9001039?: DHKeyAgreement calculates wrong TlsPremasterSecret 1 out of 256 times

Pasi Eronen eronenp at gmail.com
Fri May 10 02:00:44 PDT 2013

AKA "1 out of 256 SSL/TLS handshakes fails with DHE cipher suites"

I reported this bug over a month of ago, but for some reason, it's not
yet visible at bugs.sun.com. I've included the bug report below just in

It seems this commit from March 2012 inadvertently broke SSL/TLS DHE
cipher suites, causing the SSL/TLS handshake to fail approximately
1 out of 256 times:


The commit was done to fix this bug:


While generating a secret of the same length as modulus may be the right
choice generally speaking (and it's what e.g. IPsec uses), SSL/TLS uses
a different convention: leading zeroes must be stripped.

This is currently blocking us from updating our production systems to
Java 7, so although I have not contributed to OpenJDK before, I'd like
to submit a patch and a test case for this (I've signed the OCA
already). But before I do this, I'd like to check that the approach is

We have a separate "algorithm" value "TlsPremasterSecret", so
behavior for other cases could stay the same. Would a patch
like this:

    } else if (algorithm.equals("TlsPremasterSecret")) {
        // remove leading zero bytes per RFC 5246 Section 8.1.2
        int i = 0;
        while ((i < secret.length - 1) && (secret[i] == 0)) {
        if (i == 0) {
            return new SecretKeySpec(secret, "TlsPremasterSecret");
        } else {
            byte[] secret2 = new byte[secret.length - i];
            System.arraycopy(secret, i, secret2, 0, secret2.length);
            return new SecretKeySpec(secret2, "TlsPremasterSecret");

Plus a test case (with fixed keys) that checks that leading zero is
for TlsPremasterSecret and is not stripped otherwise, be sufficient?

Best regards,


DHKeyAgreement calculates wrong TlsPremasterSecret 1 out of 256 times

Full OS version:
Tested on Windows 7 (Microsoft Windows [Version 6.1.7601]), but occurs in
e..g OpenJDK 7 as well.

Development Kit or Runtime version:
java version "1.7.0_17"
Java(TM) SE Runtime Environment (build 1.7.0_17-b02)
Java HotSpot(TM) Client VM (build 23.7-b01, mixed mode, sharing)

When performing Diffie-Hellman key agreement for SSL/TLS, the TLS
specification (RFC 5246) says that "Leading bytes of Z that contain all zero
bits are stripped before it is used as the pre_master_secret."

However, com.sun.crypto.provider.DHKeyAgreement.java does not strip leading
zero bytes. This causes approximately 1 out 256 SSL/TLS handshakes with
DH/DHE cipher suites to fail (when the leading byte happens, by chance, to
be zero).

Steps to Reproduce:
1. Start a simple JSSE socket server with -Djavax.net.debug=all.

2. Connect to the server with e.g. OpenSSL command line tool, ensuring that
DHE cipher suite gets selected (e.g. "openssl s_client -cipher
DHE-RSA-AES128-SHA -connect") repeatedly. Other SSL
clients can be used -- this is not an OpenSSL bug (see below).

3. Repeat the connection. After a couple of hundred successful connections,
the connection will fail with handshake_failure alert.

4. Examine the JSSE debug logs produced by the server: the failed connection
will have a PreMaster secret that begins with zero byte
(while all other connections have non-zero byte here). For example:

PreMaster Secret:
0000: 00 70 C5 7E 91 38 C8 DE   ED 75 3D 76 8A B5 44 69  .p...8...u=v..Di
0010: E7 32 1C EE 80 77 50 C7   A9 51 24 2E E3 15 11 30  .2...wP..Q$....0
0020: 9D F6 9F BC 9D EB 5C 18   F7 A4 19 ED 1A AC 2E 0C  ......\.........
0030: E3 18 C5 11 B1 80 07 7D   B1 C6 70 A8 D7 EB CF DD  ..........p.....
0040: 2D B5 1D BC 01 3E 28 2A   2B 5B 38 8F EB 20 F2 A2  -....>(*+[8.. ..
0050: 00 07 47 F7 87 B8 99 CB   EF B4 13 04 C8 8B 82 FB  ..G.............

Expected Result:
Expected result is that every connection succeed.

Actual Result:
Roughly one out of 256 connections fail.

Source code for an executable test case:

Java server:

import javax.net.ssl.SSLServerSocket;
import javax.net.ssl.SSLServerSocketFactory;
import javax.net.ssl.SSLSocket;

public class TestServer {
    public static void main(String args[]) throws Exception {
        SSLServerSocketFactory ssf = (SSLServerSocketFactory)
        SSLServerSocket ss = (SSLServerSocket) ssf.createServerSocket(9999);
        System.out.println("Listening on port 9999");
        for (String cs : ss.getEnabledCipherSuites()) {
        while (true) {
            SSLSocket s = (SSLSocket) ss.accept();
            System.out.println("Connected with

Run as as follows:

keytool -storepass "password" -keypass "password" -genkey -keyalg RSA
-keystore test_keystore.jks -dname CN=test
javac TestServer.java
java -Djavax.net.debug=all -Djavax.net.ssl.keyStore=./test_keystore.jks
-Djavax.net.ssl.keyStorePassword=password TestServer

OpenSSL client:

set -e
while true; do
  openssl s_client -cipher DHE-RSA-AES128-SHA -connect -no_ign_eof < /dev/null

Disable Diffie-Hellman cipher suites.

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