Implementing SRTP Key Exchange Troubleshooting in Wireshark for Genesys Cloud CX and NICE CXone

How-To-Articles
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Implementing SRTP Key Exchange Troubleshooting in Wireshark for Genesys Cloud CX and NICE CXone

What This Guide Covers

This guide establishes a deterministic troubleshooting methodology for isolating, capturing, and decoding SRTP key exchange failures using Wireshark within Genesys Cloud CX and NICE CXone media architectures. You will configure decryption pipelines for both SDES over SIP and DTLS-SRTP over WebRTC, map cipher negotiation states to platform media policies, and validate encryption handshakes against known failure vectors. The end result is a fully instrumented capture workflow that identifies whether media drops, one-way audio, or latency spikes originate from key exchange mismatches, trunk policy conflicts, or edge NAT traversal defects.

Prerequisites, Roles & Licensing

  • Wireshark: Version 4.0 or higher with updated protocols dissector plugins.
  • Genesys Cloud CX: CX 2 or CX 3 license tier. Required permissions: Telephony > Trunk > Read, Telephony > Trunk > Edit, Analytics > Call Recording > Read. OAuth scope: telephony:trunk:read for API correlation.
  • NICE CXone: Enterprise or Cloud Contact Center license with Media Security enabled. Required permissions: Trunk Management > View, Interconnect > Configure, WebRTC Agent license for browser-based testing.
  • Network Access: SPAN port, TAP, or PCAP export capability at the edge router, SBC, or CXone/Genesys media edge.
  • Standards Reference: RFC 3711 (SRTP), RFC 4568 (SDES Key Agreement), RFC 5764 (DTLS-SRTP), RFC 8446 (TLS 1.3).

The Implementation Deep-Dive

1. Configuring Wireshark for SDES Key Extraction and Decryption

SDES (Session Description Protocol) key exchange relies on the a=crypto attribute embedded within the SIP INVITE and 200 OK SDP payloads. Genesys Cloud CX and NICE CXone both support SDES for trunk-to-trunk and SBC-to-cloud media paths. The keys are transmitted in clear text, which makes Wireshark decryption straightforward provided you configure the protocol preferences correctly.

Navigate to Edit > Preferences > Protocols > SRTP. Enable the checkbox Use SRTP decryption. Check the box Automatically add master keys. This setting instructs Wireshark to parse the a=crypto lines from SIP/SDP and populate the SRTP master key table dynamically. You must also ensure Edit > Preferences > Protocols > SIP has Follow SDP data stream enabled. This forces Wireshark to correlate the signaling port (5060/5061) with the media port (UDP 10000-20000 range) before attempting decryption.

The Trap: Leaving the Automatically add master keys option disabled while manually entering keys via the Edit > Preferences > Protocols > SRTP > Master Keys table causes key mismatch errors. SDES keys are session-specific and rotate per call direction. Manual entry fails when the platform generates a new master key for the answer leg. The catastrophic downstream effect is that Wireshark marks every RTP packet as SRTP: Invalid MAC or drops the payload entirely, leading you to falsely conclude the trunk is transmitting unencrypted RTP or that the SBC is corrupting media. Always rely on automatic SDP parsing for SDES workflows.

Architectural Reasoning: We use automatic SDP parsing because Genesys and CXone media engines generate cryptographically random master keys and salts per call leg. The platform does not reuse static keys across sessions. By letting Wireshark extract the a=crypto:1 AES_CM_128_HMAC_SHA1_80 (or equivalent) lines directly from the signaling path, we guarantee the decryption table matches the exact session parameters negotiated between the edge proxy and the remote peer. This approach eliminates human transcription errors and aligns with how the platform’s media processors validate packet authentication tags.

To validate the extraction, apply the display filter:

sip contains "a=crypto" && rtp

Expand the SRTP dissector in the packet details pane. You should see SRTP: Master Key: 0x... and SRTP: Master Salt: 0x... populated. If the payload column shows RTP: Payload with readable G.711 or Opus frames, the key exchange succeeded. If you see SRTP: Invalid MAC or SRTP: Decryption Failed, the key mismatch is confirmed at the network level.

Correlate this with the Genesys Cloud CX trunk configuration using the Admin API:

GET /api/v2/telephony/providers/edge/trunks/{trunkId}
Authorization: Bearer <access_token>

Inspect the encryption field in the JSON response. A value of "sdes" confirms the trunk expects SDES. If the trunk shows "none" but Wireshark captures a=crypto attributes, the SBC is overriding the platform policy, which creates a silent negotiation conflict.

2. Capturing and Decoding DTLS-SRTP Handshakes for WebRTC Media

DTLS-SRTP replaces SDES for all browser-to-edge, mobile-to-edge, and agent-desktop-to-cloud media paths in both Genesys Cloud CX and NICE CXone. The key exchange occurs over a DTLS handshake that mirrors TLS 1.3 but operates over UDP. Wireshark requires explicit configuration to decode DTLS-SRTP parameters and derive the SRTP master key and salt.

Navigate to Edit > Preferences > Protocols > DTLS-SRTP. Enable Use DTLS-SRTP decryption. Check Automatically add SRTP master keys. This setting tells Wireshark to watch for the DTLS ClientHello and ServerHello exchange, extract the negotiated cipher suite, and compute the SRTP key material using the SRTP Key Derivation Function (KDF) defined in RFC 5764.

You must also configure the Edit > Preferences > Protocols > TLS section. Add the server IP addresses of the Genesys or CXone media edge to the RSA keys list if certificate-based authentication is used, though most modern deployments use PSK (Pre-Shared Key) or anonymous DTLS for WebRTC. For PSK deployments, input the PSK identity and key in the PSK list. Genesys Cloud CX uses platform-generated PSKs rotated per media region, so you cannot pre-populate these. You must capture the full UDP handshake before attempting decryption.

The Trap: Capturing only the media port (UDP 10000+) without the initial DTLS handshake port (usually UDP 443 or a custom edge port) causes Wireshark to miss the key derivation entirely. DTLS-SRTP does not transmit keys in clear text. The keys are derived from the handshake master secret. If you start capturing after the call connects, Wireshark sees encrypted UDP traffic with no handshake context. The catastrophic downstream effect is that every packet is flagged as DTLS-SRTP: Unknown cipher or SRTP: Unencrypted, leading you to incorrectly assume the browser or desktop client disabled encryption. Always capture from the TCP/UDP handshake initiation, not just the media stream.

Architectural Reasoning: We mandate full handshake capture because DTLS-SRTP relies on stateful cryptographic negotiation. The ClientHello advertises supported cipher suites (e.g., TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256). The ServerHello selects the suite. The subsequent Finished messages exchange verify-data hashes that seed the SRTP KDF. Genesys Cloud CX and CXone enforce strict cipher ordering to prevent downgrade attacks. If Wireshark misses the handshake, it cannot validate whether the platform rejected a weak cipher or whether the client failed to advertise a supported suite. This directly impacts compliance audits for PCI-DSS and HIPAA, where cipher validation is mandatory.

Apply the display filter to isolate the handshake:

udp.port == 443 && dtls12

Expand the Datagram Transport Layer Security dissector. Locate the Cipher Suite field. Verify it matches the platform’s enforced list. Expand the SRTP Parameters sub-tree. You should see SRTP Master Key and SRTP Master Salt values. Switch to the RTP stream filter:

rtp && ip.addr == <agent_ip> && ip.addr == <edge_ip>

The payload column should now display decoded Opus or G.722 frames. If you see DTLS-SRTP: Alert or DTLS-SRTP: Handshake Failed, the negotiation broke before media establishment.

Cross-reference with NICE CXone WebRTC configuration via the Interconnect API:

GET /api/v2/interconnects/{interconnectId}
Authorization: Bearer <access_token>

Check the mediaSecurity object. A value of "DTLS-SRTP" confirms the interconnect expects encrypted media. If the API returns "NONE" but Wireshark captures DTLS handshakes, the CXone media engine is falling back due to client capability mismatch, which degrades security posture without generating platform alerts.

3. Mapping Negotiation Failures to Platform Media Policies

Key exchange failures rarely originate from Wireshark configuration errors. They originate from misaligned media policies between the platform, the SBC, and the endpoint. You must map the Wireshark decryption state to the exact policy configuration in Genesys Cloud CX and NICE CXone.

In Genesys Cloud CX, navigate to Admin > Telephony > Trunks. Select the target trunk. Inspect the Media Security section. The platform supports SRTP, SDES, and None. If you set SRTP, the platform expects DTLS-SRTP or SDES depending on the transport. If you set SDES, the platform inserts a=crypto attributes. If the SBC is configured for Negotiate but does not support the platform’s preferred cipher order, the SBC silently drops the a=crypto lines or replies with an incompatible suite. Wireshark will show the INVITE with a=crypto, but the 200 OK will lack matching attributes. The media stream then transmits as plain RTP, but Wireshark expects SRTP and flags invalid MACs.

In NICE CXone, navigate to Telephony > Interconnects. Select the target interconnect. Inspect Media Encryption. The platform enforces DTLS-SRTP for WebRTC and SDES for SIP trunks. CXone uses a centralized Media Security Policy that applies to all interconnects in the region. If the policy mandates AES_256_GCM but the SBC only supports AES_128_CM, the handshake fails at the DTLS level. Wireshark captures a DTLS-SRTP: Handshake Failed alert with code handshake_failure. The platform logs a MEDIA_ENCRYPTION_MISMATCH event, but without Wireshark correlation, you cannot identify the exact cipher rejection point.

The Trap: Assuming that enabling SRTP on the platform trunk automatically configures the SBC. The platform does not push encryption policies to third-party SBCs. You must manually align the SBC’s crypto policy with the platform’s cipher suite order. The catastrophic downstream effect is intermittent one-way audio. The SBC may accept the call but drop the media stream when it encounters an unsupported cipher, while the platform assumes the stream is active. This creates silent media drops that bypass standard call detail record (CDR) metrics, requiring deep packet inspection to diagnose.

Architectural Reasoning: We enforce explicit cipher suite alignment because Genesys and CXone media engines prioritize security over compatibility. The platforms reject weak ciphers (e.g., RC4, DES) by default. If the SBC advertises a weak suite, the platform terminates the handshake. By capturing the DTLS or SDES exchange in Wireshark, you verify the exact cipher selection. You can then update the SBC configuration to match the platform’s preferred order: TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 followed by TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256. This eliminates negotiation timeouts and ensures deterministic media establishment.

To automate policy validation, use the Genesys Cloud CX API to retrieve trunk media settings:

GET /api/v2/telephony/providers/edge/trunks/{trunkId}/media
Authorization: Bearer <access_token>

Parse the encryption and cipherSuites fields. Compare them against the Wireshark capture’s negotiated cipher. If they diverge, the SBC is overriding the platform policy. Update the SBC configuration to enforce strict cipher matching. In CXone, use the Interconnect API to verify mediaSecurity and allowedCiphers. Align the SBC policy accordingly. This process transforms reactive troubleshooting into proactive configuration management.

Validation, Edge Cases & Troubleshooting

Edge Case 1: Asymmetric Cipher Suite Negotiation

The Failure Condition: Wireshark captures a successful DTLS handshake, but media packets are flagged as SRTP: Invalid MAC. The call connects, but audio is garbled or silent.
The Root Cause: The SBC and platform negotiate different cipher suites for the forward and reverse directions. DTLS-SRTP requires symmetric cipher selection. If the SBC selects AES_128_GCM for the client-to-server direction but AES_256_GCM for the server-to-client direction, the SRTP KDF generates mismatched master keys. Wireshark decrypts one direction correctly but fails on the other.
The Solution: Force symmetric cipher enforcement on the SBC. Configure the crypto policy to use a single cipher suite for both directions. In Wireshark, apply the filter dtls12 && cipher_suite != 0x009c to identify non-standard selections. Update the SBC to mandate TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 for all legs. Validate by capturing both UDP streams and confirming identical SRTP Master Key values in Wireshark’s decryption table.

Edge Case 2: DTLS-SRTP Port Mismatch After NAT Traversal

The Failure Condition: The DTLS handshake completes successfully on UDP 443, but media packets arrive on UDP 10000+ and Wireshark flags them as SRTP: Unencrypted. The call establishes, but media drops after 30 seconds.
The Root Cause: NAT traversal rewriters modify the SDP c=IN IP4 and m=audio lines to reflect the public IP and port. If the SBC or firewall performs asymmetric NAT rewriting, the DTLS handshake occurs on one port pair, but the media stream routes through a different port pair. Wireshark associates the SRTP keys with the handshake port. When packets arrive on the rewritten port, Wireshark cannot match them to the key table.
The Solution: Enable Wireshark’s Edit > Preferences > Protocols > SRTP > Follow RTP streams option. This forces Wireshark to track stream re-association across port changes. Alternatively, configure the SBC to preserve the original media port in the SDP or use ICE/STUN to align the handshake and media ports. In Wireshark, apply rtp.stream == <stream_id> to verify packet continuity. If the stream ID changes mid-call, the NAT rewriter is fragmenting the session. Update the firewall to preserve UDP port consistency or enable symmetric NAT handling on the SBC.

Edge Case 3: SDES Key Rotation Timeout

The Failure Condition: Wireshark shows successful SDES decryption for the first 60 seconds, then all subsequent RTP packets are flagged as SRTP: Decryption Failed. The call audio cuts out abruptly.
The Root Cause: SDES key rotation is triggered by the a=extmap or a=rtcp-fb attributes when the platform detects packet loss or rekeying requests. Genesys Cloud CX rotates SDES keys every 60 seconds or after 10^6 packets to limit cryptographic exposure. If the SBC does not support dynamic key rotation, it continues using the initial master key. The platform switches to the new key, but the SBC encrypts with the old key. Wireshark detects the MAC mismatch and flags decryption failure.
The Solution: Configure the SBC to support RFC 4568 key rotation. Enable the a=keymib and a=crypto update handling in the SIP stack. In Wireshark, monitor for SIP: INVITE re-INVITE or SIP: UPDATE messages containing new a=crypto attributes. If the SBC ignores these updates, patch the SBC firmware or downgrade the trunk to static SDES (not recommended for production). Validate by capturing the full call duration and confirming Wireshark updates the SRTP master key table at each rotation point. Cross-reference with the Genesys Cloud CX WFM recording health metrics to ensure media continuity aligns with key rotation events.

Official References