Add a new option to require that an external Pre-Shared Key is negotiated
for a handshake to succeed, configured via the new APIs
wolfSSL_CTX_require_psk()/wolfSSL_require_psk(). When set, a handshake
that completes without negotiating an external PSK is aborted with
PSK_MISSING_ERROR instead of falling back to a certificate handshake, so
the PSK acts as an additional security factor.
This is a TLS 1.3 / DTLS 1.3 feature. In (D)TLS 1.2 the use of a PSK is
determined by the negotiated cipher suite, so a mandatory PSK is instead
configured there by restricting the cipher suite list to PSK suites; the
new APIs therefore reject non-TLS-1.3 contexts with BAD_FUNC_ARG.
To keep the requirement fail-closed, the APIs also disable version
downgrade on the object so a downgrade-capable context (e.g. one created
from a v23 method) cannot silently fall back to (D)TLS 1.2 and complete
without a PSK; a peer that does not support (D)TLS 1.3 fails to connect.
The requirement applies to external PSKs only (not session tickets):
session-ticket resumption is exempt. To preserve forward secrecy a
mandatory external PSK must also use an (EC)DHE key exchange; a pure
psk_ke handshake is rejected with PSK_KEY_ERROR. When used with
WOLFSSL_CERT_WITH_EXTERN_PSK, it also ensures that peers are properly
authenticated with both the PSK and via certificates.
The new APIs live alongside the existing wolfSSL_[CTX_]no_dhe_psk()/
only_dhe_psk() PSK options and do not depend on certificate support, so
the feature is usable in NO_CERTS (PSK-only) builds.
Added unit tests for the new APIs and enforcement.
Tobias Frauenschläger
79b30aa268