Implementing SRTP and TLS 1.3 for Hardening Global BYOC Cloud Trunks

How-To-Articles
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Implementing SRTP and TLS 1.3 for Hardening Global BYOC Cloud Trunks

What This Guide Covers

  • Architecting a secure voice transport layer for Genesys Cloud BYOC (Bring Your Own Carrier) Trunks.
  • Implementing TLS 1.3 for SIP signaling encryption and SRTP (Secure Real-time Transport Protocol) for media encryption.
  • Designing a certificate management strategy for secure communication between your SBC and Genesys Cloud.

Prerequisites, Roles & Licensing

  • Licensing: Genesys Cloud CX 1/2/3 with BYOC Cloud.
  • SBC Support: A Session Border Controller (Audiocodes, Ribbon, Cisco CUBE) that supports TLS 1.2/1.3 and SRTP.
  • Permissions:
    • Telephony > Trunk > Add/Edit
    • Telephony > Certificate > Add/Edit

The Implementation Deep-Dive

1. The Strategy: The Secure Edge

Voice traffic over the public internet is vulnerable to eavesdropping and toll fraud. Hardening your trunks with TLS and SRTP is non-negotiable for modern enterprise security (especially for PCI-DSS or HIPAA compliance).

The Strategy:

  1. Signaling (SIP over TLS): Encrypt the “Handshake” (SIP messages) to prevent attackers from seeing who is calling whom.
  2. Media (SRTP): Encrypt the “Voice Payload” (RTP) to prevent the recording or interception of actual conversations.
  3. The Protocol: Mandate TLS 1.3 to eliminate the vulnerabilities found in TLS 1.0/1.1 and the “Round-trip” overhead of TLS 1.2.

2. Implementing TLS for SIP Trunks

Genesys Cloud requires a mutual certificate trust for TLS to function.

The Implementation:

  1. Certificate Generation: Obtain a certificate from a trusted public CA (e.g., DigiCert, Sectigo) for your SBC’s FQDN.
  2. Trunk Config (Genesys):
    • Navigate to Admin > Telephony > Trunks.
    • Set Protocol to TLS.
    • Set Listen Port to 5061.
  3. Trunk Config (SBC):
    • Point your SIP Peer to Genesys Cloud’s regional FQDN (e.g., trunk.mypurecloud.com).
    • Assign your TLS Context and Certificate to the SIP interface.
  4. The Trap: Ensure the Common Name (CN) or Subject Alternative Name (SAN) on your certificate exactly matches the FQDN configured in the Genesys Trunk settings.

3. Architecting SRTP Media Encryption

SRTP uses keys exchanged during the SIP TLS handshake to encrypt the audio stream.

The Strategy:

  1. Key Exchange (SDES vs. DTLS): Genesys Cloud primarily uses SDES (Security Descriptions) for BYOC Trunks. The keys are sent inside the encrypted SIP message.
  2. The Configuration:
    • In the Genesys Trunk settings, enable Media Encryption (SRTP).
    • On the SBC, configure the media realm to “Mandatory SRTP.”
  3. The Trick: Use AES_CM_128_HMAC_SHA1_80 as the preferred crypto suite. It provides the best balance of security and hardware-offload compatibility for most SBCs.

4. Managing Certificate Rotation and Health

Expiring certificates are the #1 cause of trunk outages.

The Implementation:

  1. Monitoring: Use a monitoring tool (e.g., Nagios, Datadog) to alert when the SBC certificate is within 30 days of expiry.
  2. Zero-Downtime Rotation:
    • Upload the new certificate to the SBC as a “Standby.”
    • Update the Genesys Cloud External Trunk Configuration to trust the new CA if it has changed.
    • Perform a “Soft Restart” of the SBC SIP service during a maintenance window to pick up the new chain.
  3. The Benefit: Automated rotation using ACME (Let’s Encrypt) on the SBC can eliminate manual errors, provided your SBC vendor supports the protocol.

Validation, Edge Cases & Troubleshooting

Edge Case 1: “Cipher Suite Mismatch”

Failure Condition: The SIP TLS connection fails with a “Handshake Failure” error in the SBC logs.
Solution: Ensure the SBC supports the modern ciphers required by Genesys Cloud (e.g., ECDHE-RSA-AES256-GCM-SHA384). Disable weak ciphers like 3DES or RC4.

Edge Case 2: One-Way Audio (SRTP Specific)

Failure Condition: Call signaling works, but audio is silent.
Root Cause: The SBC is sending SRTP, but Genesys thinks it’s sending standard RTP (or vice versa).
Solution: Check the SDP (Session Description Protocol) in the SIP INVITE. Look for the m=audio line. It must specify RTP/SAVP for secure media. If it says RTP/AVP, it’s unencrypted.

Edge Case 3: Certificate Chain Issues

Failure Condition: The connection fails because Genesys “Doesn’t Trust” the SBC certificate.
Solution: Ensure the SBC is sending the Full Certificate Chain (Entity + Intermediate + Root). Many admins forget the intermediate cert, causing validation to fail on the receiving end.

Official References