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Decrypt SDES SRTP from pcap

If you have a pcap file with encrypted RTP (SDES SRTP) and have access to the SIP signalling to see the keys, these instructions will help you decrypt the RTP payload and save it as raw audio. Optionally, depending on the codec, you can then import the raw audio in Wireshark and save it as an audio file.

Steps

Build ‘srtp-decrypt’

These are the steps:

  1. Build ‘srtp-decrypt’

  2. Get crypto key for each direction

  3. Use the decrypt.sh bash script to decrypt into raw audio

  4. Import the raw audio into Wireshark as hex dump

  5. Generate the wav file

  6. Generate the wav file as two separate channels, one per direction


Follow instructions here:

https://github.com/gteissier/srtp-decrypt


Assume the binary will be produced in a path like ‘/root/code/srtp-decrypt/srtp-decrypt’


Get crypto key for each direction

Access the SIP involved and take note of the ‘crypto’ line from the SDP for each direction.

e.g.:


v=0

...

m=audio 16398 RTP/SAVP 96 8 97

c=IN IP4 xxxxx

...

a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:K7oAZZ5Fm9fePS5/t1ac00000000000000000000

...


The crypto key for the RTP stream produced by this party is K7oAZZ5Fm9fePS5/t1ac00000000000000000000


Use a bash script to decrypt into raw audio

Use this simple bash script to perform the actual decryption:


#!/bin/bash

set -x


SRTP_DECRYPT_BIN=/root/code/srtp-decrypt/srtp-decrypt

HEADER_SIZE=48


CRYPTO_KEY=5eKSVpIOePOgjjfep21R3b000000000000000000

RTP_PCAP=outbound.pcap

RAW_DATA=outbound.raw


${SRTP_DECRYPT_BIN} -d ${HEADER_SIZE} -k ${CRYPTO_KEY} < ${RTP_PCAP} > ${RAW_DATA}


Where:

CRYPTO_KEY needs the value of the crypto key seen earlier.

RTP_PCAP has the pcap with the RTP stream that you want to decrypt.

RAW_DATA is the output file where to put the decrypted raw audio.


HEADER_SIZE may vary, depending on the presence of the SLL header in the capture.

Import the raw audio into Wireshark as hex dump


In Wireshark:

  • File → Import from Hex Dump

  • Select the file with the raw audio

  • Offsets: Hexadecimal

  • Timestamp format: %M:%S.%f

  • Encapsulation type: Ethernet

  • UDP: source port 10000, destination port 20000

  • Import


Then go to:

  • Telephony → RTP → RTP Streams

  • Play

  • Select stream

  • Export as Synchronised audio file


If for example you have two RTP streams, one per direction for the same call, you can generate the wav file with two separate channels, one per direction.


Once you have the raw audio for both directions, you can use Audacity (https://www.audacityteam.org/) to import the audio.


  • Open the first file.

  • File → Import Audio for the second file.

  • Select the first audio, right click, “Set as Stereo file”

  • Save
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