Skip to content

x/crypto/ssh: validate DSA parameters in ParseDSAPrivateKey #365

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
kingljl wants to merge 1 commit into
base: master
Choose a base branch
from
+149 −7
Open

x/crypto/ssh: validate DSA parameters in ParseDSAPrivateKey #365

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
35 changes: 28 additions & 7 deletions ssh/keys.go
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1431,7 +1431,8 @@ func ParseRawPrivateKeyWithPassphrase(pemBytes, passphrase []byte) (interface{},
}

// ParseDSAPrivateKey returns a DSA private key from its ASN.1 DER encoding, as
// specified by the OpenSSL DSA man page.
// specified by the OpenSSL DSA man page. The key must use parameter size
// L1024N160; keys with other parameter sizes are rejected.
func ParseDSAPrivateKey(der []byte) (*dsa.PrivateKey, error) {
var k struct {
Version int
Expand All @@ -1449,14 +1450,34 @@ func ParseDSAPrivateKey(der []byte) (*dsa.PrivateKey, error) {
return nil, errors.New("ssh: garbage after DSA key")
}

param := dsa.Parameters{
P: k.P,
Q: k.Q,
G: k.G,
}
if err := checkDSAParams(&param); err != nil {
return nil, err
}

// The public value Y must be a non-zero element of the group, i.e.
// strictly between 0 and P. crypto/dsa.Verify does not range-check Y,
// so we reject out-of-range values here to prevent a maliciously
// oversized Y from slowing verification.
if k.Pub.Sign() <= 0 || k.Pub.Cmp(k.P) >= 0 {
return nil, errors.New("ssh: DSA public value Y out of range")
}

// The private value X must be in the range [1, Q-1]. A validly
// generated key always satisfies this, so rejecting out-of-range
// values only guards against maliciously oversized X slowing signing.
if k.Priv.Sign() <= 0 || k.Priv.Cmp(k.Q) >= 0 {
return nil, errors.New("ssh: DSA private value X out of range")
}

return &dsa.PrivateKey{
PublicKey: dsa.PublicKey{
Parameters: dsa.Parameters{
P: k.P,
Q: k.Q,
G: k.G,
},
Y: k.Pub,
Parameters: param,
Y: k.Pub,
},
X: k.Priv,
}, nil
Expand Down
121 changes: 121 additions & 0 deletions ssh/keys_test.go
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@ import (
"crypto/rsa"
"crypto/sha256"
"crypto/x509"
"encoding/asn1"
"encoding/base64"
"encoding/hex"
"encoding/pem"
Expand Down Expand Up @@ -671,6 +672,126 @@ func TestParseDSAYOutOfRange(t *testing.T) {
}
}

func TestParseDSAPrivateKeyValidation(t *testing.T) {
// Valid 1024/160 parameters (values don't need to be a real DSA group,
// they only need to pass the checkDSAParams bit-length checks and the
// G < P / G > 0 checks).
P := new(big.Int).Lsh(big.NewInt(1), 1023)
P.SetBit(P, 0, 1)
Q := new(big.Int).Lsh(big.NewInt(1), 159)
Q.SetBit(Q, 0, 1)
G := big.NewInt(2)
Y := big.NewInt(5)
X := big.NewInt(7)

type dsaKeyASN1 struct {
Version int
P, Q, G, Pub, Priv *big.Int
}

for _, tc := range []struct {
name string
P, Q, G, Y, X *big.Int
expectedErr string
}{
{
name: "huge Q",
P: P, Q: new(big.Int).Lsh(big.NewInt(1), 20000), G: G, Y: Y, X: X,
expectedErr: "ssh: unsupported DSA sub-prime size",
},
{
name: "P not 1024 bits",
P: new(big.Int).Lsh(big.NewInt(1), 1024),
Q: Q, G: G, Y: Y, X: X,
expectedErr: "ssh: unsupported DSA key size",
},
{
name: "Y zero",
P: P, Q: Q, G: G, Y: big.NewInt(0), X: X,
expectedErr: "ssh: DSA public value Y out of range",
},
{
name: "Y negative",
P: P, Q: Q, G: G, Y: big.NewInt(-1), X: X,
expectedErr: "ssh: DSA public value Y out of range",
},
{
name: "Y equals P",
P: P, Q: Q, G: G, Y: new(big.Int).Set(P), X: X,
expectedErr: "ssh: DSA public value Y out of range",
},
{
name: "Y greater than P",
P: P, Q: Q, G: G, Y: new(big.Int).Add(P, big.NewInt(1)), X: X,
expectedErr: "ssh: DSA public value Y out of range",
},
{
name: "G equals P",
P: P, Q: Q, G: new(big.Int).Set(P), Y: Y, X: X,
expectedErr: "ssh: DSA generator larger than modulus",
},
{
name: "G greater than P",
P: P, Q: Q, G: new(big.Int).Add(P, big.NewInt(1)), Y: Y, X: X,
expectedErr: "ssh: DSA generator larger than modulus",
},
{
name: "G zero",
P: P, Q: Q, G: big.NewInt(0), Y: Y, X: X,
expectedErr: "ssh: DSA generator must be positive",
},
{
name: "G negative",
P: P, Q: Q, G: big.NewInt(-1), Y: Y, X: X,
expectedErr: "ssh: DSA generator must be positive",
},
{
name: "X zero",
P: P, Q: Q, G: G, Y: Y, X: big.NewInt(0),
expectedErr: "ssh: DSA private value X out of range",
},
{
name: "X negative",
P: P, Q: Q, G: G, Y: Y, X: big.NewInt(-1),
expectedErr: "ssh: DSA private value X out of range",
},
{
name: "X equals Q",
P: P, Q: Q, G: G, Y: Y, X: new(big.Int).Set(Q),
expectedErr: "ssh: DSA private value X out of range",
},
{
name: "X greater than Q",
P: P, Q: Q, G: G, Y: Y, X: new(big.Int).Add(Q, big.NewInt(1)),
expectedErr: "ssh: DSA private value X out of range",
},
} {
t.Run(tc.name, func(t *testing.T) {
der, err := asn1.Marshal(dsaKeyASN1{0, tc.P, tc.Q, tc.G, tc.Y, tc.X})
if err != nil {
t.Fatalf("asn1.Marshal failed: %v", err)
}
_, err = ParseDSAPrivateKey(der)
if err == nil {
t.Fatal("ParseDSAPrivateKey accepted invalid DSA key")
}
if !strings.Contains(err.Error(), tc.expectedErr) {
t.Errorf("unexpected error message: got %q, want substring %q", err.Error(), tc.expectedErr)
}
})
}

// Valid key should parse successfully.
der, err := asn1.Marshal(dsaKeyASN1{0, P, Q, G, Y, X})
if err != nil {
t.Fatalf("asn1.Marshal failed: %v", err)
}
_, err = ParseDSAPrivateKey(der)
if err != nil {
t.Fatalf("ParseDSAPrivateKey rejected valid DSA key: %v", err)
}
}

func TestMarshalPrivateKey(t *testing.T) {
tests := []struct {
name string
Expand Down