Change all from_slice() constructors to return Option<T>s.

We due this in lieu of implementing `TryFrom` to allow for API
consumers to use the `?` operator to convert potential `None`s into
their own `Result<T, CustomError>` types for better error handling
with less boilerplate.

Note that this is a breaking API change.

src/edwards.rs

@@ -93,7 +93,6 @@
 #![allow(non_snake_case)]
 
 use core::borrow::Borrow;
-use core::convert::TryFrom;
 use core::fmt::Debug;
 use core::iter::Iterator;
 use core::iter::Sum;
@@ -108,8 +107,6 @@ use subtle::ConstantTimeEq;
 
 use constants;
 
-use errors::{CurveError, InternalError};
-
 use field::FieldElement;
 use scalar::Scalar;
 
@@ -338,33 +335,23 @@ impl Default for CompressedEdwardsY {
     }
 }
 
-impl TryFrom<&[u8]> for CompressedEdwardsY {
-    type Error = CurveError;
-
-    fn try_from(bytes: &[u8]) -> Result<CompressedEdwardsY, CurveError> {
-        if bytes.len() != 32 {
-            return Err(CurveError(
-                InternalError::BytesLengthError{name: "CompressedEdwardsY", length: 32}));
-        }
-
-        Ok(CompressedEdwardsY::from_slice(bytes))
-    }
-}
-
 impl CompressedEdwardsY {
     /// Construct a `CompressedEdwardsY` from a slice of bytes.
     ///
-    /// # Panics
+    /// # Returns
     ///
-    /// If the input `bytes` slice does not have a length of 32.  For
+    /// An `Option<CompressedEdwardsY>` which is `None` if the input `bytes`
-    /// a panic-safe version of this API, see the implementation of
+    /// slice does not have a length of 32.
-    /// `TryFrom<&[u8]>`.
+    pub fn from_slice(bytes: &[u8]) -> Option<CompressedEdwardsY> {
-    pub fn from_slice(bytes: &[u8]) -> CompressedEdwardsY {
+        if bytes.len() != 32 {
+            return None;
+        }
+
         let mut tmp = [0u8; 32];
 
         tmp.copy_from_slice(bytes);
 
-        CompressedEdwardsY(tmp)
+        Some(CompressedEdwardsY(tmp))
     }
 }
 

src/errors.rs

@@ -1,69 +0,0 @@
-// -*- mode: rust; -*-
-//
-// This file is part of curve25519-dalek.
-// Copyright (c) 2019 Isis Lovecruft
-// See LICENSE for licensing information.
-//
-// Authors:
-// - Isis Agora Lovecruft <isis@patternsinthevoid.net>
-
-//! Errors which may occur.
-//!
-//! Currently, these are only used in the implementations of `TryFrom`.
-//!
-//! If used with `std` support, this public types in this module implement the
-//! `std::error::Error` trait.
-
-use core::fmt;
-use core::fmt::Display;
-
-#[cfg(feature = "std")]
-use std::error::Error;
-
-/// Internal errors.  Most application-level developers will likely not
-/// need to pay any attention to these.
-#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
-pub(crate) enum InternalError {
-    /// An error in the length of bytes handed to a constructor.
-    ///
-    /// To use this, pass a string specifying the `name` of the type which is
-    /// returning the error, and the `length` in bytes which its constructor
-    /// expects.
-    BytesLengthError {
-        name: &'static str,
-        length: usize,
-    },
-}
-
-impl Display for InternalError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match *self {
-            InternalError::BytesLengthError{ name: n, length: l}
-                => write!(f, "{} must be {} bytes in length", n, l),
-        }
-    }
-}
-
-#[cfg(feature = "std")]
-impl Error for InternalError { }
-
-/// Errors which may occur.
-///
-/// This error may arise due to:
-///
-/// * Being given bytes with a length different to what was expected.
-#[derive(Clone, Copy, Eq, PartialEq, Hash, Debug)]
-pub struct CurveError(pub(crate) InternalError);
-
-impl Display for CurveError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", self.0)
-    }
-}
-
-#[cfg(feature = "std")]
-impl Error for CurveError {
-    fn source(&self) -> Option<&(dyn Error + 'static)> {
-        Some(&self.0)
-    }
-}

src/lib.rs

@@ -82,9 +82,6 @@ pub mod constants;
 // External (and internal) traits.
 pub mod traits;
 
-// Errors which may occur.
-pub mod errors;
-
 //------------------------------------------------------------------------
 // curve25519-dalek internal modules
 //------------------------------------------------------------------------

src/montgomery.rs

@@ -48,12 +48,10 @@
 // affine and projective cakes and eat both of them too.
 #![allow(non_snake_case)]
 
-use core::convert::TryFrom;
 use core::ops::{Mul, MulAssign};
 
 use constants::APLUS2_OVER_FOUR;
 use edwards::{CompressedEdwardsY, EdwardsPoint};
-use errors::{CurveError, InternalError};
 use field::FieldElement;
 use scalar::Scalar;
 
@@ -112,29 +110,6 @@ impl ValidityCheck for MontgomeryPoint {
     }
 }
 
-impl TryFrom<&[u8]> for MontgomeryPoint {
-    type Error = CurveError;
-
-    fn try_from(bytes: &[u8]) -> Result<MontgomeryPoint, CurveError> {
-        if bytes.len() != 32 {
-            return Err(CurveError(
-                InternalError::BytesLengthError{name: "MontgomeryPoint", length: 32}));
-        }
-
-        let mut array = [0u8; 32];
-        array.copy_from_slice(&bytes[..32]);
-
-        let P = MontgomeryPoint(array);
-
-        if P.is_valid() {
-            return Ok(P);
-        }
-
-        Err(CurveError(
-            InternalError::BytesLengthError{name: "MontgomeryPoint", length: 32}))
-    }
-}
-
 impl MontgomeryPoint {
     /// View this `MontgomeryPoint` as an array of bytes.
     pub fn as_bytes<'a>(&'a self) -> &'a [u8; 32] {
@@ -146,6 +121,29 @@ impl MontgomeryPoint {
         self.0
     }
 
+    /// Attempt to create a `MontgomeryPoint` from a slice of bytes.
+    ///
+    /// # Returns
+    ///
+    /// An `Option<MontgomeryPoint>` which is `None` if the length of the slice
+    /// of bytes is not 32, or if the bytes did not represent a canonical
+    /// `FieldElement`.
+    pub fn from_slice(bytes: &[u8]) -> Option<MontgomeryPoint> {
+        if bytes.len() != 32 {
+            return None;
+        }
+
+        let mut array = [0u8; 32];
+        array.copy_from_slice(&bytes[..32]);
+
+        let P = MontgomeryPoint(array);
+
+        if P.is_valid() {
+            return Some(P);
+        }
+        None
+    }
+
     /// Attempt to convert to an `EdwardsPoint`, using the supplied
     /// choice of sign for the `EdwardsPoint`.
     ///

src/ristretto.rs

@@ -158,7 +158,6 @@
 //! https://ristretto.group/
 
 use core::borrow::Borrow;
-use core::convert::TryFrom;
 use core::fmt::Debug;
 use core::iter::Sum;
 use core::ops::{Add, Neg, Sub};
@@ -181,8 +180,6 @@ use subtle::ConstantTimeEq;
 use edwards::EdwardsBasepointTable;
 use edwards::EdwardsPoint;
 
-use errors::{CurveError, InternalError};
-
 #[allow(unused_imports)]
 use prelude::*;
 
@@ -220,19 +217,6 @@ impl ConstantTimeEq for CompressedRistretto {
     }
 }
 
-impl TryFrom<&[u8]> for CompressedRistretto {
-    type Error = CurveError;
-
-    fn try_from(bytes: &[u8]) -> Result<CompressedRistretto, CurveError> {
-        if bytes.len() != 32 {
-            return Err(CurveError(
-                InternalError::BytesLengthError{name: "CompressedRistretto", length: 32}));
-        }
-
-        Ok(CompressedRistretto::from_slice(bytes))
-    }
-}
-
 impl CompressedRistretto {
     /// Copy the bytes of this `CompressedRistretto`.
     pub fn to_bytes(&self) -> [u8; 32] {
@@ -246,17 +230,20 @@ impl CompressedRistretto {
 
     /// Construct a `CompressedRistretto` from a slice of bytes.
     ///
-    /// # Panics
+    /// # Returns
     ///
-    /// If the input `bytes` slice does not have a length of 32.  For a
+    /// An `Option<CompressedRistretto>` which is `None` if the input `bytes`
-    /// panic-safe version of this API, see the implementation of
+    /// slice does not have a length of 32.
-    /// `TryFrom<&[u8]>`.
+    pub fn from_slice(bytes: &[u8]) -> Option<CompressedRistretto> {
-    pub fn from_slice(bytes: &[u8]) -> CompressedRistretto {
+        if bytes.len() != 32 {
+            return None;
+        }
+
         let mut tmp = [0u8; 32];
 
         tmp.copy_from_slice(bytes);
 
-        CompressedRistretto(tmp)
+        Some(CompressedRistretto(tmp))
     }
 
     /// Attempt to decompress to an `RistrettoPoint`.