Re: OpenLDAP and DH parameter size / LogJam vulnerability

[Howard Chu <hyc@symas.com>]

Jens Vagelpohl wrote:
> Hi all,
>
> In my setup (CentOS7, OpenLDAP 2.4.41 from the LDAP Tool Box project) I am using the following slapd.conf parameters for SSL-related configuration:
>
> TLSProtocolMin          3.1
> TLSCertificateFile      /etc/pki/tls/certs/NNN.crt
> TLSCertificateKeyFile   /etc/pki/tls/private/NNN.key
> TLSCACertificateFile    /etc/pki/tls/certs/NNN.ca.pem
> TLSDHParamFile          /usr/local/openldap/etc/openldap/dh_2048.pem
> TLSCipherSuite          AESGCM:!RSA:!DSS:!ADH:!aECDH
>
> The file /usr/local/openldap/etc/openldap/dh_2048.pem is a valid DH parameter file with size 2048:
>
> <snip>
> # openssl dh -in /usr/local/openldap/etc/openldap/dh_2048.pem  -text -noout
>      PKCS#3 DH Parameters: (2048 bit)
>          prime:
> </snip>
>
> I am now testing the actual DH parameter size used during a TLS connection with instructions from https://bettercrypto.org/blog/2015/05/20/tls-logjam/ and it only shows DH parameter size 1024:
>
> <snip>
> $ echo | openssl s_client -connect alias01.alias.ooo:636 -cipher "EDH" 2>/dev/null
> … much output …
> No client certificate CA names sent
> Peer signing digest: SHA512
> Server Temp Key: DH, 1024 bits
> </snip>
>
> I was expecting "Server Temp Key: DH, 2048 bits”. Am I just testing this the wrong way or is there an issue with DH parameter configurations in OpenLDAP?

Works for me:
###
subject=/C=US/ST=California/L=Los Angeles/O=Symas Corporation/OU=Intergalactic 
HQ/CN=localhost
issuer=/C=US/ST=California/O=Symas Corporation/OU=Intergalactic HQ/CN=Symas 
Keymaster/emailAddress=admin@symas.com
---
No client certificate CA names sent
Peer signing digest: SHA512
Server Temp Key: DH, 2048 bits
---
SSL handshake has read 3177 bytes and written 463 bytes
---
New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-GCM-SHA384
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : DHE-RSA-AES256-GCM-SHA384
###
What is your cert's public key size?

--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/