About Secure Real-Time Transport Protocol (SRTP)

For maximum call security, Acrobits Softphone and Groundwire support SDES and ZRTP: SDES protects both incoming and outgoing calls for free, and ZRTP encrypts incoming calls for free and requires an in-app purchase for outgoing calls.

SRTP is a method used to encrypt audio for VoIP calls, and it has two key exchange methods: Session Descriptions (SDES) and Zimmermann Real-time Transport Protocol (ZRTP). While both offer call security, ZRTP is considered the latest and more secure protocol, providing enhanced encryption for conversations.

Tips

If call security is vital to you, refer to some reliable sites to determine the right option for you. You may start with the general sites such as the following:

The following sections briefly describe these methods that are available on our softphones.

SDES

Acrobits Softphone and Groundwire offer free SDES encryption for both incoming and outgoing calls.

  • Requires a secure signaling channel using the Transport Layer Security (TLS) transport protocol.
  • Limited to users with their own private branch exchange (PBX) when the SIP providers do not support TLS.
  • Prone to man-in-the-middle attacks such as eavesdropping and dependent on the behavior of proxies along the SIP path.
  • Keys may be transferred in clear text between hops.
  • Disabled for pushed calls due to security concerns.

ZRTP

Acrobits Softphone and Groundwire offer free ZRTP encryption for incoming calls. You may try ZRTP for incoming calls before deciding to purchase for outgoing calls.

  • Media path key exchange method for SRTP.
  • Secures calls on insecure channels using User Datagram Protocol (UDP) transport protocol.
  • Prevents eavesdropping opportunities at proxies.

Important

For Asterisk users, using ZRTP requires a patch. Get the patch at zfoneproject.com/prod_asterisk.html.

Setting SRTP

You can configure the SRTP settings for each SIP account.

To open the SRTP settings in the application:

  • Android devices - Select Android Settings icon > Settings > Accounts > the desired SIP account > Advanced Settings > Secure Calls.
  • iOS devices - Open the Keypad tab, select iOS Setting icon > SIP Accounts or Accounts > the desired SIP account > Advanced Settings > Secure Calls.

SDES Options

The effects of the SDES settings are as follows:

  • Incoming calls:
    • Disabled - Accepts unencrypted calls only.
    • Enabled - Accepts encrypted and unencrypted calls (default).
    • Required - Accepts encrypted calls only.
  • Outgoing calls:
    • Disabled - Makes unencrypted calls (default).
    • Best Effort - Makes encrypted calls, but accepts if the other party responds with an unencrypted response.
    • Required - Makes encrypted calls and requires encrypted responses.

ZRTP Options


Tips

You may test ZRTP by making a call to your softphone from another ZRTP-enabled SIP softphone. If you can place and receive the call, your provider supports ZRTP for incoming calls. Then, you may purchase ZRTP support for outbound calls.

For a free ZRTP-compatible softphone, get one (Zfone) for your desktop at zfoneproject.com/prod_zfone.html)

The effects of the ZRTP settings are as follows:

  • Incoming calls:
    • Disabled - Accepts unencrypted calls only.
    • Enabled- Accepts encrypted and unencrypted calls (default).
    • Required - Accepts encrypted calls only.
  • Outgoing calls:
    • Disabled - Makes unencrypted calls (default).
    • Best Effort - Makes encrypted calls, but accepts if the other party responds with an unencrypted response.
    • Required - Makes encrypted calls and requires encrypted responses.