Rename the coins folder to networks (#583)

* Rename the coins folder to networks

Ethereum isn't a coin. It's a network.

Resolves #357.

* More renames of coins -> networks in orchestration

* Correct paths in tests/

* cargo fmt

networks/monero/primitives/src/unreduced_scalar.rs

@@ -0,0 +1,143 @@
+use core::cmp::Ordering;
+use std_shims::{
+  sync::OnceLock,
+  io::{self, *},
+};
+
+use zeroize::Zeroize;
+
+use curve25519_dalek::scalar::Scalar;
+
+use monero_io::*;
+
+static PRECOMPUTED_SCALARS_CELL: OnceLock<[Scalar; 8]> = OnceLock::new();
+// Precomputed scalars used to recover an incorrectly reduced scalar.
+#[allow(non_snake_case)]
+fn PRECOMPUTED_SCALARS() -> [Scalar; 8] {
+  *PRECOMPUTED_SCALARS_CELL.get_or_init(|| {
+    let mut precomputed_scalars = [Scalar::ONE; 8];
+    for (i, scalar) in precomputed_scalars.iter_mut().enumerate().skip(1) {
+      *scalar = Scalar::from(u8::try_from((i * 2) + 1).unwrap());
+    }
+    precomputed_scalars
+  })
+}
+
+/// An unreduced scalar.
+///
+/// While most of modern Monero enforces scalars be reduced, certain legacy parts of the code did
+/// not. These section can generally simply be read as a scalar/reduced into a scalar when the time
+/// comes, yet a couple have non-standard reductions performed.
+///
+/// This struct delays scalar conversions and offers the non-standard reduction.
+#[derive(Clone, PartialEq, Eq, Debug, Zeroize)]
+pub struct UnreducedScalar(pub [u8; 32]);
+
+impl UnreducedScalar {
+  /// Write an UnreducedScalar.
+  pub fn write<W: Write>(&self, w: &mut W) -> io::Result<()> {
+    w.write_all(&self.0)
+  }
+
+  /// Read an UnreducedScalar.
+  pub fn read<R: Read>(r: &mut R) -> io::Result<UnreducedScalar> {
+    Ok(UnreducedScalar(read_bytes(r)?))
+  }
+
+  fn as_bits(&self) -> [u8; 256] {
+    let mut bits = [0; 256];
+    for (i, bit) in bits.iter_mut().enumerate() {
+      *bit = core::hint::black_box(1 & (self.0[i / 8] >> (i % 8)))
+    }
+
+    bits
+  }
+
+  // Computes the non-adjacent form of this scalar with width 5.
+  //
+  // This matches Monero's `slide` function and intentionally gives incorrect outputs under
+  // certain conditions in order to match Monero.
+  //
+  // This function does not execute in constant time.
+  fn non_adjacent_form(&self) -> [i8; 256] {
+    let bits = self.as_bits();
+    let mut naf = [0i8; 256];
+    for (b, bit) in bits.into_iter().enumerate() {
+      naf[b] = i8::try_from(bit).unwrap();
+    }
+
+    for i in 0 .. 256 {
+      if naf[i] != 0 {
+        // if the bit is a one, work our way up through the window
+        // combining the bits with this bit.
+        for b in 1 .. 6 {
+          if (i + b) >= 256 {
+            // if we are at the length of the array then break out
+            // the loop.
+            break;
+          }
+          // potential_carry - the value of the bit at i+b compared to the bit at i
+          let potential_carry = naf[i + b] << b;
+
+          if potential_carry != 0 {
+            if (naf[i] + potential_carry) <= 15 {
+              // if our current "bit" plus the potential carry is less than 16
+              // add it to our current "bit" and set the potential carry bit to 0.
+              naf[i] += potential_carry;
+              naf[i + b] = 0;
+            } else if (naf[i] - potential_carry) >= -15 {
+              // else if our current "bit" minus the potential carry is more than -16
+              // take it away from our current "bit".
+              // we then work our way up through the bits setting ones to zero, when
+              // we hit the first zero we change it to one then stop, this is to factor
+              // in the minus.
+              naf[i] -= potential_carry;
+              #[allow(clippy::needless_range_loop)]
+              for k in (i + b) .. 256 {
+                if naf[k] == 0 {
+                  naf[k] = 1;
+                  break;
+                }
+                naf[k] = 0;
+              }
+            } else {
+              break;
+            }
+          }
+        }
+      }
+    }
+
+    naf
+  }
+
+  /// Recover the scalar that an array of bytes was incorrectly interpreted as by Monero's `slide`
+  /// function.
+  ///
+  /// In Borromean range proofs, Monero was not checking that the scalars used were
+  /// reduced. This lead to the scalar stored being interpreted as a different scalar.
+  /// This function recovers that scalar.
+  ///
+  /// See <https://github.com/monero-project/monero/issues/8438> for more info.
+  pub fn recover_monero_slide_scalar(&self) -> Scalar {
+    if self.0[31] & 128 == 0 {
+      // Computing the w-NAF of a number can only give an output with 1 more bit than
+      // the number, so even if the number isn't reduced, the `slide` function will be
+      // correct when the last bit isn't set.
+      return Scalar::from_bytes_mod_order(self.0);
+    }
+
+    let precomputed_scalars = PRECOMPUTED_SCALARS();
+
+    let mut recovered = Scalar::ZERO;
+    for &numb in self.non_adjacent_form().iter().rev() {
+      recovered += recovered;
+      match numb.cmp(&0) {
+        Ordering::Greater => recovered += precomputed_scalars[usize::try_from(numb).unwrap() / 2],
+        Ordering::Less => recovered -= precomputed_scalars[usize::try_from(-numb).unwrap() / 2],
+        Ordering::Equal => (),
+      }
+    }
+    recovered
+  }
+}