I started with #60 but it yeilded draft for #88 (and the issue itself)

rust-arkworks/src/error.rs

@@ -1,11 +1,27 @@
-use thiserror::Error;
+// Legacy definitions for errors which will be gone with arkworks upgrade to `>=0.4.0`.
+// `use ark_ec::hashing::HashToCurveError;`
+pub(crate) const STUB: &str = "need upgrade to `use ark_ec::hashing::HashToCurveError;`";
+
+// use thiserror::Error;
 
 /// This is an error that could occur when running a cryptograhic primitive
-#[derive(Error, Debug, PartialEq)]
-pub enum CryptoError {
-    #[error("Cannot hash to curve")]
-    CannotHashToCurve,
-
-    #[error("Cannot encode a point not on the curve")]
-    PointNotOnCurve,
-}
+// #[derive(Error, Debug, PartialEq)]
+// pub enum CryptoError {
+    // #[error("Cannot hash to curve")]
+    // ReferenceTryAndIncrement,
+
+    // #[error("Cannot encode a point not on the curve")]
+    // PointNotOnCurve,
+// }
+
+// Let's outline what errors will be in `~0.4.0`
+#[derive(Debug, Clone)]
+pub enum HashToCurveError {
+    UnsupportedCurveError(String),
+    MapToCurveError(String),
+    /* let's add two more items to absorb everything 
+    in `crate::hash_to_curve` which is 
+    subject to deprecation */
+    Legacy,
+    ReferenceTryAndIncrement
+}

rust-arkworks/src/hash_to_curve.rs

@@ -1,30 +1,29 @@
-use crate::error::CryptoError;
+use crate::error::HashToCurveError;
 use ark_ec::short_weierstrass_jacobian::GroupAffine;
 use ark_ec::{AffineCurve, ProjectiveCurve};
 use ark_ff::FromBytes;
 use elliptic_curve::hash2curve::{ExpandMsgXmd, GroupDigest};
 use elliptic_curve::sec1::ToEncodedPoint;
-use k256::sha2::Sha256;
-use k256::AffinePoint;
-use k256::{ProjectivePoint, Secp256k1};
+// TODO why not ark libs for these? oO
+use k256::{ProjectivePoint, Secp256k1, AffinePoint, sha2::Sha256};
 use secp256k1::Sec1EncodePoint;
 use tiny_keccak::{Hasher, Shake, Xof};
 
 pub fn hash_to_curve<Fp: ark_ff::PrimeField, P: ark_ec::SWModelParameters>(
     msg: &[u8],
     pk: &GroupAffine<P>,
-) -> GroupAffine<P> {
-    let pk_encoded = pk.to_encoded_point(true);
-    let b = hex::decode(pk_encoded).unwrap();
+) -> Result<GroupAffine<P>, HashToCurveError> {
+    let b = hex::decode(
+        &pk.to_encoded_point(true)
+    ).expect(super::EXPECT_MSG_DECODE);
     let x = [msg, b.as_slice()];
     let x = x.concat().clone();
     let x = x.as_slice();
 
     let pt: ProjectivePoint = Secp256k1::hash_from_bytes::<ExpandMsgXmd<Sha256>>(
         &[x],
         b"QUUX-V01-CS02-with-secp256k1_XMD:SHA-256_SSWU_RO_",
-    )
-    .unwrap();
+    ).map_err(|_| HashToCurveError::Legacy)?;
 
     let pt_affine = pt.to_affine();
 
@@ -33,13 +32,13 @@ pub fn hash_to_curve<Fp: ark_ff::PrimeField, P: ark_ec::SWModelParameters>(
 
 pub fn k256_affine_to_arkworks_secp256k1_affine<P: ark_ec::SWModelParameters>(
     k_pt: AffinePoint,
-) -> GroupAffine<P> {
+) -> Result<GroupAffine<P>, HashToCurveError> {
     let encoded_pt = k_pt.to_encoded_point(false);
 
     let num_field_bytes = 40;
 
     // extract k_pt.x
-    let k_pt_x_bytes = encoded_pt.x().unwrap();
+    let k_pt_x_bytes = encoded_pt.x().ok_or(HashToCurveError::Legacy)?;
 
     // pad x bytes
     let mut k_pt_x_bytes_vec = vec![0u8; num_field_bytes];
@@ -50,10 +49,10 @@ pub fn k256_affine_to_arkworks_secp256k1_affine<P: ark_ec::SWModelParameters>(
         );
     }
     let reader = std::io::BufReader::new(k_pt_x_bytes_vec.as_slice());
-    let g_x = P::BaseField::read(reader).unwrap();
+    let g_x = P::BaseField::read(reader).map_err(|_| HashToCurveError::Legacy)?;
 
     // extract k_pt.y
-    let k_pt_y_bytes = encoded_pt.y().unwrap();
+    let k_pt_y_bytes = encoded_pt.y().ok_or(HashToCurveError::Legacy)?;
 
     // pad y bytes
     let mut k_pt_y_bytes_vec = vec![0u8; num_field_bytes];
@@ -65,13 +64,13 @@ pub fn k256_affine_to_arkworks_secp256k1_affine<P: ark_ec::SWModelParameters>(
     }
 
     let reader = std::io::BufReader::new(k_pt_y_bytes_vec.as_slice());
-    let g_y = P::BaseField::read(reader).unwrap();
+    let g_y = P::BaseField::read(reader).map_err(|_| HashToCurveError::Legacy)?;
 
-    GroupAffine::<P>::new(g_x, g_y, false)
+    Ok(GroupAffine::<P>::new(g_x, g_y, false))
 }
 
 /// Kobi's hash_to_curve function, here for reference only
-pub fn _try_and_increment<C: ProjectiveCurve>(msg: &[u8]) -> Result<C::Affine, CryptoError> {
+pub fn _try_and_increment<C: ProjectiveCurve>(msg: &[u8]) -> Result<C::Affine, HashToCurveError> {
     for nonce in 0u8..=255 {
         let mut h = Shake::v128();
         h.update(&[nonce]);
@@ -85,5 +84,5 @@ pub fn _try_and_increment<C: ProjectiveCurve>(msg: &[u8]) -> Result<C::Affine, C
         }
     }
 
-    Err(CryptoError::CannotHashToCurve)
+    Err(HashToCurveError::ReferenceTryAndIncrement)
 }