PublicKeyParts: provide serialization helpers

When serializing the exponent or the modulus from a key,
`BoxedUint::to_be_bytes` will return a slice padded with zeroes on the
left because of its inner representation. Not every implementation out
there will handle leading zeroes.

This provides a `PublicKeyParts::n_bytes`/`n_bytes` that will strip out those
leading zeros.

Author: baloo

src/key.rs

@@ -718,6 +718,8 @@ mod tests {
         let n_limbs: &[u64] = PublicKeyParts::n(&public_key).as_ref().as_ref();
         assert_eq!(n_limbs, &[101u64]);
         assert_eq!(PublicKeyParts::e(&public_key), &BoxedUint::from(200u64));
+        assert_eq!(PublicKeyParts::e_bytes(&public_key), [200].into());
+        assert_eq!(PublicKeyParts::n_bytes(&public_key), [101].into());
     }
 
     fn test_key_basics(private_key: &RsaPrivateKey) {

src/traits/keys.rs

@@ -1,5 +1,6 @@
 //! Traits related to the key components
 
+use alloc::boxed::Box;
 use crypto_bigint::{
     modular::{BoxedMontyForm, BoxedMontyParams},
     BoxedUint, NonZero,
@@ -27,6 +28,22 @@ pub trait PublicKeyParts {
     fn n_bits_precision(&self) -> u32 {
         self.n().bits_precision()
     }
+
+    /// Returns the big endian serialization of the modulus of the key
+    fn n_bytes(&self) -> Box<[u8]> {
+        let modulus = self.n();
+        let modulus_zeros = modulus.leading_zeros() as usize / 8;
+        let modulus = modulus.to_be_bytes();
+        (&modulus[modulus_zeros..]).into()
+    }
+
+    /// Returns the big endian serialization of the public exponent of the key
+    fn e_bytes(&self) -> Box<[u8]> {
+        let exponent = self.e();
+        let exponent_zeros = exponent.leading_zeros() as usize / 8;
+        let exponent = exponent.to_be_bytes();
+        (&exponent[exponent_zeros..]).into()
+    }
 }
 
 /// Components of an RSA private key.