Net-Entropy
An entropy checker for ciphered network connections- Check that statistical entropy property of ciphered data seems correct
- Detect attacks on cryptographic software that create low entropy network traffic (e.g. a shellcode)
- Reports low entropy alarms via syslog
- Configurable entropy threshold
- Configurable verification method (cumulative / packet entropy, server side, client side, or cumulative client+server)
- Very small resources requirment, can run on routers, ids hosts, etc... (< 1% CPU on 100 Mbps Ethernet, < 5 Mo RAM)
- Can verify various kind of cryptocraphic layers: SSH, all SSL secured protocols, IPSec/ESP (in future version)
- Net-Entropy requires libpcap for network frame capture, Libnet and libnids for IP defragmentation and TCP stream assembly
See https://orchids.dnsalias.net/trac/olivain/net-entropy/ for updates and downloads.
Sample figures:
The principle of functioning of Net-Entropy relies on the randomness property of cryptographic algorithms. A perfectly random infinite Byte string have a statistical entropy that tends to 8 bits per Byte. The figure 1 shows the average estimated statistical entropy computed from small size random messages.
Figure 1: Average statistical entropy estimated from small random messages
The perfect case shown in the figure 1 is not quitely exact in real world cryptographic applications. This is essentially due to cryptographic protocols, which insert plain text messages for connection setup and key establishment. These messages insert a bias in the Byte distribution of the whole exchanged data, so which decrease the entropy. The figure 2 shows the entropy of a HTTPS connection (HTTP secured with SSL/TLS). The key establishment explains the initial low entropy, and the slower growth, once the content data are ciphered.
Figure 2: Statistical entropy for a HTTPS connection
The figure 3 shows the entropy of a HTTPS connection attacked with an exploitation of an OpenSSL flaw (BugTraq ID 5363). Data generated by the attack reduces the connection entropy.
Figure 3: Statistical entropy for an Apache/SSL attack connection
For additional information, the figure 4 shows the entropy of a connection of plain text protocols such as HTTP, SMTP and TELNET.
Figure 4: Statistical entropy plain data (http, smtp, telnet)