Reference13r3:Concept DTLS-SRTP

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There are also other versions of this article available: Reference11r1 | Reference13r3 (this version)

Applies To

This information applies to

  • all innovaphone devices from v13r3


SRTP is a standard for encrypting RTP media streams. Before two endpoints can do that encryption they need to exchange secret keys. DTLS-SRTP is a new method how to do so.


In order to understand the advantages of DTLS-SRTP let's go one step back and take a look SDES - the standard key exchange method for SRTP. SDES transmits the secret keys as part of the signalling messages. That messages go from one endpoint over several PBXes to the other endpoint, hop-by-hop. As the secret keys have to be protected from eavesdropping the signalling messages have to be encrypted as well. But the signalling messages are only encrypted hop-by-hop, so each PBX can see or even modify the keys.

Srtp key exchange sdes.png

It is a design problem of SDES that endpoints have to trust the PBXes. This is a manageable risk when all PBXes are under control of the customer. But with internet calls across company boundaries it becomes an actual threat. Possible attacks include corrupted signalling nodes that give away the keys together with the call details to an attacker that is then capable of decrypting captured SRTP calls on a large scale.


DTLS-SRTP was designed to solve that issue. It does not transmit the keys in signalling messages but inband as part of the media stream using end-to-end encryption. Before the media stream starts both endpoints perform a mutual DTLS handshake on the media ports. The shared symmetric key that was established for the resulting DTLS session is then used to derive an SRTP key. Then the encrypted SRTP media stream is started.

Srtp key exchange dtls srtp.png

The protocol does not rely on the trustability of the certificates that are used for DTLS. They are just considered as a container format for the public key of the remote endpoint.

Instead fingerprints are used to allow users for manually checking if the call is encrypted end-to-end. In this context a fingerprint is a hash value calculated over the certificate. Each endpoint sends the fingerprint of its own certificate to the remote party over the signalling. The VOIP stack checks if the remote fingerprint actually matches the certificate that was used for DTLS. If it is not, the call is disconnected right away. Users that need end-to-end encryption can read the fingerprints that are displayed on the phone and compare them to the fingerprints that are displayed on the other phone. If they match, end-to-end-encryption is running for this call. If the fingerprints do not match, it means the call is decrypted and encrypted again somewhere in the middle. For example this happens with media relay, ISDN legs or when there is an actual man-in-the-middle attack.

Pros and cons

At a glance DTLS-SRTP offers the following advantages:

  • End-to-end encryption
  • Detection of man-in-the-middle attacks
  • No need to trust PBXes
  • No need to trust certification authorities

On the other hand there are also some negative aspects:

  • Adds an additional delay at the begin of calls
  • Causes significantly higher CPU load at the begin of calls
  • Less SRTP Cipher available, what means lower encryption level on the Media compared to SDES that could have a SRTP Cipher with AES 256/80.


Priority of SDES and DTLS-SRTP

If nothing is configured, the device offers both SDES and DTLS-SRTP for outgoing calls. For incoming calls it selects DTLS if offered. Otherwise it selects SDES or unencrypted RTP, as a fall-back. This allows for compatibility with most endpoints.

The admin can change that behaviour at the configuration of the registration or interface. There the key exchange mechanisms (SDES, DTLS-SRTP) and their priority can be selected. For example on phones this can be done on page Phone/User/General. Please consult the help pages for details.


No special configuration is needed regarding certificates. DTLS-SRTP does not require endpoints to have the certificate of the remote endpoint in the trust list. Also it doesn't check the names inside certificates.

Proof of secure connection

Conversations may be routed via several hops.

If the call is encrypted with SDES / SDES-DTLS only, the keys are included in the signalling - but each hop would be able to read them. The endpoint therefore does not know whether there is a real end-to-end encryption.

Use of DTLS or at least DTLS-SDES eliminates the possibility for hops to read the keys.

However, there is one exception: If between the endpoints there's a a hop doing MediaRelay, RTP and thus the keys could be read and changed there.

Therefore one can see on the phone how and especially with which keys a call is transmitted.

For this the phone must always do DTLS or at least alternatively DTLS-SDES.

When a call is active, the local / remote keys used can be checked via the callscreen (opening the call on the callscreen) and compared verbally.

If these match, the call is end-to-end encrypted (without man-in-the-middle) and can be marked as such by clicking on the green lock symbol.

DTLS key check1.png -> DTLS key check2.png -> DTLS key check3.png

Important: this can only be done manually using the 4-eye-principle - an automatic check would of course take away any basis for the security aspect!



Traces for debugging DTLS-SRTP can be activated at the signalling module. The trace flags are also available on the debug.xml page.

config add H323 /dtls-trace on
config add SIP /dtls-trace on
config add TSIP /dtls-trace on
config add SIPS /dtls-trace on

Reading traces

When creating an offer for a call DTLS-SRTP is initialized for both SRTP and SRTCP. The Initialized lines show the local fingerprints.

 DTLS.SRTP.0: Initialize
 DTLS.SRTP.0: Initialized fingerprint=447af6c5cfe3b66cf0901d9143f6824d4c5d0ba2ee8bd9d0ed9b9f69680277d0
 DTLS.SRTCP.0: Initialize
 DTLS.SRTCP.0: Initialized fingerprint=447af6c5cfe3b66cf0901d9143f6824d4c5d0ba2ee8bd9d0ed9b9f69680277d0

If DTLS is selected as the key exchange mechanism by the signalling protocol, DTLS is connected in the client or server role. A successful connection flow starts with the Connect line that shows the local role and the remote fingerprint. It is followed by a sequence of sent and received messages. When the connection was established successfully, the trace shows a Connected line followed by a Use SRTP line that shows the negotiated cipher suite.

 DTLS.SRTP.0: Connect CLIENT options=4 fingerprint=447af6c5cfe3b66cf0901d9143f6824d4c5d0ba2ee8bd9d0ed9b9f69680277d0
 DTLS.SRTP.0: Add ClientHello extension "use_srtp" (AES_CM_128_HMAC_SHA1_80, AES_CM_128_HMAC_SHA1_32, no MKI)
 DTLS.SRTP.0: Write ClientHello
 DTLS.SRTCP.0: Connect CLIENT options=4 fingerprint=447af6c5cfe3b66cf0901d9143f6824d4c5d0ba2ee8bd9d0ed9b9f69680277d0
 DTLS.SRTP.0: Read HelloVerifyRequest
 DTLS.SRTP.0: Write ClientHello
 DTLS.SRTP.0: Read ServerHello
 DTLS.SRTP.0: Read ServerHello extension "use_srtp"
 DTLS.SRTP.0: Read Certificate
 DTLS.SRTP.0: Read CertificateRequest
 DTLS.SRTP.0: Read ServerHelloDone
 DTLS.SRTP.0: Write Certificate
 DTLS.SRTP.0: Calculate ClientKeyExchange
 DTLS.SRTP.0: Write ClientKeyExchange
 DTLS.SRTP.0: Sign CertificateVerify
 DTLS.SRTP.0: Write CertificateVerify
 DTLS.SRTP.0: Write ChangeCipherSpec
 DTLS.SRTP.0: Write Finished
 DTLS.SRTP.0: Buffer ChangeCipherSpec
 DTLS.SRTP.0: Buffer handshake message of next epoch 1
 DTLS.SRTP.0: Read ChangeCipherSpec
 DTLS.SRTP.0: Read Finished
 DTLS.SRTP.0: Store session e1a6cafeadcae22a79eef9994613114a000fed7400000001
 DTLS.SRTP.0: Connected
 DTLS.SRTCP.0: Unpause
 DTLS.SRTCP.0: Add ClientHello extension "use_srtp" (AES_CM_128_HMAC_SHA1_80, AES_CM_128_HMAC_SHA1_32, no MKI)
 DTLS.SRTCP.0: Try resume session e1a6cafeadcae22a79eef9994613114a000fed7400000001
 DTLS.SRTCP.0: Write ClientHello
 DTLS.SRTCP.0: Read HelloVerifyRequest
 DTLS.SRTCP.0: Write ClientHello
 DTLS.SRTCP.0: Buffer ChangeCipherSpec
 DTLS.SRTCP.0: Buffer handshake message of next epoch 1
 DTLS.SRTCP.0: Read ServerHello
 DTLS.SRTCP.0: Read ServerHello extension "use_srtp"
 DTLS.SRTCP.0: Resume session e1a6cafeadcae22a79eef9994613114a000fed7400000001
 DTLS.SRTCP.0: Read ChangeCipherSpec
 DTLS.SRTCP.0: Read Finished
 DTLS.SRTCP.0: Write ChangeCipherSpec
 DTLS.SRTCP.0: Write Finished
 DTLS.SRTCP.0: Connected

There are some trailing Timeout and Ignore record lines. They reflect retransmissions that are done to be robust to packet loss and can be ignored.

 DTLS.SRTCP.0: Timeout
 DTLS.SRTP.0: Ignore record (unexpected epoch 0)

If the connection fails for some reason the trace shows an error followed by a Close line.

 DTLS.SRTP.0: Disconnect (fingerprint mismatch)
 DTLS.SRTP.0: Close error=4



innovaphone devices create SHA256 fingerprints. But from remote parties innovaphone devices also accept the following fingerprint types:

  • MD5
  • SHA1
  • SHA224
  • SHA256
  • SHA384
  • SHA512

SRTP cipher suites

The following cipher suites are supported with DTLS-SRTP:

  • SRTP_AES128_CM_HMAC_SHA1_80 (preferred)

Unencrypted RTP is never negotiated using DTLS-SRTP.


  • RFC5764 - Datagram Transport Layer Security (DTLS) Extension to Establish Keys for the Secure Real-time Transport Protocol (SRTP)