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fix(error): return TLS 1.2 PRF setup errors #14

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Merged
thieman merged 1 commit into from
Jun 24, 2026
Merged

fix(error): return TLS 1.2 PRF setup errors #14

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21 changes: 11 additions & 10 deletions src/alg.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -39,20 +39,21 @@ pub(crate) fn ecdh_x25519() -> Result<BCRYPT_ALG_HANDLE, Error> {
}

#[cfg(feature = "tls12")]
pub(crate) fn tls12_kdf() -> BCRYPT_ALG_HANDLE {
static ALG_HANDLE: OnceCell<Handle> = OnceCell::new();
pub(crate) fn tls12_kdf() -> Result<BCRYPT_ALG_HANDLE, Error> {
static ALG_HANDLE: OnceCell<Option<Handle>> = OnceCell::new();
ALG_HANDLE
.get_or_init(|| {
Handle(
load_algorithm(
BCRYPT_TLS1_2_KDF_ALGORITHM,
BCRYPT_OPEN_ALGORITHM_PROVIDER_FLAGS::default(),
None,
)
.unwrap(),
load_algorithm(
BCRYPT_TLS1_2_KDF_ALGORITHM,
BCRYPT_OPEN_ALGORITHM_PROVIDER_FLAGS::default(),
None,
)
.ok()
.map(Handle)
})
.0
.as_ref()
.map(|handle| handle.0)
.ok_or_else(|| Error::General("CNG TLS 1.2 KDF algorithm provider unavailable".into()))
}

/// Load an algorithm provider with specified flags, and optional property.
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65 changes: 47 additions & 18 deletions src/prf.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -16,42 +16,42 @@ use windows::{

pub(crate) struct Prf<const HASH_SIZE: usize>(pub(crate) Algorithm<HASH_SIZE>);

impl<const HASH_SIZE: usize> rustls::crypto::tls12::Prf for Prf<HASH_SIZE> {
fn for_key_exchange(
fn u32_len(len: usize, name: &str) -> Result<u32, rustls::Error> {
u32::try_from(len).map_err(|_| rustls::Error::General(format!("{name} is too large for CNG")))
}

impl<const HASH_SIZE: usize> Prf<HASH_SIZE> {
fn try_for_secret(
&self,
output: &mut [u8; 48],
kx: Box<dyn ActiveKeyExchange>,
peer_pub_key: &[u8],
output: &mut [u8],

@tobz tobz Jun 23, 2026

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I'm not sure I understand why we're switching to a variable-length slice here... 🤔

@thieman thieman Jun 23, 2026

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[GPT 5.5] This is shared with for_secret, whose rustls trait signature is already &mut [u8]. The for_key_exchange method still receives &mut [u8; 48]; before this PR it immediately called for_secret, which coerced that fixed array to the same slice type. So this does not broaden the key-exchange caller contract, it just avoids duplicating the CNG KDF implementation. I checked the PR diff and did not find another introduced fixed-array-output helper pattern that needs the same treatment.

secret: &[u8],
label: &[u8],
seed: &[u8],
) -> Result<(), rustls::Error> {
let secret = kx.complete(peer_pub_key)?;
self.for_secret(output, secret.secret_bytes(), label, seed);
Ok(())
}

fn for_secret(&self, output: &mut [u8], secret: &[u8], label: &[u8], seed: &[u8]) {
let mut key = Owned::default();
let tls12_kdf = tls12_kdf()?;

unsafe {
BCryptGenerateSymmetricKey(tls12_kdf(), &mut *key, None, secret, 0)
BCryptGenerateSymmetricKey(tls12_kdf, &mut *key, None, secret, 0)
.ok()
.unwrap();
.map_err(|e| {
rustls::Error::General(format!("TLS 1.2 PRF key import failed: {e}"))
})?;
}

let buffers = [
BCryptBuffer {
cbBuffer: label.len() as u32,
cbBuffer: u32_len(label.len(), "TLS 1.2 PRF label")?,
BufferType: KDF_TLS_PRF_LABEL,
pvBuffer: label.as_ptr() as *mut _,
},
BCryptBuffer {
cbBuffer: seed.len() as u32,
cbBuffer: u32_len(seed.len(), "TLS 1.2 PRF seed")?,
BufferType: KDF_TLS_PRF_SEED,
pvBuffer: seed.as_ptr() as *mut _,
},
BCryptBuffer {
cbBuffer: self.0.id_bytes.len() as u32,
cbBuffer: u32_len(self.0.id_bytes.len(), "TLS 1.2 PRF hash algorithm id")?,
BufferType: KDF_HASH_ALGORITHM,
pvBuffer: self.0.id_bytes.as_ptr() as *mut _,
},
Expand All @@ -67,8 +67,31 @@ impl<const HASH_SIZE: usize> rustls::crypto::tls12::Prf for Prf<HASH_SIZE> {
unsafe {
BCryptKeyDerivation(*key, Some(&params), output, &mut size, 0)
.ok()
.unwrap();
.map_err(|e| {
rustls::Error::General(format!("TLS 1.2 PRF derivation failed: {e}"))
})?;
};

Ok(())
}
}

impl<const HASH_SIZE: usize> rustls::crypto::tls12::Prf for Prf<HASH_SIZE> {
fn for_key_exchange(
&self,
output: &mut [u8; 48],
kx: Box<dyn ActiveKeyExchange>,
peer_pub_key: &[u8],
label: &[u8],
seed: &[u8],
) -> Result<(), rustls::Error> {
let secret = kx.complete(peer_pub_key)?;
self.try_for_secret(output, secret.secret_bytes(), label, seed)
}

fn for_secret(&self, output: &mut [u8], secret: &[u8], label: &[u8], seed: &[u8]) {
self.try_for_secret(output, secret, label, seed)
.expect("rustls only calls TLS 1.2 PRF for advertised CNG-backed cipher suites")
}

fn fips(&self) -> bool {
Expand All @@ -83,7 +106,13 @@ mod test {

use super::super::hash::{SHA256, SHA384};

use super::Prf;
use super::{u32_len, Prf};

#[test]
fn cng_buffer_lengths_fit_in_u32() {
assert_eq!(u32_len(42, "test").unwrap(), 42);
assert!(u32_len(usize::MAX, "test").is_err());
}

#[test]
fn test_sha256() {
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