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I want to encrypt data between a Java Server backend and a GWT Client. On the GWT client I use the sbn.js library. It works very fast and is much faster that gwt-crypto.

Here is How I encrypt on the client side given (e,n) of RSA. I created a JSFiddle:

var n = "BC86E3DC782C446EE756B874ACECF2A115E613021EAF1ED5EF295BEC2BED899D26FE2EC896BF9DE84FE381AF67A7B7CBB48D85235E72AB595ABF8FE840D5F8DB";

var e = "3";
var d = "7daf4292fac82d9f44e47af87348a1c0b9440cac1474bf394a1b929d729e5bbcf402f29a9300e11b478c091f7e5dacd3f8edae2effe3164d7e0eeada87ee817b";

function do_encrypt() {
    var before = new Date();
    var rsa = new RSAKey();
    rsa.setPublic(n, e);
    var res = rsa.encrypt($("#plaintext").val());

    $("#ciphertext").val(res);
    $("#cipherb64").val(hex2b64(res));
    console.log("res");



}

$("#encrypt").click(function () {
    do_encrypt();
});

I use the hex representation of the encrypted plaintext to be decrypted on the server. Here is how I decrypt on the server.

I use the following libs:

compile 'org.bouncycastle:bcprov-jdk15on:1.51'
compile 'org.bouncycastle:bcprov-ext-jdk15on:1.51'

Here is how I decrypt on the server using (d,n) of RSA:

    try {
        BigInteger modulus = new BigInteger("BC86E3DC782C446EE756B874ACECF2A115E613021EAF1ED5EF295BEC2BED899D26FE2EC896BF9DE84FE381AF67A7B7CBB48D85235E72AB595ABF8FE840D5F8DB",16);
        BigInteger exponent = new BigInteger("3");
        RSAKeyParameters publicKey = new RSAKeyParameters(false, modulus, exponent)

        BigInteger exponent2 = new BigInteger("7daf4292fac82d9f44e47af87348a1c0b9440cac1474bf394a1b929d729e5bbcf402f29a9300e11b478c091f7e5dacd3f8edae2effe3164d7e0eeada87ee817b", 16);
        RSAKeyParameters privateKey = new RSAKeyParameters(true, modulus, exponent2)

        String encryptedData = "a7f7d5c77c246729141cdfcc77f1f7b38d5f8066b0bc53b2e85119f3f1784f43be2140b5c382ad483bb57cc1b586962cbb1e831e6070a27e4880bbc549e20a372571d09c6b1269ddd7288916f10c96a9138f4165569c4767bfb489de2d44b450ed1495c99da985dc264dabadd9709ccd950ae55095373ccbc3344a26b3efd2dc";

        ////// decrypt
        AsymmetricBlockCipher d = new RSAEngine();
        d = new PKCS1Encoding(d);
        d.init(false, privateKey);

        byte[] messageBytes2 = new BigInteger(encryptedData,16).toByteArray();
        byte[] hexEncodedCipher2 = d.processBlock(messageBytes2, 0, messageBytes2.length); 


        println("encrypted:"+new String(hexEncodedCipher2));

    }
    catch(Exception e) {
        e.printStackTrace()
        println "#################### error"
    }

I got the following exception: 

Error |
org.bouncycastle.crypto.DataLengthException: input too large for RSA cipher.

I suppose that the line println("encrypted:"+new String(hexEncodedCipher2)); is the problem.

  1. How can I decrypt on the client side?

  2. Why do I get different encryptions every time I ran the client side encryption with the same (e,n) and the same plaintext?

Michael
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    1) Your cipher looks to be too large. It should never be larger than your modulus. 2) That is how it's supposed to work, RSA encryption using some random padding so every encryption, even of the same plaintext, will be different. Also, you should only use the bcprov-jdk15on library, the -ext vesion is almost identical except it includes some legacy ciphers (like IDEA) you won't need. – President James K. Polk Sep 11 '14 at 02:31
  • @GregS Could you please correct my code example. – Michael Sep 11 '14 at 11:35
  • RSA ( and any public-key scheme ) is widely used to cipher a symetric key that will be used to cipher a huge amount of data. symetric ciphers are really faster than public key ones. hybrid scheme is the good choice. http://stackoverflow.com/questions/118463/what-is-the-performance-difference-of-pki-to-symmetric-encryption – philippe lhardy Sep 20 '14 at 20:25

1 Answers1

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RSA can only encrypt data blocks that are shorter than the key length.

So you have to use hybrid scheme which is to cipher in RSA a random key that will be use with a symmetric cipher like AES.

There are many topics i SO for this ex : how to use RSA to encrypt files (huge data) in C#

where to go next ? Javascript <-> Java AES

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philippe lhardy
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