Why does SSL handshake give ‘Could not generate DH keypair’ exception?

When I make an SSL connection with some IRC servers (but not others – presumably due to the server’s preferred encryption method) I get the following exception:

Caused by: java.lang.RuntimeException: Could not generate DH keypair
    at com.sun.net.ssl.internal.ssl.DHCrypt.<init>(DHCrypt.java:106)
    at com.sun.net.ssl.internal.ssl.ClientHandshaker.serverKeyExchange(ClientHandshaker.java:556)
    at com.sun.net.ssl.internal.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:183)
    at com.sun.net.ssl.internal.ssl.Handshaker.processLoop(Handshaker.java:593)
    at com.sun.net.ssl.internal.ssl.Handshaker.process_record(Handshaker.java:529)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:893)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1138)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1165)
    ... 3 more

Final cause:

Caused by: java.security.InvalidAlgorithmParameterException: Prime size must be multiple of 64, and can only range from 512 to 1024 (inclusive)
    at com.sun.crypto.provider.DHKeyPairGenerator.initialize(DashoA13*..)
    at java.security.KeyPairGenerator$Delegate.initialize(KeyPairGenerator.java:627)
    at com.sun.net.ssl.internal.ssl.DHCrypt.<init>(DHCrypt.java:100)
    ... 10 more

An example of a server that demonstrates this problem is aperture.esper.net:6697 (this is an IRC server). An example of a server that does not demonstrate the problem is kornbluth.freenode.net:6697. [Not surprisingly, all servers on each network share the same respective behaviour.]

My code (which as noted does work when connecting to some SSL servers) is:

    SSLContext sslContext = SSLContext.getInstance("SSL");
    sslContext.init(null, trustAllCerts, new SecureRandom());
    s = (SSLSocket)sslContext.getSocketFactory().createSocket();
    s.connect(new InetSocketAddress(host, port), timeout);
    s.setSoTimeout(0);
    ((SSLSocket)s).startHandshake();

It’s that last startHandshake that throws the exception. And yes there is some magic going on with the ‘trustAllCerts’; that code forces the SSL system not to validate certs. (So… not a cert problem.)

Obviously one possibility is that esper’s server is misconfigured, but I searched and didn’t find any other references to people having problems with esper’s SSL ports, and ‘openssl’ connects to it (see below). So I’m wondering if this is a limitation of Java default SSL support, or something. Any suggestions?

Here’s what happens when I connect to aperture.esper.net 6697 using ‘openssl’ from commandline:

~ $ openssl s_client -connect aperture.esper.net:6697
CONNECTED(00000003)
depth=0 /C=GB/ST=England/L=London/O=EsperNet/OU=aperture.esper.net/CN=*.esper.net/emailAddress=[e-mail]
verify error:num=18:self signed certificate
verify return:1
depth=0 /C=GB/ST=England/L=London/O=EsperNet/OU=aperture.esper.net/CN=*.esper.net/emailAddress=[e-mail]
verify return:1
---
Certificate chain
 0 s:/C=GB/ST=England/L=London/O=EsperNet/OU=aperture.esper.net/CN=*.esper.net/emailAddress=[e-mail]
   i:/C=GB/ST=England/L=London/O=EsperNet/OU=aperture.esper.net/CN=*.esper.net/emailAddress=[e-mail]
---
Server certificate
-----BEGIN CERTIFICATE-----
[There was a certificate here, but I deleted it to save space]
-----END CERTIFICATE-----
subject=/C=GB/ST=England/L=London/O=EsperNet/OU=aperture.esper.net/CN=*.esper.net/emailAddress=[e-mail]
issuer=/C=GB/ST=England/L=London/O=EsperNet/OU=aperture.esper.net/CN=*.esper.net/emailAddress=[e-mail]
---
No client certificate CA names sent
---
SSL handshake has read 2178 bytes and written 468 bytes
---
New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-SHA
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
    Protocol  : TLSv1
    Cipher    : DHE-RSA-AES256-SHA
    Session-ID: 51F1D40A1B044700365D3BD1C61ABC745FB0C347A334E1410946DCB5EFE37AFD
    Session-ID-ctx: 
    Master-Key: DF8194F6A60B073E049C87284856B5561476315145B55E35811028C4D97F77696F676DB019BB6E271E9965F289A99083
    Key-Arg   : None
    Start Time: 1311801833
    Timeout   : 300 (sec)
    Verify return code: 18 (self signed certificate)
---

As noted, after all that, it does connect successfully which is more than you can say for my Java app.

Should it be relevant, I’m using OS X 10.6.8, Java version 1.6.0_26.

Answer

The problem is the prime size. The maximum-acceptable size that Java accepts is 1024 bits. This is a known issue (see JDK-6521495).

The bug report that I linked to mentions a workaround using BouncyCastle’s JCE implementation. Hopefully that should work for you.

UPDATE

This was reported as bug JDK-7044060 and fixed recently.

Note, however, that the limit was only raised to 2048 bit. For sizes > 2048 bit, there is JDK-8072452 – Remove the maximum prime size of DH Keys; the fix appears to be for 9.