rustls/crypto/

mod.rs

use alloc::borrow::Cow;
use alloc::boxed::Box;
use alloc::vec::Vec;
use core::borrow::Borrow;
use core::fmt::{self, Debug};
use core::hash::{Hash, Hasher};
use core::time::Duration;

use pki_types::{FipsStatus, PrivateKeyDer, SignatureVerificationAlgorithm};

use crate::enums::ProtocolVersion;
#[cfg(feature = "webpki")]
use crate::error::PeerMisbehaved;
use crate::error::{ApiMisuse, Error};
use crate::msgs::ALL_KEY_EXCHANGE_ALGORITHMS;
use crate::sync::Arc;
#[cfg(feature = "webpki")]
pub use crate::webpki::{verify_tls12_signature, verify_tls13_signature};
#[cfg(doc)]
use crate::{ClientConfig, ConfigBuilder, ServerConfig, client, crypto, server};
use crate::{SupportedCipherSuite, Tls12CipherSuite, Tls13CipherSuite};

/// TLS message encryption/decryption interfaces.
pub mod cipher;

mod enums;
pub use enums::{CipherSuite, HashAlgorithm, SignatureAlgorithm, SignatureScheme};

/// Hashing interfaces.
pub mod hash;

/// HMAC interfaces.
pub mod hmac;

/// Key exchange interfaces.
pub mod kx;
use kx::{NamedGroup, SupportedKxGroup};

/// Cryptography specific to TLS1.2.
pub mod tls12;

/// Cryptography specific to TLS1.3.
pub mod tls13;

/// Hybrid public key encryption (RFC 9180).
pub mod hpke;

#[cfg(any(doc, test))]
pub(crate) mod test_provider;
#[cfg(test)]
pub(crate) use test_provider::TEST_PROVIDER;
#[cfg(doc)]
#[doc(hidden)]
pub use test_provider::TEST_PROVIDER;
#[cfg(all(test, any(target_arch = "aarch64", target_arch = "x86_64")))]
pub(crate) use test_provider::TLS13_TEST_SUITE;

// Message signing interfaces.
mod signer;
pub use signer::{
    CertificateIdentity, Credentials, Identity, InconsistentKeys, SelectedCredential, Signer,
    SigningKey, SingleCredential, public_key_to_spki,
};

pub use crate::suites::CipherSuiteCommon;

/// Controls core cryptography used by rustls.
///
/// This structure provides defaults. Everything in it can be overridden at
/// runtime by replacing field values as needed.
///
/// # Using the per-process default `CryptoProvider`
///
/// If it is hard to pass a specific `CryptoProvider` to all callers that need to establish
/// TLS connections, you can store a per-process `CryptoProvider` default via
/// [`CryptoProvider::install_default()`]. When initializing a `ClientConfig` or `ServerConfig` via
/// [`ClientConfig::builder()`] or [`ServerConfig::builder()`], you can obtain the installed
/// provider via [`CryptoProvider::get_default()`].
///
/// The intention is that an application can specify the [`CryptoProvider`] they wish to use
/// once, and have that apply to the variety of places where their application does TLS
/// (which may be wrapped inside other libraries).
/// They should do this by calling [`CryptoProvider::install_default()`] early on.
///
/// To achieve this goal:
///
/// - _libraries_ should use [`ClientConfig::builder()`]/[`ServerConfig::builder()`]
///   or otherwise rely on the [`CryptoProvider::get_default()`] provider.
/// - _applications_ should call [`CryptoProvider::install_default()`] early
///   in their `fn main()`.
///
/// # Using a specific `CryptoProvider`
///
/// Supply the provider when constructing your [`ClientConfig`] or [`ServerConfig`]:
///
/// - [`ClientConfig::builder()`][crate::ClientConfig::builder()]
/// - [`ServerConfig::builder()`][crate::ServerConfig::builder()]
///
/// When creating and configuring a webpki-backed client or server certificate verifier, a choice of
/// provider is also needed to start the configuration process:
///
/// - [`WebPkiServerVerifier::builder()`][crate::client::WebPkiServerVerifier::builder()]
/// - [`WebPkiClientVerifier::builder()`][crate::server::WebPkiClientVerifier::builder()]
///
/// # Making a custom `CryptoProvider`
///
/// Your goal will be to populate an instance of this `CryptoProvider` struct.
///
/// ## Which elements are required?
///
/// There is no requirement that the individual elements ([`SupportedCipherSuite`], [`SupportedKxGroup`],
/// [`SigningKey`], etc.) come from the same crate.  It is allowed and expected that uninteresting
/// elements would be delegated back to one of the default providers (statically) or a parent
/// provider (dynamically).
///
/// For example, if we want to make a provider that just overrides key loading in the config builder
/// API (with [`ConfigBuilder::with_single_cert`], etc.), it might look like this:
///
/// ```
/// # #[cfg(feature = "aws-lc-rs")] {
/// # use std::sync::Arc;
/// # mod fictitious_hsm_api { pub fn load_private_key(key_der: pki_types::PrivateKeyDer<'static>) -> ! { unreachable!(); } }
/// use rustls::crypto::aws_lc_rs;
///
/// pub fn provider() -> rustls::crypto::CryptoProvider {
///   rustls::crypto::CryptoProvider{
///     key_provider: &HsmKeyLoader,
///     ..aws_lc_rs::DEFAULT_PROVIDER
///   }
/// }
///
/// #[derive(Debug)]
/// struct HsmKeyLoader;
///
/// impl rustls::crypto::KeyProvider for HsmKeyLoader {
///     fn load_private_key(&self, key_der: pki_types::PrivateKeyDer<'static>) -> Result<Box<dyn rustls::crypto::SigningKey>, rustls::Error> {
///          fictitious_hsm_api::load_private_key(key_der)
///     }
/// }
/// # }
/// ```
///
/// ## References to the individual elements
///
/// The elements are documented separately:
///
/// - **Random** - see [`SecureRandom::fill()`].
/// - **Cipher suites** - see [`SupportedCipherSuite`], [`Tls12CipherSuite`], and
///   [`Tls13CipherSuite`].
/// - **Key exchange groups** - see [`SupportedKxGroup`].
/// - **Signature verification algorithms** - see [`WebPkiSupportedAlgorithms`].
/// - **Authentication key loading** - see [`KeyProvider::load_private_key()`] and
///   [`SigningKey`].
///
/// # Example code
///
/// See custom [`provider-example/`] for a full client and server example that uses
/// cryptography from the [`RustCrypto`] and [`dalek-cryptography`] projects.
///
/// ```shell
/// $ cargo run --example client | head -3
/// Current ciphersuite: TLS13_CHACHA20_POLY1305_SHA256
/// HTTP/1.1 200 OK
/// Content-Type: text/html; charset=utf-8
/// Content-Length: 19899
/// ```
///
/// [`provider-example/`]: https://github.com/rustls/rustls/tree/main/provider-example/
/// [`RustCrypto`]: https://github.com/RustCrypto
/// [`dalek-cryptography`]: https://github.com/dalek-cryptography
///
/// # FIPS-approved cryptography
///
/// Each element of a `CryptoProvider` may be implemented using FIPS-approved cryptography,
/// and the FIPS status of the overall provider is derived from the status of its elements.
/// Call [`CryptoProvider::fips()`] to determine the FIPS status of a given provider.
///
/// You can verify the configuration at runtime by checking
/// [`ServerConfig::fips()`]/[`ClientConfig::fips()`] return `true`.
#[expect(clippy::exhaustive_structs)]
#[derive(Debug, Clone)]
pub struct CryptoProvider {
    /// List of supported TLS1.2 cipher suites, in preference order -- the first element
    /// is the highest priority.
    ///
    /// Note that the protocol version is negotiated before the cipher suite.
    ///
    /// The `Tls12CipherSuite` type carries both configuration and implementation.
    ///
    /// A valid `CryptoProvider` must ensure that all cipher suites are accompanied by at least
    /// one matching key exchange group in [`CryptoProvider::kx_groups`].
    pub tls12_cipher_suites: Cow<'static, [&'static Tls12CipherSuite]>,

    /// List of supported TLS1.3 cipher suites, in preference order -- the first element
    /// is the highest priority.
    ///
    /// Note that the protocol version is negotiated before the cipher suite.
    ///
    /// The `Tls13CipherSuite` type carries both configuration and implementation.
    pub tls13_cipher_suites: Cow<'static, [&'static Tls13CipherSuite]>,

    /// List of supported key exchange groups, in preference order -- the
    /// first element is the highest priority.
    ///
    /// The first element in this list is the _default key share algorithm_,
    /// and in TLS1.3 a key share for it is sent in the client hello.
    ///
    /// The `SupportedKxGroup` type carries both configuration and implementation.
    pub kx_groups: Cow<'static, [&'static dyn SupportedKxGroup]>,

    /// List of signature verification algorithms for use with webpki.
    ///
    /// These are used for both certificate chain verification and handshake signature verification.
    ///
    /// This is called by [`ConfigBuilder::with_root_certificates()`],
    /// [`server::WebPkiClientVerifier::builder()`] and
    /// [`client::WebPkiServerVerifier::builder()`].
    pub signature_verification_algorithms: WebPkiSupportedAlgorithms,

    /// Source of cryptographically secure random numbers.
    pub secure_random: &'static dyn SecureRandom,

    /// Provider for loading private [`SigningKey`]s from [`PrivateKeyDer`].
    pub key_provider: &'static dyn KeyProvider,

    /// Provider for creating [`TicketProducer`]s for stateless session resumption.
    pub ticketer_factory: &'static dyn TicketerFactory,
}

impl CryptoProvider {
    /// Sets this `CryptoProvider` as the default for this process.
    ///
    /// This can be called successfully at most once in any process execution.
    ///
    /// After calling this, other callers can obtain a reference to the installed
    /// default via [`CryptoProvider::get_default()`].
    pub fn install_default(self) -> Result<(), Arc<Self>> {
        static_default::install_default(self)
    }
}

impl CryptoProvider {
    /// Returns the default `CryptoProvider` for this process.
    ///
    /// This will be `None` if no default has been set yet.
    pub fn get_default() -> Option<&'static Arc<Self>> {
        static_default::get_default()
    }

    /// Return the FIPS validation status for this `CryptoProvider`.
    ///
    /// This covers only the cryptographic parts of FIPS approval.  There are
    /// also TLS protocol-level recommendations made by NIST.  You should
    /// prefer to call [`ClientConfig::fips()`] or [`ServerConfig::fips()`]
    /// which take these into account.
    pub fn fips(&self) -> FipsStatus {
        let Self {
            tls12_cipher_suites,
            tls13_cipher_suites,
            kx_groups,
            signature_verification_algorithms,
            secure_random,
            key_provider,
            ticketer_factory,
        } = self;

        let mut status = Ord::min(
            signature_verification_algorithms.fips(),
            secure_random.fips(),
        );
        status = Ord::min(status, key_provider.fips());
        status = Ord::min(status, ticketer_factory.fips());
        for cs in tls12_cipher_suites.iter() {
            status = Ord::min(status, cs.fips());
        }
        for cs in tls13_cipher_suites.iter() {
            status = Ord::min(status, cs.fips());
        }
        for kx in kx_groups.iter() {
            status = Ord::min(status, kx.fips());
        }

        status
    }

    pub(crate) fn consistency_check(&self) -> Result<(), Error> {
        if self.tls12_cipher_suites.is_empty() && self.tls13_cipher_suites.is_empty() {
            return Err(ApiMisuse::NoCipherSuitesConfigured.into());
        }

        if self.kx_groups.is_empty() {
            return Err(ApiMisuse::NoKeyExchangeGroupsConfigured.into());
        }

        // verifying cipher suites have matching kx groups
        let mut supported_kx_algos = Vec::with_capacity(ALL_KEY_EXCHANGE_ALGORITHMS.len());
        for group in self.kx_groups.iter() {
            let kx = group.name().key_exchange_algorithm();
            if !supported_kx_algos.contains(&kx) {
                supported_kx_algos.push(kx);
            }
            // Small optimization. We don't need to go over other key exchange groups
            // if we already cover all supported key exchange algorithms
            if supported_kx_algos.len() == ALL_KEY_EXCHANGE_ALGORITHMS.len() {
                break;
            }
        }

        for cs in self.tls12_cipher_suites.iter() {
            if supported_kx_algos.contains(&cs.kx) {
                continue;
            }
            let suite_name = cs.common.suite;
            return Err(Error::General(alloc::format!(
                "TLS1.2 cipher suite {suite_name:?} requires {0:?} key exchange, but no {0:?}-compatible \
                key exchange groups were present in `CryptoProvider`'s `kx_groups` field",
                cs.kx,
            )));
        }

        Ok(())
    }

    pub(crate) fn iter_cipher_suites(&self) -> impl Iterator<Item = SupportedCipherSuite> + '_ {
        self.tls13_cipher_suites
            .iter()
            .copied()
            .map(SupportedCipherSuite::Tls13)
            .chain(
                self.tls12_cipher_suites
                    .iter()
                    .copied()
                    .map(SupportedCipherSuite::Tls12),
            )
    }

    /// We support a given TLS version if at least one ciphersuite for the version
    /// is available.
    pub(crate) fn supports_version(&self, v: ProtocolVersion) -> bool {
        match v {
            ProtocolVersion::TLSv1_2 => !self.tls12_cipher_suites.is_empty(),
            ProtocolVersion::TLSv1_3 => !self.tls13_cipher_suites.is_empty(),
            _ => false,
        }
    }

    pub(crate) fn find_kx_group(
        &self,
        name: NamedGroup,
        version: ProtocolVersion,
    ) -> Option<&'static dyn SupportedKxGroup> {
        if !name.usable_for_version(version) {
            return None;
        }
        self.kx_groups
            .iter()
            .find(|skxg| skxg.name() == name)
            .copied()
    }
}

impl Borrow<[&'static Tls12CipherSuite]> for CryptoProvider {
    fn borrow(&self) -> &[&'static Tls12CipherSuite] {
        &self.tls12_cipher_suites
    }
}

impl Borrow<[&'static Tls13CipherSuite]> for CryptoProvider {
    fn borrow(&self) -> &[&'static Tls13CipherSuite] {
        &self.tls13_cipher_suites
    }
}

/// Describes which `webpki` signature verification algorithms are supported and
/// how they map to TLS [`SignatureScheme`]s.
#[expect(clippy::exhaustive_structs)]
#[derive(Clone, Copy)]
pub struct WebPkiSupportedAlgorithms {
    /// A list of all supported signature verification algorithms.
    ///
    /// Used for verifying certificate chains.
    ///
    /// The order of this list is not significant.
    pub all: &'static [&'static dyn SignatureVerificationAlgorithm],

    /// A mapping from TLS `SignatureScheme`s to matching webpki signature verification algorithms.
    ///
    /// This is one (`SignatureScheme`) to many ([`SignatureVerificationAlgorithm`]) because
    /// (depending on the protocol version) there is not necessary a 1-to-1 mapping.
    ///
    /// For TLS1.2, all `SignatureVerificationAlgorithm`s are tried in sequence.
    ///
    /// For TLS1.3, only the first is tried.
    ///
    /// The supported schemes in this mapping is communicated to the peer and the order is significant.
    /// The first mapping is our highest preference.
    pub mapping: &'static [(
        SignatureScheme,
        &'static [&'static dyn SignatureVerificationAlgorithm],
    )],
}

impl WebPkiSupportedAlgorithms {
    /// Return all the `scheme` items in `mapping`, maintaining order.
    pub fn supported_schemes(&self) -> Vec<SignatureScheme> {
        self.mapping
            .iter()
            .map(|item| item.0)
            .collect()
    }

    /// Return the FIPS validation status of this implementation.
    pub fn fips(&self) -> FipsStatus {
        let algs = self
            .all
            .iter()
            .map(|alg| alg.fips_status())
            .min();
        let mapped = self
            .mapping
            .iter()
            .flat_map(|(_, algs)| algs.iter().map(|alg| alg.fips_status()))
            .min();

        match (algs, mapped) {
            (Some(algs), Some(mapped)) => Ord::min(algs, mapped),
            (Some(status), None) | (None, Some(status)) => status,
            (None, None) => FipsStatus::Unvalidated,
        }
    }

    /// Return the first item in `mapping` that matches `scheme`.
    #[cfg(feature = "webpki")]
    pub(crate) fn convert_scheme(
        &self,
        scheme: SignatureScheme,
    ) -> Result<&[&'static dyn SignatureVerificationAlgorithm], Error> {
        self.mapping
            .iter()
            .filter_map(|item| if item.0 == scheme { Some(item.1) } else { None })
            .next()
            .ok_or_else(|| PeerMisbehaved::SignedHandshakeWithUnadvertisedSigScheme.into())
    }
}

impl Debug for WebPkiSupportedAlgorithms {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "WebPkiSupportedAlgorithms {{ all: [ .. ], mapping: ")?;
        f.debug_list()
            .entries(self.mapping.iter().map(|item| item.0))
            .finish()?;
        write!(f, " }}")
    }
}

impl Hash for WebPkiSupportedAlgorithms {
    fn hash<H: Hasher>(&self, state: &mut H) {
        let Self { all, mapping } = self;

        write_algs(state, all);
        state.write_usize(mapping.len());
        for (scheme, algs) in *mapping {
            state.write_u16(u16::from(*scheme));
            write_algs(state, algs);
        }

        fn write_algs<H: Hasher>(
            state: &mut H,
            algs: &[&'static dyn SignatureVerificationAlgorithm],
        ) {
            state.write_usize(algs.len());
            for alg in algs {
                state.write(alg.public_key_alg_id().as_ref());
                state.write(alg.signature_alg_id().as_ref());
            }
        }
    }
}

pub(crate) mod rand {
    use super::{GetRandomFailed, SecureRandom};

    /// Make an array of size `N` containing random material.
    pub(crate) fn random_array<const N: usize>(
        secure_random: &dyn SecureRandom,
    ) -> Result<[u8; N], GetRandomFailed> {
        let mut v = [0; N];
        secure_random.fill(&mut v)?;
        Ok(v)
    }

    /// Return a uniformly random [`u32`].
    pub(crate) fn random_u32(secure_random: &dyn SecureRandom) -> Result<u32, GetRandomFailed> {
        Ok(u32::from_be_bytes(random_array(secure_random)?))
    }

    /// Return a uniformly random [`u16`].
    pub(crate) fn random_u16(secure_random: &dyn SecureRandom) -> Result<u16, GetRandomFailed> {
        Ok(u16::from_be_bytes(random_array(secure_random)?))
    }
}

/// Random material generation failed.
#[expect(clippy::exhaustive_structs)]
#[derive(Debug)]
pub struct GetRandomFailed;

/// A source of cryptographically secure randomness.
pub trait SecureRandom: Send + Sync + Debug {
    /// Fill the given buffer with random bytes.
    ///
    /// The bytes must be sourced from a cryptographically secure random number
    /// generator seeded with good quality, secret entropy.
    ///
    /// This is used for all randomness required by rustls, but not necessarily
    /// randomness required by the underlying cryptography library.  For example:
    /// [`SupportedKxGroup::start()`] requires random material to generate
    /// an ephemeral key exchange key, but this is not included in the interface with
    /// rustls: it is assumed that the cryptography library provides for this itself.
    fn fill(&self, buf: &mut [u8]) -> Result<(), GetRandomFailed>;

    /// Return the FIPS validation status of this implementation.
    fn fips(&self) -> FipsStatus {
        FipsStatus::Unvalidated
    }
}

/// A mechanism for loading private [`SigningKey`]s from [`PrivateKeyDer`].
///
/// This trait is intended to be used with private key material that is sourced from DER,
/// such as a private-key that may be present on-disk. It is not intended to be used with
/// keys held in hardware security modules (HSMs) or physical tokens. For these use-cases
/// see the Rustls manual section on [customizing private key usage].
///
/// [customizing private key usage]: <https://docs.rs/rustls/latest/rustls/manual/_03_howto/index.html#customising-private-key-usage>
pub trait KeyProvider: Send + Sync + Debug {
    /// Decode and validate a private signing key from `key_der`.
    ///
    /// This is used by [`ConfigBuilder::with_client_auth_cert()`], [`ConfigBuilder::with_single_cert()`],
    /// and [`ConfigBuilder::with_single_cert_with_ocsp()`].  The key types and formats supported by this
    /// function directly defines the key types and formats supported in those APIs.
    ///
    /// Return an error if the key type encoding is not supported, or if the key fails validation.
    fn load_private_key(
        &self,
        key_der: PrivateKeyDer<'static>,
    ) -> Result<Box<dyn SigningKey>, Error>;

    /// Return the FIPS validation status for this key provider.
    ///
    /// The returned status must cover all possible key types supported by
    /// [`KeyProvider::load_private_key()`].
    fn fips(&self) -> FipsStatus {
        FipsStatus::Unvalidated
    }
}

/// A factory that builds [`TicketProducer`]s.
///
/// These can be used in [`ServerConfig::ticketer`] to enable stateless resumption.
///
/// [`ServerConfig::ticketer`]: crate::server::ServerConfig::ticketer
pub trait TicketerFactory: Debug + Send + Sync {
    /// Build a new `TicketProducer`.
    fn ticketer(&self) -> Result<Arc<dyn TicketProducer>, Error>;

    /// Return the FIPS validation status of ticketers produced from here.
    fn fips(&self) -> FipsStatus {
        FipsStatus::Unvalidated
    }
}

/// A trait for the ability to encrypt and decrypt tickets.
pub trait TicketProducer: Debug + Send + Sync {
    /// Encrypt and authenticate `plain`, returning the resulting
    /// ticket.  Return None if `plain` cannot be encrypted for
    /// some reason: an empty ticket will be sent and the connection
    /// will continue.
    fn encrypt(&self, plain: &[u8]) -> Option<Vec<u8>>;

    /// Decrypt `cipher`, validating its authenticity protection
    /// and recovering the plaintext.  `cipher` is fully attacker
    /// controlled, so this decryption must be side-channel free,
    /// panic-proof, and otherwise bullet-proof.  If the decryption
    /// fails, return None.
    fn decrypt(&self, cipher: &[u8]) -> Option<Vec<u8>>;

    /// Returns the lifetime of tickets produced now.
    /// The lifetime is provided as a hint to clients that the
    /// ticket will not be useful after the given time.
    ///
    /// This lifetime must be implemented by key rolling and
    /// erasure, *not* by storing a lifetime in the ticket.
    ///
    /// The objective is to limit damage to forward secrecy caused
    /// by tickets, not just limiting their lifetime.
    fn lifetime(&self) -> Duration;
}

mod static_default {
    #[cfg(not(feature = "std"))]
    use alloc::boxed::Box;
    #[cfg(feature = "std")]
    use std::sync::OnceLock;

    #[cfg(not(feature = "std"))]
    use once_cell::race::OnceBox;

    use super::CryptoProvider;
    use crate::sync::Arc;

    #[cfg(feature = "std")]
    pub(crate) fn install_default(
        default_provider: CryptoProvider,
    ) -> Result<(), Arc<CryptoProvider>> {
        PROCESS_DEFAULT_PROVIDER.set(Arc::new(default_provider))
    }

    #[cfg(not(feature = "std"))]
    pub(crate) fn install_default(
        default_provider: CryptoProvider,
    ) -> Result<(), Arc<CryptoProvider>> {
        PROCESS_DEFAULT_PROVIDER
            .set(Box::new(Arc::new(default_provider)))
            .map_err(|e| *e)
    }

    pub(crate) fn get_default() -> Option<&'static Arc<CryptoProvider>> {
        PROCESS_DEFAULT_PROVIDER.get()
    }

    #[cfg(feature = "std")]
    static PROCESS_DEFAULT_PROVIDER: OnceLock<Arc<CryptoProvider>> = OnceLock::new();
    #[cfg(not(feature = "std"))]
    static PROCESS_DEFAULT_PROVIDER: OnceBox<Arc<CryptoProvider>> = OnceBox::new();
}

#[cfg(test)]
#[track_caller]
pub(crate) fn tls13_suite(
    suite: CipherSuite,
    provider: &CryptoProvider,
) -> &'static Tls13CipherSuite {
    provider
        .tls13_cipher_suites
        .iter()
        .find(|cs| cs.common.suite == suite)
        .unwrap()
}

#[cfg(test)]
#[track_caller]
pub(crate) fn tls12_suite(
    suite: CipherSuite,
    provider: &CryptoProvider,
) -> &'static Tls12CipherSuite {
    provider
        .tls12_cipher_suites
        .iter()
        .find(|cs| cs.common.suite == suite)
        .unwrap()
}

#[cfg(test)]
#[track_caller]
pub(crate) fn tls13_only(provider: CryptoProvider) -> CryptoProvider {
    CryptoProvider {
        tls12_cipher_suites: Cow::default(),
        ..provider
    }
}

#[cfg(test)]
#[track_caller]
pub(crate) fn tls12_only(provider: CryptoProvider) -> CryptoProvider {
    CryptoProvider {
        tls13_cipher_suites: Cow::default(),
        ..provider
    }
}