Re: (ITS#7506) Diffie-Hellman parameters (DHParamFile) are improperly handled

[ben@bjencks.net]

I suppose I should attach a test case:

cd $WORKDIR
openssl dhparam 1536 > dh1536.pem
openssl req -new -newkey rsa:2048 -keyout server.key \
  -out server.crt -sha256 -nodes -subj '/CN=localhost/' -x509

For GnuTLS, load the following config:
dn: cn=config
objectClass: olcGlobal
cn: config
olcTLSCertificateFile: $WORKDIR/server.crt
olcTLSCertificateKeyFile: $WORKDIR/server.key
olcTLSCipherSuite: NONE:+VERS-TLS1.1:+VERS-TLS1.0:+AES-128-CBC:
 +DHE-RSA:+SHA1:+COMP-NULL
olcTLSDHParamFile: $WORKDIR/dh1536.pem

dn: olcDatabase={0}config,cn=config
objectClass: olcDatabaseConfig
olcDatabase: {0}config

For Openssl, change olcTLSCipherSuite to:
olcTLSCipherSuite: aRSA+AES128+kEDH

Start up slapd with the config, listening on ldaps://localhost:6636.

openssl s_client -connect localhost:6636

With GnuTLS this will fail negotiation; with OpenSSL it will use 1024
bit parameters rather than the 1536 bits params we specified.

The only way I've found to find the actual parameter size is to tcpdump
the negotiation. First, verify that the negotiated cipher suite is
DHE-RSA-AES128-SHA according to s_client, then:

tcpdump -i lo -wldap.pcap port 6636
wireshark ldap.pcap
Apply the SSL decode to the traffic (right click on packet, Decode As,
Transport tab, SSL), find the Server Key Exchange packet, select the
Server Key Exchange portion of the handshake message, and look at the
hex dump.

It should start with 0c, followed by a three byte size, then the next
two bytes are the length of the DH parameters: 0080 for 1024 bits, 00c0
for 1536 bits.

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