Friday, 19 February 2021

Extracting RTP streams from network captures

I needed an efficient way to programmatically extract RTP streams from a network capture.

In addition I wanted to:

  • save each stream into a separate pcap file.
  • extract SRTP-negotiated keys if present and available in the trace, associating them to the related RTP (or SRTP if the negotiation succeeded) stream.

Some caveats:

  • In normal conditions the negotiation of SRTP sessions happens via a secure transport, typically SIP over TLS, so the exchanged crypto information may not be available from a simple network capture.
  • There are ways to extract RTP streams using Wireshark or tcpdump; it’s not necessary to do it programmatically.

All this said I wrote a small tool ( that parses a network capture and tries to interpret each packet as either RTP/SRTP or SIP, and does two main things:

  • save each detected RTP/SRTP stream into a dedicated pcap file, which name contains the related SSRC.
  • print a summary of the crypto information exchanged, if available.

With those two elements, it’s then possible to decrypt an SRTP stream, depending on the availability of the exchanged crypto information, and also decode it into audio, depending on the codec.

Decryption and decoding is not part of my tool, but can be achieved easily with other tools, like pjsip’s pcaputil.

I might integrate that part into pcap_tool in the future. Again not because it’s strictly necessary, but to start getting more control on the parsing and manipulation. This may reveal to be useful in the future.

pcap_tool is available here for anybody interested in using it and may perhaps wish to change or extend some parts.

You can just clone it and build it as described in the README.

An example output:

./pcap_tool -d ../../trace_20210218_1.pcap


Extracted 1092 RTP frames

Detected RTP Stream: 0x7a2179fa Source port:22248 - Destination port:4000 - Packets: 544 (./stream-0x7a2179fa.pcap)

Detected RTP Stream: 0x772dc5d7 Source port:4000 - Destination port:22248 - Packets: 548 (./stream-0x772dc5d7.pcap)

source port: 22248 - tag: 3 - suite: AES_CM_128_HMAC_SHA1_80 - key: /1TI6DJWHk7fBJY1yBp7L51uEz1JJ2n6CcQAAsJM


source port: 4000 - tag: 4 - suite: AES_CM_128_HMAC_SHA1_32 - key: mPytX24bRmyNgMaqQSxP8dMMqdkkmQeHgC2Ttb3v

source port: 4000 - tag: 3 - suite: AES_CM_128_HMAC_SHA1_80 - key: J1YS1owJDKAFdq5cRF+JtektYDf6IiowCAeijeal

source port: 4000 - tag: 2 - suite: AES_256_CM_HMAC_SHA1_32 - key: 5A9R8O8MCzbuGvJ08WWNJcNHsPaEcEp1ZDp5DunknZ+bZ2JQaVpZ2qmqraTmgQ==

source port: 4000 - tag: 1 - suite: AES_256_CM_HMAC_SHA1_80 - key: ZcZn1IY++2xsSIk/U1GsHSGp+OI/BYIocv/40ldJB28bcNeMmYzs4z4ozrNQ5Q==


That network capture contained 2 SRTP streams, which have been saved separately into stream-0x7a2179fa.pcap and stream-0x772dc5d7.pcap files respectively.

For the negotiation it’s visible what the sender from port 22248 (owner of the 0x7a2179fa stream) used as crypto information, and looking at the same tag (3 in this case) it’s possible to see what crypto information was used by the sender of 0x772dc5d7 stream from port 4000.

With this it’s possible to decrypt (and decode since G.711 was used) with pjsip’s pcaputil with something like:

pcaputil -c AES_CM_128_HMAC_SHA1_80 -k /1TI6DJWHk7fBJY1yBp7L51uEz1JJ2n6CcQAAsJM stream-0x7a2179fa.pcap stream-0x7a2179fa.wav

and have the audio from that stream into a WAV file.

How to build pcaputil (in fact all pjsip’s applications) is widely documented but I also described it in the appendix of 

The call in the example was generated in fact with pjsua.

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