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[DTLS 1.3] Prepare (D)TLS 1.2 implementation - #6001
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reneme merged 2 commits intoOct 5, 2026
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This avoids a hard-to-read boolean flag in various constructors or function invocations. Co-Authored-By: René Meusel <rene.meusel@rohde-schwarz.com>
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🟡 Changes recommended
Downgrade replay can discard early fragments, and the replacement DTLS state cannot retransmit the original ClientHello.
Review effort: Balanced
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What changed in this PR
Prepares TLS/DTLS 1.2 for DTLS 1.3 downgrade handoff and replaces internal datagram booleans with TLS_Flavor.
Changes:
- Transfers epoch-zero sequence numbers during downgrade.
- Propagates
TLS_Flavorthrough TLS 1.2 APIs. - Updates downgrade detection and ServerHello generation.
| File | Description |
|---|---|
src/lib/tls/tls13/tls_types_13.h |
Defines downgrade sequence-number state. |
src/lib/tls/tls13/tls_channel_impl_13.cpp |
Initializes downgrade flavor metadata. |
src/lib/tls/tls13/msg_server_hello_13.cpp |
Supplies TLS flavor for random generation. |
src/lib/tls/tls12/tls_server_impl_12.h |
Adopts TLS_Flavor in server construction. |
src/lib/tls/tls12/tls_server_impl_12.cpp |
Propagates flavor through server behavior. |
src/lib/tls/tls12/tls_seq_numbers.h |
Adds initial DTLS sequence counters. |
src/lib/tls/tls12/tls_record.h |
Changes record API to use flavor. |
src/lib/tls/tls12/tls_record.cpp |
Selects record parsing by flavor. |
src/lib/tls/tls12/tls_messages_12.h |
Extends ServerHello constructors with flavor. |
src/lib/tls/tls12/tls_client_impl_12.h |
Adopts flavor in client construction. |
src/lib/tls/tls12/tls_client_impl_12.cpp |
Adds DTLS downgrade state restoration. |
src/lib/tls/tls12/tls_channel_impl_12.h |
Stores flavor and accepts sequence counters. |
src/lib/tls/tls12/tls_channel_impl_12.cpp |
Applies flavor and initializes counters. |
src/lib/tls/tls12/msg_server_hello_12.cpp |
Passes flavor into random generation. |
src/lib/tls/tls_server.cpp |
Maps public datagram selection to flavor. |
src/lib/tls/tls_messages.h |
Exposes TLS 1.3 offer detection. |
src/lib/tls/tls_messages_internal.h |
Updates ServerHello random API. |
src/lib/tls/tls_client.cpp |
Maps offered version to flavor. |
src/lib/tls/tls_channel_impl.h |
Expands downgrade handoff metadata. |
src/lib/tls/msg_server_hello.cpp |
Accepts flavor during random generation. |
src/lib/tls/msg_client_hello.cpp |
Detects TLS/DTLS 1.3 offers. |
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randombit
approved these changes
Oct 5, 2026
This sets up the protocol downgrade mechanics to handle downgrades from DTLS 1.3. Most notably, this requires handing over the current read/write fragment sequence numbers (e.g. if a ClientHello was transmitted as multiple fragments). Co-Authored-By: René Meusel <rene.meusel@rohde-schwarz.com>
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This prepares the (D)TLS 1.2 stack for DTLS 1.3 support. Most notably: The downgrade mechanics must be prepared to receive fragment sequence numbers from a downgrading (D)TLS 1.3 stack. If we started off as DTLS 1.3 and sent/received a ClientHello that needed fragmentation, these fragment sequence numbers don't start at "0" for the incoming DTLS 1.2 implementation.
Additionally, this introduces a
TLS_Flavorenum to replace variousis_datagramboolean flags in function signatures. That is mostly a readability improvement that we introduced in DTLS 1.3 and applied to DTLS 1.2 for consistency. At some point in the future, we might do the same with theis_serverflags and replace them with theConnection_Sideenum.#5000