| <?% config.freshness.reviewed = '2021-05-13' %?> |
| <?% config.freshness.owner = 'hta' %?> |
| |
| # SRTP in WebRTC |
| |
| WebRTC mandates encryption of media by means of the Secure Realtime Protocol, or |
| SRTP, which is described in |
| [RFC 3711](https://datatracker.ietf.org/doc/html/rfc3711). |
| |
| The key negotiation in WebRTC happens using DTLS-SRTP which is described in |
| [RFC 5764](https://datatracker.ietf.org/doc/html/rfc5764). The older |
| [SDES protocol](https://datatracker.ietf.org/doc/html/rfc4568) is implemented |
| but not enabled by default. |
| |
| Unencrypted RTP can be enabled for debugging purposes by setting the |
| PeerConnections [`disable_encryption`][1] option to true. |
| |
| ## Supported cipher suites |
| |
| The implementation supports the following cipher suites: |
| |
| * SRTP_AES128_CM_HMAC_SHA1_80 |
| * SRTP_AEAD_AES_128_GCM |
| * SRTP_AEAD_AES_256_GCM |
| |
| The SRTP_AES128_CM_HMAC_SHA1_32 cipher suite is accepted for audio-only |
| connections if offered by the other side. It is not actively supported, see |
| [SelectCrypto][2] for details. |
| |
| The cipher suite ordering allows a non-WebRTC peer to prefer GCM cipher suites, |
| however they are not selected as default by two instances of the WebRTC library. |
| |
| ## cricket::SrtpSession |
| |
| The [`cricket::SrtpSession`][3] is providing encryption and decryption of SRTP |
| packets using [`libsrtp`](https://github.com/cisco/libsrtp). Keys will be |
| provided by `SrtpTransport` or `DtlsSrtpTransport` in the [`SetSend`][4] and |
| [`SetRecv`][5] methods. |
| |
| Encryption and decryption happens in-place in the [`ProtectRtp`][6], |
| [`ProtectRtcp`][7], [`UnprotectRtp`][8] and [`UnprotectRtcp`][9] methods. The |
| `SrtpSession` class also takes care of initializing and deinitializing `libsrtp` |
| by keeping track of how many instances are being used. |
| |
| ## webrtc::SrtpTransport and webrtc::DtlsSrtpTransport |
| |
| The [`webrtc::SrtpTransport`][10] class is controlling the `SrtpSession` |
| instances for RTP and RTCP. When |
| [rtcp-mux](https://datatracker.ietf.org/doc/html/rfc5761) is used, the |
| `SrtpSession` for RTCP is not needed. |
| |
| [`webrtc:DtlsSrtpTransport`][11] is a subclass of the `SrtpTransport` that |
| extracts the keying material when the DTLS handshake is done and configures it |
| in its base class. It will also become writable only once the DTLS handshake is |
| done. |
| |
| ## cricket::SrtpFilter |
| |
| The [`cricket::SrtpFilter`][12] class is used to negotiate SDES. |
| |
| [1]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/peer_connection_interface.h;l=1413;drc=f467b445631189557d44de86a77ca6a0c3e2108d |
| [2]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/media_session.cc;l=297;drc=3ac73bd0aa5322abee98f1ff8705af64a184bf61 |
| [3]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=33;drc=be66d95ab7f9428028806bbf66cb83800bda9241 |
| [4]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=40;drc=be66d95ab7f9428028806bbf66cb83800bda9241 |
| [5]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=51;drc=be66d95ab7f9428028806bbf66cb83800bda9241 |
| [6]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=62;drc=be66d95ab7f9428028806bbf66cb83800bda9241 |
| [7]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=69;drc=be66d95ab7f9428028806bbf66cb83800bda9241 |
| [8]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=72;drc=be66d95ab7f9428028806bbf66cb83800bda9241 |
| [9]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=73;drc=be66d95ab7f9428028806bbf66cb83800bda9241 |
| [10]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_transport.h;l=37;drc=a4d873786f10eedd72de25ad0d94ad7c53c1f68a |
| [11]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/dtls_srtp_transport.h;l=31;drc=2f8e0536eb97ce2131e7a74e3ca06077aa0b64b3 |
| [12]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_filter.h;drc=d15a575ec3528c252419149d35977e55269d8a41 |
