Tls

Decryption methods

linux

VoIPmonitor version >= 27 can decrypt any application by using ssl key logger which logs keys directly in the openssl library (for openssl >= 1.1.0). Ssl key logger is small sslkeylog.so library which uses LD_PRELOAD to intercept session keys. Those keys are then sent over UDP to voipmonitor sniffer. The behaviour of applications (like asterisk / kamailio / freeswitch and all software using openssl) is not affected or changed - it only logs keys.

This library is located in voipmonitor source tree: tools/ssl_keylogger/sslkeylog/sslkeylog.c

Installation

Compiling sslkeylogger

cd /usr/local/src
git clone https://github.com/voipmonitor/sniffer.git voipmonitor-git
cd voipmonitor-git/tools/ssl_keylogger/
make

running keylogger =

Testing keylogger

You should always test if the library / keylogger is working by this command:

env SSLKEYLOG_UDP='127.0.0.1:1234' LD_PRELOAD="/usr/local/src/sniffer-git/tools/ssl_keylogger/sslkeylog.so" openssl

The output should show similar output:

* SSL KEYLOG : OK detect pointer to function SSL_new : 0x7fe9d6e96540
* SSL KEYLOG : OK detect pointer to function SSL_CTX_set_keylog_callback : 0x7fe9d6e97870
* SSL KEYLOG : log to : 127.0.0.1:1234
OpenSSL> root@voipmonitor

If you see SSL KEYLOG messages - the keylog is working

Asterisk

Asterisk binary is directly linking with libssl so we just need to preload our sslkeylog

env SSLKEYLOG_UDP='127.0.0.1:1234' LD_PRELOAD="/usr/local/src/voipmonitor-git/tools/ssl_keylogger/sslkeylog.so" asterisk -vvvgcd

you need to find your starting script and modify it

SSLKEYLOG_UDP parameter tells to what IP and port keys should be sent (this is voipmonitor IP and port) - in this example it runs on the same host as asterisk (127.0.0.1:1234)

Kamailio

Kamailio uses tls.so module (which is linked to openssl.so) thus the LD_PRELOAD needs to load openssl.so first

env SSLKEYLOG_UDP='127.0.0.1:1234' LD_PRELOAD="/usr/local/src/voipmonitor-git/tools/ssl_keylogger/sslkeylog.so /usr/local/lib64/libssl.so.1.1" kamailio

In this example, our system uses compiled openssl from sources, thus the path to the libssl is /usr/local/lib64/libssl.so.1.1 (on debian stock library is located /usr/src/openssl-1.1.1g/libssl.so)

You need to modify your kamailio start scripts

on our debian we modified this line in /etc/init.d/kamailio

env SSLKEYLOG_UDP='127.0.0.1:1234' LD_PRELOAD="/usr/local/src/voipmonitor-git/tools/ssl_keylogger/sslkeylog.so /usr/local/lib64/libssl.so.1.1" start-stop-daemon --start --quiet --pidfile $PIDFILE --exec $DAEMON -- $OPTIONS || log_failure_msg " already running"

configuring voipmonitor

Lets assume that SIP proxy is running on 192.168.0.1:5061

edit : /etc/voipmonitor.conf 

enabling ssl_sessionkey UDP receiver

ssl_sessionkey_udp = yes
ssl_sessionkey_udp_port = 1234
ssl_sessionkey_udp_ip = 192.168.178.0/24  (this is not mandatory)
ssl_sessionkey_udp_maxwait_ms = 10000

enabling ssl decryption

ssl = yes
ssl_ipport = 192.168.0.1:5061

(ssl_ipport can be specified multiple times in case voipmonitor should decrypt multiple SIP proxies)

Run the voipmonitor in usual way.

generic

Since version 11 VoIPmonitor sniffer is able to decode and decrypt TLS SIP protocol by providing private key.

Please note that the sniffer only supports TLS layer and SSLv3 (not SSLv1 or SSLv2) layer which you can verify in CLIENT HELLO packet in wireshark where you can see SSL or TLS in header.

Decrypted SIP packets are converted to virtual UDP packets with the same ethernet headers replacing the IP TCP layer with UDP so you will not see the TCP stream in stored pcap files.

TLS feature is still in beta - if you will have any problems which are reproducible we need to see pcap file with the TLS packets (no need for RTP) and of course the private key.

Please note that TLS where cipher suite is set to Diffie–Hellman key exchange is not possible to decode in any way. It is possible only if your software (PBX/SBC) is storing key for each TLS session but still this feature is not supported. The only solution is to change the cipher suite to use anything else than diffie hellman cipher suites.

VoIPmonitor is able to decrypt SRTP.

Configuration

add to the voipmonitor.conf

ssl = yes
ssl_ipport = 10.0.0.1 : 5061 /etc/private.key

where 10.0.0.1 is server with TLS port 5061. Private key is in /etc/private.key and it is in PEM format (starting with -----BEGIN RSA PRIVATE KEY-----)