phpseclib/jsbn: encrypt with public key in PHP, decrypt with private key in jsbn

问题

I can successfully generate a public/private keypair using jsbn on the client side. Encrypting and decrypting client side with these keys is fine. I can also transmit that public key to the server and store it in MySQL via PHP.

I cannot encrypt something in PHP using the jsbn public key, and then decrypt it client side using the jsbn private key.

// attempting to encrypt in PHP using the jsbn public key.
// (this public key came from jsbn client side)
$jsbn_public_key = '763989d1f75a779dae752ac236b011e85f9496bb414d72f5e89bf44274a942277fab2d4f5c58a57634d4000eecc8009d2efaeff17aa4a0efae2c4d41f3423be88be043628c6bac86f97deaadf23231793e6fa02550fb2ca65b2600e074205d23338e28ab3c5e92265e6bd7995c173085e3dc042e59ef464c5ed058c3ad863911';
$rsa = new Crypt_RSA();
$rsa->setPublicKeyFormat(CRYPT_RSA_PUBLIC_FORMAT_PKCS1); // tried various options here.
$public_key = array(
    'n' => new Math_BigInteger($jsbn_public_key, 16),
    'e' => new Math_BigInteger('65537', 10), // same value as jsbn
);
$rsa->loadKey($public_key,CRYPT_RSA_PUBLIC_FORMAT_RAW);  // tried various options here.
$ciphertext = $rsa->encrypt($value);
$ciphertext_web_safe = bin2hex($ciphertext); // I think this is where the problem is!
// $ciphertext_web_safe looks something like this:
// 1b02bee0422028accba74f37e6e6974125fd16bdf83a72b8d6462e140ee6e85adae869fddc5e83635aaf90bc6074a3128890eeadf9537c33ebdfd665e16a3f1b617fa2fa5454f469e84f86d77ffcbc234dfc8a32291fbc84df61a0098c97fd90bb10204f68e783d9996678cd7853f3cbd932a4a067cb7f4f9eb62ca0542964f6
// which is exactly the same length as an encrypted string generated by jsbn

And here's the client side encryption:

var my_rsa = {
    ....
    e: "65537",
    public_key: '763989d1f75a779dae752ac236b011e85f9496bb414d72f5e89bf44274a942277fab2d4f5c58a57634d4000eecc8009d2efaeff17aa4a0efae2c4d41f3423be88be043628c6bac86f97deaadf23231793e6fa02550fb2ca65b2600e074205d23338e28ab3c5e92265e6bd7995c173085e3dc042e59ef464c5ed058c3ad863911',
    encrypt: function(value){
        var rsakey = new RSAKey();
        rsakey.setPublic(this.public_key, this.e);
        return rsakey.encrypt(value);
    },
    decrypt: function(ciphertext){
        var rsakey = new RSAKey();
        rsakey.setPrivateEx(this.public_key, this.e, this.private_key.d, this.private_key.p, this.private_key.q, this.private_key.dmp1, this.private_key.dmq1, this.private_key.coeff);
        return rsakey.decrypt(ciphertext);
    },
    ....
};

When I try to decrypt the '$ciphertext_web_safe' value in JavaScript I simply get an empty string. No errors in console log etc..

Is there a problem with the "bin2hex" used in PHP? Should there be some sort of padding etc.. applied before converting the encrypted binary data to hex?

Cheers, Dave