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Add more robust key processing: DSA #14992

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f231f16
Add more robust processing for DSA keys.
sjudson Jun 12, 2026
997469f
Move all errors up front and move checks to consistent level.
sjudson Jun 16, 2026
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163 changes: 115 additions & 48 deletions src/rust/src/backend/dsa.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,8 @@
// 2.0, and the BSD License. See the LICENSE file in the root of this repository
// for complete details.

use std::cmp::Ordering;

use pyo3::types::PyAnyMethods;

use crate::backend::utils;
Expand Down Expand Up @@ -38,18 +40,22 @@ struct DsaParameters {

pub(crate) fn private_key_from_pkey(
pkey: &openssl::pkey::PKeyRef<openssl::pkey::Private>,
) -> DsaPrivateKey {
DsaPrivateKey {
) -> CryptographyResult<DsaPrivateKey> {
let dsa = pkey.dsa()?;
check_dsa_private_key(dsa.p(), dsa.q(), dsa.g(), dsa.pub_key(), dsa.priv_key())?;
Ok(DsaPrivateKey {
pkey: pkey.to_owned(),
}
})
}

pub(crate) fn public_key_from_pkey(
pkey: &openssl::pkey::PKeyRef<openssl::pkey::Public>,
) -> DsaPublicKey {
DsaPublicKey {
) -> CryptographyResult<DsaPublicKey> {
let dsa = pkey.dsa()?;
check_dsa_public_key(dsa.p(), dsa.q(), dsa.g(), dsa.pub_key())?;
Ok(DsaPublicKey {
pkey: pkey.to_owned(),
}
})
}

#[pyo3::pyfunction]
Expand Down Expand Up @@ -270,67 +276,128 @@ fn check_dsa_parameters(
py: pyo3::Python<'_>,
parameters: &DsaParameterNumbers,
) -> CryptographyResult<()> {
if ![1024, 2048, 3072, 4096].contains(
&parameters
.p
.bind(py)
.call_method0("bit_length")?
.extract::<usize>()?,
) {
return Err(CryptographyError::from(
pyo3::exceptions::PyValueError::new_err(
"p must be exactly 1024, 2048, 3072, or 4096 bits long",
),
let p = py_int_to_signed_bn(py, parameters.p.bind(py))?;
let q = py_int_to_signed_bn(py, parameters.q.bind(py))?;
let g = py_int_to_signed_bn(py, parameters.g.bind(py))?;

check_dsa_parameters_bignums(&p, &q, &g)
}

fn dsa_value_error(message: &'static str) -> CryptographyError {
CryptographyError::from(pyo3::exceptions::PyValueError::new_err(message))
}

fn py_int_to_signed_bn(
py: pyo3::Python<'_>,
value: &pyo3::Bound<'_, pyo3::PyAny>,
) -> CryptographyResult<openssl::bn::BigNum> {
let negative = value.lt(0)?;
let magnitude = value.call_method0(pyo3::intern!(py, "__abs__"))?;
let mut bn = utils::py_int_to_bn(py, &magnitude)?;
bn.set_negative(negative);
Ok(bn)
}

fn check_dsa_parameters_bignums(
p: &openssl::bn::BigNumRef,
q: &openssl::bn::BigNumRef,
g: &openssl::bn::BigNumRef,
) -> CryptographyResult<()> {
if p.is_negative() || ![1024, 2048, 3072, 4096].contains(&(p.num_bits() as usize)) {
return Err(dsa_value_error(
"p must be exactly 1024, 2048, 3072, or 4096 bits long",
));
}

if ![160, 224, 256].contains(
&parameters
.q
.bind(py)
.call_method0("bit_length")?
.extract::<usize>()?,
) {
return Err(CryptographyError::from(
pyo3::exceptions::PyValueError::new_err("q must be exactly 160, 224, or 256 bits long"),
if q.is_negative() || ![160, 224, 256].contains(&(q.num_bits() as usize)) {
return Err(dsa_value_error(
"q must be exactly 160, 224, or 256 bits long",
));
}

if parameters.g.bind(py).le(1)? || parameters.g.bind(py).ge(parameters.p.bind(py))? {
return Err(CryptographyError::from(
pyo3::exceptions::PyValueError::new_err("g, p don't satisfy 1 < g < p."),
));
let one = openssl::bn::BigNum::from_u32(1)?;
if g.cmp(&one) != Ordering::Greater || g.cmp(p) != Ordering::Less {
return Err(dsa_value_error("g, p don't satisfy 1 < g < p."));
}

Ok(())
}

fn check_dsa_private_numbers(
py: pyo3::Python<'_>,
numbers: &DsaPrivateNumbers,
fn check_dsa_public_key(
p: &openssl::bn::BigNumRef,
q: &openssl::bn::BigNumRef,
g: &openssl::bn::BigNumRef,
y: &openssl::bn::BigNumRef,
) -> CryptographyResult<()> {
let params = numbers.public_numbers.get().parameter_numbers.get();
check_dsa_parameters(py, params)?;
check_dsa_parameters_bignums(p, q, g)?;

if numbers.x.bind(py).le(0)? || numbers.x.bind(py).ge(params.q.bind(py))? {
return Err(CryptographyError::from(
pyo3::exceptions::PyValueError::new_err("x must be > 0 and < q."),
));
let one = openssl::bn::BigNum::from_u32(1)?;
if y.cmp(&one) != Ordering::Greater || y.cmp(p) != Ordering::Less {
return Err(dsa_value_error("y must be > 1 and < p."));
}

if (**numbers.public_numbers.get().y.bind(py)).ne(params
.g
.bind(py)
.pow(numbers.x.bind(py), Some(params.p.bind(py)))?)?
{
return Err(CryptographyError::from(
pyo3::exceptions::PyValueError::new_err("y must be equal to (g ** x % p)."),
));
let mut ctx = openssl::bn::BigNumContext::new()?;
let mut result = openssl::bn::BigNum::new()?;
result.mod_exp(y, q, p, &mut ctx)?;
if result.cmp(&one) != Ordering::Equal {
return Err(dsa_value_error("y ** q mod p must be 1."));
}

Ok(())
}

fn check_dsa_private_key(
p: &openssl::bn::BigNumRef,
q: &openssl::bn::BigNumRef,
g: &openssl::bn::BigNumRef,
y: &openssl::bn::BigNumRef,
x: &openssl::bn::BigNumRef,
) -> CryptographyResult<()> {
check_dsa_public_key(p, q, g, y)?;

let zero = openssl::bn::BigNum::from_u32(0)?;
if x.cmp(&zero) != Ordering::Greater || x.cmp(q) != Ordering::Less {
return Err(dsa_value_error("x must be > 0 and < q."));
}

let mut ctx = openssl::bn::BigNumContext::new()?;
let mut expected_y = openssl::bn::BigNum::new()?;
expected_y.mod_exp(g, x, p, &mut ctx)?;
if y.cmp(&expected_y) != Ordering::Equal {
return Err(dsa_value_error("y must be equal to (g ** x % p)."));
}

Ok(())
}

fn check_dsa_public_numbers(
py: pyo3::Python<'_>,
numbers: &DsaPublicNumbers,
) -> CryptographyResult<()> {
let params = numbers.parameter_numbers.get();
let p = py_int_to_signed_bn(py, params.p.bind(py))?;
let q = py_int_to_signed_bn(py, params.q.bind(py))?;
let g = py_int_to_signed_bn(py, params.g.bind(py))?;
let y = py_int_to_signed_bn(py, numbers.y.bind(py))?;

check_dsa_public_key(&p, &q, &g, &y)
}

fn check_dsa_private_numbers(
py: pyo3::Python<'_>,
numbers: &DsaPrivateNumbers,
) -> CryptographyResult<()> {
let public_numbers = numbers.public_numbers.get();
let params = public_numbers.parameter_numbers.get();
let p = py_int_to_signed_bn(py, params.p.bind(py))?;
let q = py_int_to_signed_bn(py, params.q.bind(py))?;
let g = py_int_to_signed_bn(py, params.g.bind(py))?;
let y = py_int_to_signed_bn(py, public_numbers.y.bind(py))?;
let x = py_int_to_signed_bn(py, numbers.x.bind(py))?;

check_dsa_private_key(&p, &q, &g, &y, &x)
}

#[pyo3::pyclass(
frozen,
module = "cryptography.hazmat.primitives.asymmetric.dsa",
Expand Down Expand Up @@ -440,7 +507,7 @@ impl DsaPublicNumbers {

let parameter_numbers = self.parameter_numbers.get();

check_dsa_parameters(py, parameter_numbers)?;
check_dsa_public_numbers(py, self)?;

let dsa = openssl::dsa::Dsa::from_public_components(
utils::py_int_to_bn(py, parameter_numbers.p.bind(py))?,
Expand Down
4 changes: 2 additions & 2 deletions src/rust/src/backend/keys.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -165,7 +165,7 @@ fn private_key_from_pkey<'p>(
openssl::pkey::Id::ED448 => Ok(crate::backend::ed448::private_key_from_pkey(pkey)
.into_pyobject(py)?
.into_any()),
openssl::pkey::Id::DSA => Ok(crate::backend::dsa::private_key_from_pkey(pkey)
openssl::pkey::Id::DSA => Ok(crate::backend::dsa::private_key_from_pkey(pkey)?
.into_pyobject(py)?
.into_any()),
openssl::pkey::Id::DH => Ok(crate::backend::dh::private_key_from_pkey(pkey)
Expand Down Expand Up @@ -363,7 +363,7 @@ fn public_key_from_pkey<'p>(
.into_pyobject(py)?
.into_any()),

openssl::pkey::Id::DSA => Ok(crate::backend::dsa::public_key_from_pkey(pkey)
openssl::pkey::Id::DSA => Ok(crate::backend::dsa::public_key_from_pkey(pkey)?
.into_pyobject(py)?
.into_any()),
openssl::pkey::Id::DH => Ok(crate::backend::dh::public_key_from_pkey(pkey)
Expand Down
74 changes: 59 additions & 15 deletions tests/hazmat/primitives/test_dsa.py
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Original file line number Diff line number Diff line change
Expand Up @@ -270,6 +270,24 @@ def test_invalid_parameters_values(self, p, q, g, backend):
DSA_KEY_1024.public_numbers.y,
2**200,
),
(
DSA_KEY_1024.public_numbers.parameter_numbers.p,
DSA_KEY_1024.public_numbers.parameter_numbers.q,
DSA_KEY_1024.public_numbers.parameter_numbers.g,
-1,
DSA_KEY_1024.x,
),
(
DSA_KEY_1024.public_numbers.parameter_numbers.p,
DSA_KEY_1024.public_numbers.parameter_numbers.q,
DSA_KEY_1024.public_numbers.parameter_numbers.g,
pow(
DSA_KEY_1024.public_numbers.parameter_numbers.g,
DSA_KEY_1024.x + 1,
DSA_KEY_1024.public_numbers.parameter_numbers.p,
),
DSA_KEY_1024.x,
),
(
DSA_KEY_1024.public_numbers.parameter_numbers.p,
DSA_KEY_1024.public_numbers.parameter_numbers.q,
Expand Down Expand Up @@ -352,6 +370,30 @@ def test_invalid_dsa_private_key_arguments(self, p, q, g, y, x, backend):
2**1200,
DSA_KEY_1024.public_numbers.y,
),
(
DSA_KEY_1024.public_numbers.parameter_numbers.p,
DSA_KEY_1024.public_numbers.parameter_numbers.q,
DSA_KEY_1024.public_numbers.parameter_numbers.g,
-1,
),
(
DSA_KEY_1024.public_numbers.parameter_numbers.p,
DSA_KEY_1024.public_numbers.parameter_numbers.q,
DSA_KEY_1024.public_numbers.parameter_numbers.g,
1,
),
(
DSA_KEY_1024.public_numbers.parameter_numbers.p,
DSA_KEY_1024.public_numbers.parameter_numbers.q,
DSA_KEY_1024.public_numbers.parameter_numbers.g,
DSA_KEY_1024.public_numbers.parameter_numbers.p + 1,
),
(
DSA_KEY_1024.public_numbers.parameter_numbers.p,
DSA_KEY_1024.public_numbers.parameter_numbers.q,
DSA_KEY_1024.public_numbers.parameter_numbers.g,
DSA_KEY_1024.public_numbers.parameter_numbers.p - 1,
),
],
)
def test_invalid_dsa_public_key_arguments(self, p, q, g, y, backend):
Expand Down Expand Up @@ -478,12 +520,17 @@ def test_dsa_verification(self, backend, subtests):
backend, algorithm, vector["p"], vector["q"], vector["g"]
)

public_key = dsa.DSAPublicNumbers(
parameter_numbers=dsa.DSAParameterNumbers(
vector["p"], vector["q"], vector["g"]
),
y=vector["y"],
).public_key(backend)
try:
public_key = dsa.DSAPublicNumbers(
parameter_numbers=dsa.DSAParameterNumbers(
vector["p"], vector["q"], vector["g"]
),
y=vector["y"],
).public_key(backend)
except ValueError:
assert vector["result"] == "F"
continue

sig = encode_dss_signature(vector["r"], vector["s"])

if vector["result"] == "F":
Expand Down Expand Up @@ -610,16 +657,13 @@ def test_prehashed_digest_mismatch(self, backend):
skip_message="Requires OpenSSL 3.0.9+, LibreSSL, BoringSSL, or AWS-LC",
)
def test_nilpotent(self):
key = load_vectors_from_file(
os.path.join("asymmetric", "DSA", "custom", "nilpotent.pem"),
lambda pemfile: serialization.load_pem_private_key(
pemfile.read().encode(), password=None
),
)
assert isinstance(key, dsa.DSAPrivateKey)

with pytest.raises(ValueError):
key.sign(b"anything", hashes.SHA256())
load_vectors_from_file(
os.path.join("asymmetric", "DSA", "custom", "nilpotent.pem"),
lambda pemfile: serialization.load_pem_private_key(
pemfile.read().encode(), password=None
),
)


class TestDSANumbers:
Expand Down
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