Expose conversions to and from integer representations to allow usage…

src/private_key.rs

@@ -25,11 +25,12 @@ impl<const LIMBS: usize, const N: usize, MOD: ConstMontyParams<LIMBS>> PrivateKe
         Self { params, key }
     }
 
-    pub(crate) const fn params(&self) -> CsidhParams<LIMBS, N, MOD> {
+    /// Obtains the parameters used by this `PrivateKey`
+    pub const fn params(&self) -> CsidhParams<LIMBS, N, MOD> {
         self.params
     }
-
-    pub(crate) const fn key(&self) -> [u32; N] {
+    ///Obtains the key material in this `PrivateKey`
+    pub const fn key(&self) -> [u32; N] {
         self.key
     }
 }

src/public_key.rs

@@ -54,6 +54,11 @@ where
         }
     }
 
+    ///Creates a foreign public key from this `PublicKey`
+    pub fn to_repr(&self) -> Uint<SAT_LIMBS> {
+        return self.key().to_montgomery();
+    }
+
     pub(crate) const fn key(&self) -> ConstMontyForm<MOD, SAT_LIMBS> {
         self.key
     }

src/shared_secret.rs

@@ -4,7 +4,10 @@ use crypto_bigint::{
     rand_core::CryptoRngCore,
 };
 
-use crate::{csidh::csidh, private_key::PrivateKey, public_key::PublicKey};
+use crate::{
+    CsidhParams, csidh::csidh, montgomery_curve::MontgomeryCurve, private_key::PrivateKey,
+    public_key::PublicKey,
+};
 
 /// A shared secret created with the CSIDH key exchange.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -36,4 +39,24 @@ where
             ),
         }
     }
+
+    /// Constructs a `SharedSecret` from the foreign shared secret, if the secret is valid.
+    #[must_use]
+    pub fn new<const N: usize>(
+        params: CsidhParams<SAT_LIMBS, N, MOD>,
+        shared_secret: Uint<SAT_LIMBS>,
+        rng: &mut impl CryptoRngCore,
+    ) -> Option<Self> {
+        let shared_secret = ConstMontyForm::new(&shared_secret);
+        if MontgomeryCurve::new(params, shared_secret).is_supersingular(rng) {
+            Some(Self { shared_secret })
+        } else {
+            None
+        }
+    }
+
+    ///Creates a foreign shared secret from this `SharedSecret`
+    pub fn to_repr(&self) -> Uint<SAT_LIMBS> {
+        return self.shared_secret.to_montgomery();
+    }
 }