Add more transparency into some EC structures. (#641)

This adds the ability to create ECDH keys from raw bytes and export
signatures as raw bytes.

libraries/crypto/src/ecdh.rs

@@ -78,6 +78,17 @@ impl SecKey {
         p.getx().to_int().to_bin(&mut x);
         x
     }
+
+    /// Creates a private key from the exponent's bytes, or None if checks fail.
+    pub fn from_bytes(bytes: &[u8; 32]) -> Option<SecKey> {
+        let a = NonZeroExponentP256::from_int_checked(Int256::from_bin(bytes));
+        // The branching here is fine because all this reveals is whether the key was invalid.
+        if bool::from(a.is_none()) {
+            return None;
+        }
+        let a = a.unwrap();
+        Some(SecKey { a })
+    }
 }
 
 impl PubKey {

libraries/crypto/src/ecdsa.rs

@@ -19,9 +19,7 @@ use super::ec::point::PointP256;
 use super::Hash256;
 use alloc::vec;
 use alloc::vec::Vec;
-#[cfg(feature = "std")]
-use arrayref::array_mut_ref;
-use arrayref::{array_ref, mut_array_refs};
+use arrayref::{array_mut_ref, array_ref, mut_array_refs};
 use core::marker::PhantomData;
 use rand_core::RngCore;
 use zeroize::Zeroize;
@@ -220,7 +218,6 @@ impl Signature {
         Some(Signature { r, s })
     }
 
-    #[cfg(feature = "std")]
     pub fn to_bytes(&self, bytes: &mut [u8; Signature::BYTES_LENGTH]) {
         self.r
             .to_int()