Non-canonical infinity point & bad flags in BLS12-381 serialization should fail (#176)

Co-authored-by: Kevaundray Wedderburn <kevtheappdev@gmail.com>

CHANGELOG.md

@@ -14,6 +14,8 @@
 
 ### Bugfixes
 
+- [\#176](https://github.com/arkworks-rs/curves/pull/176) Non-canonical infinity point and bad flags in BLS12-381 serialization should fail.
+
 ## v0.4.0
 - [\#76](https://github.com/arkworks-rs/curves/pull/76) twisted Edwards parameters for bls12-377
 - Fixed curve benches

bls12_381/src/curves/g1.rs

@@ -180,6 +180,7 @@ mod test {
 
     use super::*;
     use crate::g1;
+    use ark_serialize::CanonicalDeserialize;
     use ark_std::{rand::Rng, UniformRand};
 
     fn sample_unchecked() -> Affine<g1::Config> {
@@ -204,4 +205,55 @@ mod test {
             assert!(p.is_in_correct_subgroup_assuming_on_curve());
         }
     }
+
+    #[test]
+    fn non_canonical_identity_point() {
+        let non_canonical_hex = "c01000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000";
+        let non_canonical_bytes = hex::decode(non_canonical_hex).unwrap();
+        assert_eq!(non_canonical_bytes.len(), 48);
+
+        let maybe_affine_point: Result<G1Affine, ark_serialize::SerializationError> =
+            CanonicalDeserialize::deserialize_compressed(&non_canonical_bytes[..]);
+
+        assert!(maybe_affine_point.is_err());
+
+        let non_canonical_hex_uncompressed = "c00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001";
+        let non_canonical_bytes = hex::decode(non_canonical_hex_uncompressed).unwrap();
+        assert_eq!(non_canonical_bytes.len(), 96);
+
+        let maybe_affine_point: Result<G1Affine, ark_serialize::SerializationError> =
+            CanonicalDeserialize::deserialize_uncompressed(&non_canonical_bytes[..]);
+
+        assert!(maybe_affine_point.is_err())
+    }
+
+    #[test]
+    fn bad_flag_combination() {
+        // See https://github.com/zkcrypto/pairing/tree/fa8103764a07bd273927447d434de18aace252d3/src/bls12_381#serialization
+        // - Bit 1 is compressed/uncompressed
+        // - Bit 2 is infinity
+        // - Bit 3 is lexicographical order for compressed point deserialization
+        // Hence `0b1110` ("e" in hex) or `0b0110` ("6" in hex") are both nonsensical.
+
+        // uncompressed, but lexicographically largest flag is set
+        let non_canonical_hex = "600000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000";
+        let non_canonical_bytes = hex::decode(non_canonical_hex).unwrap();
+        assert_eq!(non_canonical_bytes.len(), 48);
+
+        let maybe_affine_point: Result<G1Affine, ark_serialize::SerializationError> =
+            CanonicalDeserialize::deserialize_compressed(&non_canonical_bytes[..]);
+
+        assert!(maybe_affine_point.is_err());
+
+        // compressed, but infinity flag is set and lexicographically largest flag is
+        // set
+        let non_canonical_hex_2 = "e00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000";
+
+        let non_canonical_bytes = hex::decode(non_canonical_hex_2).unwrap();
+        assert_eq!(non_canonical_bytes.len(), 48);
+
+        let maybe_affine_point: Result<G1Affine, ark_serialize::SerializationError> =
+            CanonicalDeserialize::deserialize_compressed(&non_canonical_bytes[..]);
+        assert!(maybe_affine_point.is_err());
+    }
 }

bls12_381/src/curves/util.rs

@@ -14,17 +14,28 @@ pub struct EncodingFlags {
 }
 
 impl EncodingFlags {
-    pub fn get_flags(bytes: &[u8]) -> Self {
+    /// Fetches the flags from the byte-string
+    pub fn get_flags(bytes: &[u8]) -> Result<Self, SerializationError> {
         let compression_flag_set = (bytes[0] >> 7) & 1;
         let infinity_flag_set = (bytes[0] >> 6) & 1;
         let sort_flag_set = (bytes[0] >> 5) & 1;
 
-        Self {
-            is_compressed: compression_flag_set == 1,
-            is_infinity: infinity_flag_set == 1,
-            is_lexographically_largest: sort_flag_set == 1,
+        let is_compressed = compression_flag_set == 1;
+        let is_infinity = infinity_flag_set == 1;
+        let is_lexographically_largest = sort_flag_set == 1;
+
+        if is_lexographically_largest && (!is_compressed || is_infinity) {
+            return Err(SerializationError::InvalidData);
         }
+
+        Ok(Self {
+            is_compressed,
+            is_infinity,
+            is_lexographically_largest,
+        })
     }
+
+    /// Encodes the flags into the byte-string
     pub fn encode_flags(&self, bytes: &mut [u8]) {
         if self.is_compressed {
             bytes[0] |= 1 << 7;
@@ -38,6 +49,13 @@ impl EncodingFlags {
             bytes[0] |= 1 << 5;
         }
     }
+
+    /// Removes the flags from the byte-string.
+    ///
+    /// This reverses the effects of `encode_flags`.
+    pub fn remove_flags(bytes: &mut [u8]) {
+        bytes[0] &= 0b0001_1111;
+    }
 }
 
 pub(crate) fn deserialize_fq(bytes: [u8; 48]) -> Option<Fq> {
@@ -71,20 +89,15 @@ pub(crate) fn serialize_fq(field: Fq) -> [u8; 48] {
     result
 }
 
-pub(crate) fn read_fq_with_offset(
-    bytes: &[u8],
-    offset: usize,
-    mask: bool,
-) -> Result<Fq, ark_serialize::SerializationError> {
+fn read_bytes_with_offset(bytes: &[u8], offset: usize, mask: bool) -> [u8; G1_SERIALIZED_SIZE] {
     let mut tmp = [0; G1_SERIALIZED_SIZE];
     // read `G1_SERIALIZED_SIZE` bytes
     tmp.copy_from_slice(&bytes[offset * G1_SERIALIZED_SIZE..G1_SERIALIZED_SIZE * (offset + 1)]);
 
     if mask {
-        // Mask away the flag bits
-        tmp[0] &= 0b0001_1111;
+        EncodingFlags::remove_flags(&mut tmp);
     }
-    deserialize_fq(tmp).ok_or(SerializationError::InvalidData)
+    tmp
 }
 
 pub(crate) fn read_g1_compressed<R: ark_serialize::Read>(
@@ -97,20 +110,26 @@ pub(crate) fn read_g1_compressed(
         .ok_or(SerializationError::InvalidData)?;
 
     // Obtain the three flags from the start of the byte sequence
-    let flags = EncodingFlags::get_flags(&bytes[..]);
+    let flags = EncodingFlags::get_flags(&bytes[..])?;
 
-    // we expect to be deserializing a compressed point
+    // We expect to be deserializing a compressed point
     if !flags.is_compressed {
         return Err(SerializationError::UnexpectedFlags);
     }
 
+    // Attempt to obtain the x-coordinate
+    let x_bytes = read_bytes_with_offset(&bytes, 0, true);
+
     if flags.is_infinity {
+        // Check that the `x` co-ordinate was `0`
+        if x_bytes != [0u8; 48] {
+            return Err(SerializationError::InvalidData);
+        }
+
         return Ok(G1Affine::zero());
     }
 
-    // Attempt to obtain the x-coordinate
-    let x = read_fq_with_offset(&bytes, 0, true)?;
-
+    let x = deserialize_fq(x_bytes).ok_or(SerializationError::InvalidData)?;
     let p = G1Affine::get_point_from_x_unchecked(x, flags.is_lexographically_largest)
         .ok_or(SerializationError::InvalidData)?;
 
@@ -126,22 +145,27 @@ pub(crate) fn read_g1_uncompressed(
         .map_err(|_| SerializationError::InvalidData)?;
 
     // Obtain the three flags from the start of the byte sequence
-    let flags = EncodingFlags::get_flags(&bytes[..]);
+    let flags = EncodingFlags::get_flags(&bytes[..])?;
 
     // we expect to be deserializing an uncompressed point
     if flags.is_compressed {
         return Err(SerializationError::UnexpectedFlags);
     }
 
+    let x_bytes = read_bytes_with_offset(&bytes, 0, true);
+    let y_bytes = read_bytes_with_offset(&bytes, 1, false);
+
     if flags.is_infinity {
+        if x_bytes != [0u8; 48] || y_bytes != [0u8; 48] {
+            return Err(SerializationError::InvalidData);
+        }
         return Ok(G1Affine::zero());
     }
 
     // Attempt to obtain the x-coordinate
-    let x = read_fq_with_offset(&bytes, 0, true)?;
+    let x = deserialize_fq(x_bytes).ok_or(SerializationError::InvalidData)?;
     // Attempt to obtain the y-coordinate
-    let y = read_fq_with_offset(&bytes, 1, false)?;
-
+    let y = deserialize_fq(y_bytes).ok_or(SerializationError::InvalidData)?;
     let p = G1Affine::new_unchecked(x, y);
 
     Ok(p)
@@ -156,21 +180,26 @@ pub(crate) fn read_g2_compressed(
         .map_err(|_| SerializationError::InvalidData)?;
 
     // Obtain the three flags from the start of the byte sequence
-    let flags = EncodingFlags::get_flags(&bytes);
+    let flags = EncodingFlags::get_flags(&bytes)?;
 
     // we expect to be deserializing a compressed point
     if !flags.is_compressed {
         return Err(SerializationError::UnexpectedFlags);
     }
 
+    let xc1_bytes = read_bytes_with_offset(&bytes, 0, true);
+    let xc0_bytes = read_bytes_with_offset(&bytes, 1, false);
+
     if flags.is_infinity {
+        if xc1_bytes != [0u8; 48] || xc0_bytes != [0u8; 48] {
+            return Err(SerializationError::InvalidData);
+        }
         return Ok(G2Affine::zero());
     }
 
     // Attempt to obtain the x-coordinate
-    let xc1 = read_fq_with_offset(&bytes, 0, true)?;
-    let xc0 = read_fq_with_offset(&bytes, 1, false)?;
-
+    let xc1 = deserialize_fq(xc1_bytes).ok_or(SerializationError::InvalidData)?;
+    let xc0 = deserialize_fq(xc0_bytes).ok_or(SerializationError::InvalidData)?;
     let x = Fq2::new(xc0, xc1);
 
     let p = G2Affine::get_point_from_x_unchecked(x, flags.is_lexographically_largest)
@@ -188,25 +217,39 @@ pub(crate) fn read_g2_uncompressed(
         .map_err(|_| SerializationError::InvalidData)?;
 
     // Obtain the three flags from the start of the byte sequence
-    let flags = EncodingFlags::get_flags(&bytes);
+    let flags = EncodingFlags::get_flags(&bytes)?;
 
     // we expect to be deserializing an uncompressed point
     if flags.is_compressed {
         return Err(SerializationError::UnexpectedFlags);
     }
 
+    let xc1_bytes = read_bytes_with_offset(&bytes, 0, true);
+    let xc0_bytes = read_bytes_with_offset(&bytes, 1, false);
+
+    let yc1_bytes = read_bytes_with_offset(&bytes, 2, false);
+    let yc0_bytes = read_bytes_with_offset(&bytes, 3, false);
+
     if flags.is_infinity {
+        if xc1_bytes != [0u8; 48]
+            || xc0_bytes != [0u8; 48]
+            || yc1_bytes != [0u8; 48]
+            || yc0_bytes != [0u8; 48]
+        {
+            return Err(SerializationError::InvalidData);
+        }
         return Ok(G2Affine::zero());
     }
 
+    let xc1 = deserialize_fq(xc1_bytes).ok_or(SerializationError::InvalidData)?;
+    let xc0 = deserialize_fq(xc0_bytes).ok_or(SerializationError::InvalidData)?;
+    let yc1 = deserialize_fq(yc1_bytes).ok_or(SerializationError::InvalidData)?;
+    let yc0 = deserialize_fq(yc0_bytes).ok_or(SerializationError::InvalidData)?;
+
     // Attempt to obtain the x-coordinate
-    let xc1 = read_fq_with_offset(&bytes, 0, true)?;
-    let xc0 = read_fq_with_offset(&bytes, 1, false)?;
     let x = Fq2::new(xc0, xc1);
 
     // Attempt to obtain the y-coordinate
-    let yc1 = read_fq_with_offset(&bytes, 2, false)?;
-    let yc0 = read_fq_with_offset(&bytes, 3, false)?;
     let y = Fq2::new(yc0, yc1);
 
     let p = G2Affine::new_unchecked(x, y);