBase64(pidCryptUtil.convertFromHex(strHexSensitiveDataEncrypted)), 64))

console.log(strBase64SensitiveDataEncrypted);

phpseclib decryption

require_once("Crypt/RSA.php");

function decrypt($strBase64CipherText)
{
    //CRYPT_RSA_MODE_INTERNAL is slow
    //CRYPT_RSA_MODE_OPENSSL is fast, but requires openssl to be installed, configured and accessible.
    define("CRYPT_RSA_MODE", CRYPT_RSA_MODE_INTERNAL);

    $rsa=new Crypt_RSA();


    //$strPrivateKey=file_get_contents("private.pem");
    //This private key is for example purposes
    //DO NOT REUSE
    $strPrivateKey="-----BEGIN RSA PRIVATE KEY-----
        MIICXQIBAAKBgQDBNHK7R2CCYGqljipbPoj3Pwyz4cF4bL5rsm1t8S30gbEbMnKn
        1gpzteoPlKp7qp0TnsgKab13Fo1d+Yy8u3m7JUd/sBrUa9knY6dpreZ9VTNul8Bs
        p2LNnAXOIA5xwT10PU4uoWOo1v/wn8eMeBS7QsDFOzIm+dptHYorB3DOUQIDAQAB
        AoGBAKgwGyxy702v10b1omO55YuupEU3Yq+NopqoQeCyUnoGKIHvgaYfiwu9sdsM
        ZPiwxnqc/7Eo6Zlw1XGYWu61GTrOC8MqJKswJvzZ0LrO3oEb8IYRaPxvuRn3rrUz
        K7WnPJyQ2FPL+/D81NK6SH1eHZjemb1jV9d8uGb7ifvha5j9AkEA+4/dZV+dZebL
        dRKtyHLfbXaUhJcNmM+04hqN1DUhdLAfnFthoiSDw3i1EFixvPSiBfwuWC6h9mtL
        CeKgySaOkwJBAMSdBhn3C8NHhsJA8ihQbsPa6DyeZN+oitiU33HfuggO3SVIBN/7
        HmnuLibqdxpnDOtJT+9A+1D29TkNENlTWgsCQGjVIC8xtFcV4e2s1gz1ihSE2QmU
        JU9sJ3YeGMK5TXLiPpobHsnCK8LW16WzQIZ879RMrkeDT21wcvnwno6U6c8CQQCl
        dsiVvXUmyOE+Rc4F43r0VRwxN9QI7hy7nL5XZUN4WJoAMBX6Maos2Af7NEM78xHK
        SY59+aAHSW6irr5JR351AkBA+o7OZzHIhvJfaZLUSwTPsRhkdE9mx44rEjXoJsaT
        e8DYZKr84Cbm+OSmlApt/4d6M4YA581Os1eC8kopewpy
        -----END RSA PRIVATE KEY-----
    ";
    $strPrivateKey=preg_replace("/[ \t]/", "", $strPrivateKey);//this won't be necessary when loading from PEM


    $rsa->loadKey($strPrivateKey);

    $binaryCiphertext=base64_decode($strBase64CipherText);

    $rsa->setEncryptionMode(CRYPT_RSA_ENCRYPTION_PKCS1);
    $strBase64DecryptedData=$rsa->decrypt($binaryCiphertext);

    return base64_decode($strBase64DecryptedData);
}

//The pidCrypt example implementation will output a base64 string of an encrypted base64 string which contains the original data, like this one:
$strBase64CipherText="JDlK7L/nGodDJodhCj4uMw0/LW329HhO2EvxNXNUuhe+C/PFcJBE7Gp5GWZ835fNekJDbotsUFpLvP187AFAcNEfP7VAH1xLhhlB2a9Uj/z4Hulr4E2EPs6XgvmLBS3MwiHALX2fES5hSKY/sfSUssRH10nBHHO9wBLHw5mRaeg=";

$binaryDecrypted=decrypt($strBase64CipherText);

//should output '4111111111111111'
var_export($binaryDecrypted);

Check out node-rsa.

It's a node.js module

This module provides access to RSA public-key routines from OpenSSL. Support is limited to RSAES-OAEP and encryption with a public key, decryption with a private key.

Maybe you can port it to run in the browser.

UPDATE

RSA client side library for javascript: (pidcrypt has been officially discontinued and the website domain is expired - see @jack's answer which contains the same libraries as pidcrypt contained). https://www.pidder.com/pidcrypt/?page=rsa

PHP server side component: http://phpseclib.sourceforge.net/

Good luck!

Encrypt in Javascript, decrypt in PHP, using public-key cryptography

The Best Thing to Do, with Libsodium

I need to use RSA to solve this problem.

But I really want to use RSA!

The EasyRSA Encryption Protocol

Bonus: Encryption with JavaScript, Decryption with PHP

Tags:

Javascript

Php

Rsa

Pgp

Public Key

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