Add using_privkey and using_pubkey convenience functions to trait CMZCredential

These functions act like default(), but also load the given private or
public key

cmzcred_derive/src/lib.rs

@@ -77,17 +77,20 @@ fn impl_cmzcred_derive(ast: &syn::DeriveInput, group_ident: &Ident) -> TokenStre
                 }
             }
 
-            fn set_pubkey(&mut self, pubkey: &CMZPubkey<Self::Point>) {
+            fn set_pubkey(&mut self, pubkey: &CMZPubkey<Self::Point>) -> &mut Self {
                 self.pubkey = pubkey.clone();
+                self
             }
 
             fn get_pubkey(&self) -> CMZPubkey<Self::Point> {
                 self.pubkey.clone()
             }
 
-            fn set_privkey(&mut self, privkey: &CMZPrivkey<Self::Point>) {
+            fn set_privkey(&mut self, privkey: &CMZPrivkey<Self::Point>)
+            -> &mut Self {
                 self.pubkey = cmz_privkey_to_pubkey(&privkey);
                 self.privkey = privkey.clone();
+                self
             }
 
             fn get_privkey(&self) -> CMZPrivkey<Self::Point> {

src/lib.rs

@@ -169,7 +169,10 @@ pub fn cmz_privkey_to_pubkey<G: PrimeGroup>(privkey: &CMZPrivkey<G>) -> CMZPubke
 }
 
 /// The CMZCredential trait implemented by all CMZ credential struct types.
-pub trait CMZCredential<G: PrimeGroup> {
+pub trait CMZCredential<G: PrimeGroup>
+where
+    Self: Default + Sized,
+{
     /// The type of attributes for this credential
     type Scalar: PrimeField;
 
@@ -192,7 +195,7 @@ pub trait CMZCredential<G: PrimeGroup> {
     fn attr_mut(&mut self, name: &str) -> &mut Option<Self::Scalar>;
 
     /// Set the public key for this credential.
-    fn set_pubkey(&mut self, pubkey: &CMZPubkey<G>);
+    fn set_pubkey(&mut self, pubkey: &CMZPubkey<G>) -> &mut Self;
 
     /// Get a copy of the public key for this credential.  If the public
     /// key has not yet been set or computed, a pubkey with X0 == None
@@ -201,7 +204,7 @@ pub trait CMZCredential<G: PrimeGroup> {
 
     /// Set the private key for this credential.  The public key will
     /// automatically be computed from the private key.
-    fn set_privkey(&mut self, privkey: &CMZPrivkey<G>);
+    fn set_privkey(&mut self, privkey: &CMZPrivkey<G>) -> &mut Self;
 
     /// Get a copy of the private key for this credential.  If the
     /// private key has not yet been set, a privkey with an empty x
@@ -211,6 +214,22 @@ pub trait CMZCredential<G: PrimeGroup> {
     /// Generate random private and public keys for this credential
     /// type.
     fn gen_keys(rng: &mut impl RngCore) -> (CMZPrivkey<G>, CMZPubkey<G>);
+
+    /// Convenience function for creating a new Self, and loading the
+    /// given private key (which will also compute the public key).
+    fn using_privkey(privkey: &CMZPrivkey<G>) -> Self {
+        let mut slf = Self::default();
+        slf.set_privkey(privkey);
+        slf
+    }
+
+    /// Convenience function for creating a new Self, and loading the
+    /// given public key.
+    fn using_pubkey(pubkey: &CMZPubkey<G>) -> Self {
+        let mut slf = Self::default();
+        slf.set_pubkey(pubkey);
+        slf
+    }
 }
 
 /** The CMZ macro for declaring CMZ credentials.

tests/basic.rs

@@ -15,12 +15,9 @@ fn test_basic() {
     cmz_group_init(RistrettoPoint::hash_from_bytes::<Sha512>(
         b"CMZ Generator A",
     ));
-    let mut basic_cred = Basic::default();
-
-    println!("{:#?}", basic_cred);
 
     let (privkey, pubkey) = Basic::gen_keys(&mut rng);
-    basic_cred.set_privkey(&privkey);
+    let basic_cred = Basic::using_privkey(&privkey);
 
     println!("{:#?}", basic_cred);
 }