Merge #194

194: Fix checks on curve parameters r=jethrogb a=zugzwang

On instantiating a curve by parameters, the code checks that all
field elements are non-zero in 𝔽ₚ. This is not necessary, and forbids
useful curves such as e.g. the Barreto-Naehrig pairing-friendly curves,
of the form y² = x³ + 257 (e.g. `a = 0`), and secp256k1 (used in Bitcoin, but can be instantiated by name).

Co-authored-by: Francisco Vial-Prado <[email protected]>

Author: bors[bot]

mbedtls/src/ecp/mod.rs

@@ -79,6 +79,13 @@ impl EcGroup {
         Ok(ret)
     }
 
+    /// Initialize an EcGroup with custom group parameters.
+    ///
+    /// HAZMAT: This function DOES NOT perform a full check on parameters
+    /// against all known attacks. The caller MUST make sure that parameters are
+    /// trusted. Failing to comply with this requirement may result in the use
+    /// of INSECURE curves. Prefer [EcGroup::new] with known curves listed in
+    /// [EcGroupId].
     pub fn from_parameters(
         p: Mpi,
         a: Mpi,
@@ -96,15 +103,16 @@ impl EcGroup {
         let zero = Mpi::new(0)?;
 
         // basic bounds checking
-        if &a <= &zero
+        if &a < &zero
             || &a >= &p
-            || &b <= &zero
+            || &b < &zero
             || &b >= &p
-            || &g_x <= &zero
+            || &g_x < &zero
             || &g_x >= &p
-            || &g_y <= &zero
+            || &g_y < &zero
             || &g_y >= &p
             || &order <= &zero
+            || (&a == &zero && &b == &zero)
         {
             return Err(Error::EcpBadInputData);
         }
@@ -191,6 +199,8 @@ impl EcGroup {
         match self.group_id()? {
             EcGroupId::Curve25519 => Ok(8),
             EcGroupId::Curve448 => Ok(4),
+            // Requires a point-counting algorithm such as SEA.
+            EcGroupId::None => Err(Error::EcpFeatureUnavailable),
             _ => Ok(1),
         }
     }
@@ -754,4 +764,154 @@ mod tests {
         let pt3 = pt1.clone();
         assert_eq!(pt2.eq(&pt3).unwrap(), true);
     }
+
+    #[cfg(feature = "std")]
+    struct Params<'a> {
+        p:   &'a str,
+        a:   &'a str,
+        b:   &'a str,
+        g_x: &'a str,
+        g_y: &'a str,
+        n:   &'a str,
+    }
+
+    #[cfg(feature = "std")]
+    impl Into<super::Result<EcGroup>> for Params<'_> {
+        fn into(self) -> super::Result<EcGroup> {
+            use std::str::FromStr;
+            EcGroup::from_parameters(
+                Mpi::from_str(self.p)?,
+                Mpi::from_str(self.a)?,
+                Mpi::from_str(self.b)?,
+                Mpi::from_str(self.g_x)?,
+                Mpi::from_str(self.g_y)?,
+                Mpi::from_str(self.n)?,
+            )
+        }
+    }
+
+    #[test]
+    #[cfg(feature = "std")]
+    fn pathological_parameters() {
+        // y² = x³ mod 7 (note  a == b == 0)
+        let singular: super::Result<_> = Params {
+            p:   "0x07",
+            a:   "0x00",
+            b:   "0x00",
+            g_x: "0x01",
+            g_y: "0x02",
+            n:   "0x0b",
+        }.into();
+        assert!(singular.is_err());
+    }
+
+    #[test]
+    #[cfg(feature = "std")]
+    fn bad_generators() {
+        // y² = x³ + x + 6 (mod 7) with bad generator (1, 2) and prime order 11
+        let small_curve: super::Result<_> = Params {
+            p:   "0x07",
+            a:   "0x01",
+            b:   "0x06",
+            g_x: "0x01",
+            g_y: "0x02",
+            n:   "0x0b",
+        }.into();
+        assert!(small_curve.is_err());
+
+        // y² = x³ + x + 6 (mod 7) with bad generator (0, 0) and prime order 11
+        let small_curve_zero_gen: super::Result<_> = Params {
+            p:   "0x07",
+            a:   "0x01",
+            b:   "0x06",
+            g_x: "0x00",
+            g_y: "0x00",
+            n:   "0x0b",
+        }.into();
+        assert!(small_curve_zero_gen.is_err());
+    }
+
+    #[test]
+    #[cfg(feature = "std")]
+    fn unknown_cofactor() {
+        // y² = x³ + x + 6 (mod 7)  with generator (1, 6) and prime order 11
+        let small_curve: super::Result<_> = Params {
+            p:   "0x07",
+            a:   "0x01",
+            b:   "0x06",
+            g_x: "0x01",
+            g_y: "0x06",
+            n:   "0x0b",
+        }.into();
+        assert!(small_curve.unwrap().cofactor().is_err());
+    }
+
+    #[test]
+    #[cfg(feature = "std")]
+    fn zero_params_curves() {
+        use super::Result;
+        // Barreto-Naehrig 254, note a = 0
+        let bn254: Result<_> = Params {
+            p:   "0x2523648240000001BA344D80000000086121000000000013A700000000000013",
+            a:   "0x0000000000000000000000000000000000000000000000000000000000000000",
+            b:   "0x0000000000000000000000000000000000000000000000000000000000000002",
+            g_x: "0x2523648240000001BA344D80000000086121000000000013A700000000000012",
+            g_y: "0x0000000000000000000000000000000000000000000000000000000000000001",
+            n:   "0x2523648240000001BA344D8000000007FF9F800000000010A10000000000000D",
+        }.into();
+        assert!(bn254.is_ok());
+
+        // Prescribed embedded degree of 12, BLS12-381
+        let bls12_381: Result<_> = Params {
+            p:   "0x1a0111ea397fe69a4b1ba7b6434bacd764774b84f38512bf6730d2a0f6b0f6241eabfffeb153ffffb9feffffffffaaab",
+            a:   "0x00",
+            b:   "0x04",
+            g_x: "0x17F1D3A73197D7942695638C4FA9AC0FC3688C4F9774B905A14E3A3F171BAC586C55E83FF97A1AEFFB3AF00ADB22C6BB",
+            g_y: "0x08B3F481E3AAA0F1A09E30ED741D8AE4FCF5E095D5D00AF600DB18CB2C04B3EDD03CC744A2888AE40CAA232946C5E7E1",
+            n:   "0x73EDA753299D7D483339D80809A1D80553BDA402FFFE5BFEFFFFFFFF00000001",
+        }.into();
+        assert!(bls12_381.is_ok());
+
+        // Fp256BN
+        let fp256_bn: Result<_> = Params {
+            p:   "0xfffffffffffcf0cd46e5f25eee71a49f0cdc65fb12980a82d3292ddbaed33013",
+            a:   "0x00",
+            b:   "0x03",
+            g_x: "0x01",
+            g_y: "0x02",
+            n:   "0xfffffffffffcf0cd46e5f25eee71a49e0cdc65fb1299921af62d536cd10b500d",
+        }.into();
+        assert!(fp256_bn.is_ok());
+
+        // id-GostR3410-2001-CryptoPro-C-ParamSet, note g_x = 0
+        let gost_r3410: Result<_> = Params {
+            p:   "0x9b9f605f5a858107ab1ec85e6b41c8aacf846e86789051d37998f7b9022d759b",
+            a:   "0x9b9f605f5a858107ab1ec85e6b41c8aacf846e86789051d37998f7b9022d7598",
+            b:   "0x805a",
+            g_x: "0x00",
+            g_y: "0x41ece55743711a8c3cbf3783cd08c0ee4d4dc440d4641a8f366e550dfdb3bb67",
+            n:   "0x9b9f605f5a858107ab1ec85e6b41c8aa582ca3511eddfb74f02f3a6598980bb9",
+        }.into();
+        assert!(gost_r3410.is_ok());
+
+        // secp256k1 (Bitcoin), note a = 0
+        let my_secp256k1: Result<EcGroup> = Params {
+            p:   "0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f",
+            a:   "0x0000000000000000000000000000000000000000000000000000000000000000",
+            b:   "0x0000000000000000000000000000000000000000000000000000000000000007",
+            g_x: "0x79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798",
+            g_y: "0x483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8",
+            n:   "0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141",
+        }.into();
+        assert!(my_secp256k1.is_ok());
+        let my_secp256k1 = my_secp256k1.unwrap();
+
+        // We compare against the known SecP256K1
+        let secp256k1 = EcGroup::new(EcGroupId::SecP256K1).unwrap();
+        assert!(my_secp256k1.p()         == secp256k1.p());
+        assert!(my_secp256k1.a()         == secp256k1.a());
+        assert!(my_secp256k1.b()         == secp256k1.b());
+        assert!(my_secp256k1.generator() == secp256k1.generator());
+        assert!(my_secp256k1.order()     == secp256k1.order());
+    }
 }