Malfunctioning example

Some reference code to be saved at this point. It has two properties
which are have to be dropped.
* I struggled to preserve `CurveGroup::Affine` output, but seems
there's no safe and generic way to do it.
* Also tried to add `const fn` to switch to arrays, but I'm just
not there yet.

rust-arkworks/examples/reference_hash_to_curve.rs

@@ -0,0 +1,60 @@
+// use ark_ec::{AffineCurve, ProjectiveCurve};
+use ark_ec::{AffineRepr, CurveGroup, CurveConfig, short_weierstrass::SWCurveConfig};
+use tiny_keccak::{Hasher, Shake, Xof};
+use elliptic_curve::hash2curve::{ExpandMsgXmd, GroupDigest};
+use k256::AffinePoint;
+use k256::sha2::Sha256;
+use elliptic_curve::sec1::ToEncodedPoint;
+// use ark_ec::short_weierstrass_jacobian::GroupAffine;
+use k256::{ProjectivePoint, Secp256k1};
+use ark_ff::{BigInteger, BigInt, fields::{Field, PrimeField}};
+
+const fn digest_len(modulus_bit_size: u32) -> usize {(modulus_bit_size/8 + if modulus_bit_size % 8 != 0 {1} else {0}) as usize}
+const fn bigint_len(bytearray_size: usize) -> usize {bytearray_size/8 + if bytearray_size % 8 != 0 {1} else {0}}
+
+/// Kobi's hash_to_curve function, here for reference only
+struct NaiveBytes(Vec<u8>);
+impl Default for NaiveBytes {fn default() -> Self {Self(vec![0])}}
+impl NaiveBytes {fn incement(&mut self) {
+    // TODO would be nice to make field private so that `unwrap` had no chance to panic
+    if self.0.iter().last().unwrap() == &u8::MAX {self.0.push(0);}
+    else {
+        self.0[self.0.len() - 1] += 1;
+    }
+}}
+
+pub fn _try_and_increment<C: CurveGroup + SWCurveConfig>(msg: &[u8]) -> C::Affine {
+// pub fn _try_and_increment<C: SWCurveConfig>(msg: &[u8]) -> Affine<C> {
+// `SWCurveConfig` chosen here just as a most general curve description, which can be tuned further with more appropriate to the task modules
+    let nonce = NaiveBytes::default();
+    loop {
+        let mut h = Shake::v128();
+        h.update(&nonce.0);
+        h.update(msg.as_ref());
+        // let width_bits = C::Affine::MODULUS_BIT_SIZE /* + 1 */;
+        // let output_size = width_bits / 8 + if width_bits % 8 != 0 {1} else {0};
+
+        // as this one isn't intended to work with greater than 256 bits groups, checks for digest being long enough are omitted here
+        assert!(<<C as CurveGroup>::BaseField as Field>::BasePrimeField::MODULUS_BIT_SIZE <= 256u32); // just to be sure
+
+        let mut output_u8 = [0u8; digest_len(<<C as CurveGroup>::BaseField as Field>::BasePrimeField::MODULUS_BIT_SIZE)];
+        h.squeeze(&mut output_u8);
+
+        // `from_bytes_be(sign: Sign, bytes: &[u8])`
+        // `assert_eq!(BigInt::from_bytes_be(Sign::Plus, b"A"),
+        //    BigInt::parse_bytes(b"65", 10).unwrap());`
+
+        let output_u64 = output_u8.into_iter().chunk(8).map(|(i, chunk)| {chunk as u64});
+        // TODO check that `BigInt::new` is actually BE
+        if BigInt::new(output_u64).into() < <<C as CurveGroup>::BaseField as Field>::BasePrimeField::MODULUS {
+            if let Some(
+                result
+            ) = ark_ec::short_weierstrass::Affine::get_point_from_x_unchecked(
+            // ) = ark_ec::models::short_weierstrass::Affine::get_point_from_x_unchecked(
+                <<C as CurveConfig>::BaseField as Field>::BasePrimeField::from_be_bytes_mod_order(&output_u8),
+                nonce.0.iter().last().unwrap() % 2 == 1
+            ) {return result.into_group().into_affine();}
+        }
+        // else {dbg!(nonce.0)}
+    }
+}

rust-arkworks/src/hash_to_curve.rs

@@ -11,6 +11,8 @@ use k256::{ProjectivePoint, Secp256k1};
 use ark_ff::FromBytes;
 use secp256k1::Sec1EncodePoint;
 
+// for reference see <../examples/>
+
 pub fn hash_to_curve<
     Fp: ark_ff::PrimeField,
     // P: ark_ec::SWModelParameters,
@@ -69,22 +71,3 @@ pub fn k256_affine_to_arkworks_secp256k1_affine<
 
     GroupAffine::<P>::new(g_x, g_y, false)
 }
-
-/// Kobi's hash_to_curve function, here for reference only
-pub fn _try_and_increment<C: CurveGroup>(msg: &[u8]) -> Result<C::Affine, EcError> {
-    for nonce in 0u8..=255 {
-        let mut h = Shake::v128();
-        h.update(&[nonce]);
-        h.update(msg.as_ref());
-        let output_size = C::zero().serialized_size(ark_serialize::Compress::Yes);
-        // TODO try to replace with an array sized with a generic number
-        let mut output = vec![0u8; output_size];
-        h.squeeze(&mut output);
-
-        if let Some(p) = C::Affine::from_random_bytes(&output) {
-            return Ok(p);
-        }
-    }
-
-    Err(EcError::CannotHashToCurve)
-}