Introduce generic-based capability handling

Add type parameter to Secp256k1
Add PhantomData for C
Separate into structs and traits
Move constructors to own impl blocks

src/ecdh.rs

@@ -29,7 +29,7 @@ pub struct SharedSecret(ffi::SharedSecret);
 impl SharedSecret {
     /// Creates a new shared secret from a pubkey and secret key
     #[inline]
-    pub fn new(secp: &Secp256k1, point: &PublicKey, scalar: &SecretKey) -> SharedSecret {
+    pub fn new<C>(secp: &Secp256k1<C>, point: &PublicKey, scalar: &SecretKey) -> SharedSecret {
         unsafe {
             let mut ss = ffi::SharedSecret::blank();
             let res = ffi::secp256k1_ecdh(secp.ctx, &mut ss, point.as_ptr(), scalar.as_ptr());

src/key.rs

@@ -21,6 +21,8 @@ use std::mem;
 
 use super::{Secp256k1, ContextFlag};
 use super::Error::{self, IncapableContext, InvalidPublicKey, InvalidSecretKey};
+use Signing;
+use Verification;
 use constants;
 use ffi;
 
@@ -63,7 +65,7 @@ impl SecretKey {
     /// Creates a new random secret key
     #[inline]
     #[cfg(any(test, feature = "rand"))]
-    pub fn new<R: Rng>(secp: &Secp256k1, rng: &mut R) -> SecretKey {
+    pub fn new<R: Rng, C>(secp: &Secp256k1<C>, rng: &mut R) -> SecretKey {
         let mut data = random_32_bytes(rng);
         unsafe {
             while ffi::secp256k1_ec_seckey_verify(secp.ctx, data.as_ptr()) == 0 {
@@ -75,7 +77,7 @@ impl SecretKey {
 
     /// Converts a `SECRET_KEY_SIZE`-byte slice to a secret key
     #[inline]
-    pub fn from_slice(secp: &Secp256k1, data: &[u8])
+    pub fn from_slice<C>(secp: &Secp256k1<C>, data: &[u8])
                         -> Result<SecretKey, Error> {
         match data.len() {
             constants::SECRET_KEY_SIZE => {
@@ -94,7 +96,7 @@ impl SecretKey {
 
     #[inline]
     /// Adds one secret key to another, modulo the curve order
-    pub fn add_assign(&mut self, secp: &Secp256k1, other: &SecretKey)
+    pub fn add_assign<C>(&mut self, secp: &Secp256k1<C>, other: &SecretKey)
                      -> Result<(), Error> {
         unsafe {
             if ffi::secp256k1_ec_privkey_tweak_add(secp.ctx, self.as_mut_ptr(), other.as_ptr()) != 1 {
@@ -107,7 +109,7 @@ impl SecretKey {
 
     #[inline]
     /// Multiplies one secret key by another, modulo the curve order
-    pub fn mul_assign(&mut self, secp: &Secp256k1, other: &SecretKey)
+    pub fn mul_assign<C>(&mut self, secp: &Secp256k1<C>, other: &SecretKey)
                      -> Result<(), Error> {
         unsafe {
             if ffi::secp256k1_ec_privkey_tweak_mul(secp.ctx, self.as_mut_ptr(), other.as_ptr()) != 1 {
@@ -142,12 +144,9 @@ impl PublicKey {
 
     /// Creates a new public key from a secret key.
     #[inline]
-    pub fn from_secret_key(secp: &Secp256k1,
+    pub fn from_secret_key<C: Signing>(secp: &Secp256k1<C>,
                            sk: &SecretKey)
                            -> Result<PublicKey, Error> {
-        if secp.caps == ContextFlag::VerifyOnly || secp.caps == ContextFlag::None {
-            return Err(IncapableContext);
-        }
         let mut pk = unsafe { ffi::PublicKey::blank() };
         unsafe {
             // We can assume the return value because it's not possible to construct
@@ -160,7 +159,7 @@ impl PublicKey {
 
     /// Creates a public key directly from a slice
     #[inline]
-    pub fn from_slice(secp: &Secp256k1, data: &[u8])
+    pub fn from_slice<C>(secp: &Secp256k1<C>, data: &[u8])
                       -> Result<PublicKey, Error> {
 
         let mut pk = unsafe { ffi::PublicKey::blank() };
@@ -179,7 +178,7 @@ impl PublicKey {
     /// the y-coordinate is represented by only a single bit, as x determines
     /// it up to one bit.
     pub fn serialize(&self) -> [u8; constants::PUBLIC_KEY_SIZE] {
-        let secp = Secp256k1::with_caps(ContextFlag::None);
+        let secp = Secp256k1::without_caps();
         let mut ret = [0; constants::PUBLIC_KEY_SIZE];
 
         unsafe {
@@ -199,7 +198,7 @@ impl PublicKey {
 
     /// Serialize the key as a byte-encoded pair of values, in uncompressed form
     pub fn serialize_uncompressed(&self) -> [u8; constants::UNCOMPRESSED_PUBLIC_KEY_SIZE] {
-        let secp = Secp256k1::with_caps(ContextFlag::None);
+        let secp = Secp256k1::without_caps();
         let mut ret = [0; constants::UNCOMPRESSED_PUBLIC_KEY_SIZE];
 
         unsafe {
@@ -219,11 +218,8 @@ impl PublicKey {
 
     #[inline]
     /// Adds the pk corresponding to `other` to the pk `self` in place
-    pub fn add_exp_assign(&mut self, secp: &Secp256k1, other: &SecretKey)
+    pub fn add_exp_assign<C: Verification>(&mut self, secp: &Secp256k1<C>, other: &SecretKey)
                          -> Result<(), Error> {
-        if secp.caps == ContextFlag::SignOnly || secp.caps == ContextFlag::None {
-            return Err(IncapableContext);
-        }
         unsafe {
             if ffi::secp256k1_ec_pubkey_tweak_add(secp.ctx, &mut self.0 as *mut _,
                                                   other.as_ptr()) == 1 {
@@ -236,11 +232,8 @@ impl PublicKey {
 
     #[inline]
     /// Muliplies the pk `self` in place by the scalar `other`
-    pub fn mul_assign(&mut self, secp: &Secp256k1, other: &SecretKey)
+    pub fn mul_assign<C: Verification>(&mut self, secp: &Secp256k1<C>, other: &SecretKey)
                          -> Result<(), Error> {
-        if secp.caps == ContextFlag::SignOnly || secp.caps == ContextFlag::None {
-            return Err(IncapableContext);
-        }
         unsafe {
             if ffi::secp256k1_ec_pubkey_tweak_mul(secp.ctx, &mut self.0 as *mut _,
                                                   other.as_ptr()) == 1 {
@@ -254,7 +247,7 @@ impl PublicKey {
     /// Adds a second key to this one, returning the sum. Returns an error if
     /// the result would be the point at infinity, i.e. we are adding this point
     /// to its own negation
-    pub fn combine(&self, secp: &Secp256k1, other: &PublicKey) -> Result<PublicKey, Error> {
+    pub fn combine<C>(&self, secp: &Secp256k1<C>, other: &PublicKey) -> Result<PublicKey, Error> {
         unsafe {
             let mut ret = mem::uninitialized();
             let ptrs = [self.as_ptr(), other.as_ptr()];

src/lib.rs

@@ -56,6 +56,7 @@ pub mod schnorr;
 
 pub use key::SecretKey;
 pub use key::PublicKey;
+use std::marker::PhantomData;
 
 /// A tag used for recovering the public key from a compact signature
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
@@ -89,7 +90,7 @@ impl RecoveryId {
 impl Signature {
     #[inline]
     /// Converts a DER-encoded byte slice to a signature
-    pub fn from_der(secp: &Secp256k1, data: &[u8]) -> Result<Signature, Error> {
+    pub fn from_der<C>(secp: &Secp256k1<C>, data: &[u8]) -> Result<Signature, Error> {
         let mut ret = unsafe { ffi::Signature::blank() };
 
         unsafe {
@@ -103,7 +104,7 @@ impl Signature {
     }
 
     /// Converts a 64-byte compact-encoded byte slice to a signature
-    pub fn from_compact(secp: &Secp256k1, data: &[u8]) -> Result<Signature, Error> {
+    pub fn from_compact<C>(secp: &Secp256k1<C>, data: &[u8]) -> Result<Signature, Error> {
         let mut ret = unsafe { ffi::Signature::blank() };
         if data.len() != 64 {
             return Err(Error::InvalidSignature)
@@ -123,7 +124,7 @@ impl Signature {
     /// only useful for validating signatures in the Bitcoin blockchain from before
     /// 2016. It should never be used in new applications. This library does not
     /// support serializing to this "format"
-    pub fn from_der_lax(secp: &Secp256k1, data: &[u8]) -> Result<Signature, Error> {
+    pub fn from_der_lax<C>(secp: &Secp256k1<C>, data: &[u8]) -> Result<Signature, Error> {
         unsafe {
             let mut ret = ffi::Signature::blank();
             if ffi::ecdsa_signature_parse_der_lax(secp.ctx, &mut ret,
@@ -152,7 +153,7 @@ impl Signature {
     /// valid. (For example, parsing the historic Bitcoin blockchain requires
     /// this.) For these applications we provide this normalization function,
     /// which ensures that the s value lies in the lower half of its range.
-    pub fn normalize_s(&mut self, secp: &Secp256k1) {
+    pub fn normalize_s<C>(&mut self, secp: &Secp256k1<C>) {
         unsafe {
             // Ignore return value, which indicates whether the sig
             // was already normalized. We don't care.
@@ -175,7 +176,7 @@ impl Signature {
 
     #[inline]
     /// Serializes the signature in DER format
-    pub fn serialize_der(&self, secp: &Secp256k1) -> Vec<u8> {
+    pub fn serialize_der<C>(&self, secp: &Secp256k1<C>) -> Vec<u8> {
         let mut ret = Vec::with_capacity(72);
         let mut len: size_t = ret.capacity() as size_t;
         unsafe {
@@ -189,7 +190,7 @@ impl Signature {
 
     #[inline]
     /// Serializes the signature in compact format
-    pub fn serialize_compact(&self, secp: &Secp256k1) -> [u8; 64] {
+    pub fn serialize_compact<C>(&self, secp: &Secp256k1<C>) -> [u8; 64] {
         let mut ret = [0; 64];
         unsafe {
             let err = ffi::secp256k1_ecdsa_signature_serialize_compact(secp.ctx, ret.as_mut_ptr(),
@@ -214,7 +215,7 @@ impl RecoverableSignature {
     /// Converts a compact-encoded byte slice to a signature. This
     /// representation is nonstandard and defined by the libsecp256k1
     /// library.
-    pub fn from_compact(secp: &Secp256k1, data: &[u8], recid: RecoveryId) -> Result<RecoverableSignature, Error> {
+    pub fn from_compact<C>(secp: &Secp256k1<C>, data: &[u8], recid: RecoveryId) -> Result<RecoverableSignature, Error> {
         let mut ret = unsafe { ffi::RecoverableSignature::blank() };
 
         unsafe {
@@ -237,7 +238,7 @@ impl RecoverableSignature {
 
     #[inline]
     /// Serializes the recoverable signature in compact format
-    pub fn serialize_compact(&self, secp: &Secp256k1) -> (RecoveryId, [u8; 64]) {
+    pub fn serialize_compact<C>(&self, secp: &Secp256k1<C>) -> (RecoveryId, [u8; 64]) {
         let mut ret = [0u8; 64];
         let mut recid = 0i32;
         unsafe {
@@ -251,7 +252,7 @@ impl RecoverableSignature {
     /// Converts a recoverable signature to a non-recoverable one (this is needed
     /// for verification
     #[inline]
-    pub fn to_standard(&self, secp: &Secp256k1) -> Signature {
+    pub fn to_standard<C>(&self, secp: &Secp256k1<C>) -> Signature {
         let mut ret = unsafe { ffi::Signature::blank() };
         unsafe {
             let err = ffi::secp256k1_ecdsa_recoverable_signature_convert(secp.ctx, &mut ret, self.as_ptr());
@@ -376,14 +377,28 @@ impl error::Error for Error {
     }
 }
 
+pub trait Signing {}
+pub trait Verification {}
+
+pub struct None {}
+pub struct SignOnly {}
+pub struct VerifyOnly {}
+pub struct All {}
+
+impl Signing for SignOnly {}
+impl Signing for All {}
+
+impl Verification for VerifyOnly {}
+impl Verification for All {}
+
 /// The secp256k1 engine, used to execute all signature operations
-pub struct Secp256k1 {
+pub struct Secp256k1<C> {
     ctx: *mut ffi::Context,
-    caps: ContextFlag
+    phantom: PhantomData<C>
 }
 
-unsafe impl Send for Secp256k1 {}
+unsafe impl<C> Send for Secp256k1<C> {}
-unsafe impl Sync for Secp256k1 {}
+unsafe impl<C> Sync for Secp256k1<C> {}
 
 /// Flags used to determine the capabilities of a `Secp256k1` object;
 /// the more capabilities, the more expensive it is to create.
@@ -407,54 +422,56 @@ impl fmt::Display for ContextFlag {
     }
 }
 
-impl Clone for Secp256k1 {
+impl<C> Clone for Secp256k1<C> {
-    fn clone(&self) -> Secp256k1 {
+    fn clone(&self) -> Secp256k1<C> {
         Secp256k1 {
             ctx: unsafe { ffi::secp256k1_context_clone(self.ctx) },
-            caps: self.caps
+            phantom: self.phantom
         }
     }
 }
 
-impl PartialEq for Secp256k1 {
+impl<C> PartialEq for Secp256k1<C> {
-    fn eq(&self, other: &Secp256k1) -> bool { self.caps == other.caps }
+    fn eq(&self, other: &Secp256k1<C>) -> bool { true }
-}
-impl Eq for Secp256k1 { }
-
-impl fmt::Debug for Secp256k1 {
-    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-        write!(f, "Secp256k1 {{ [private], caps: {:?} }}", self.caps)
-    }
 }
 
-impl Drop for Secp256k1 {
+impl<C> Eq for Secp256k1<C> { }
+
+impl<C> Drop for Secp256k1<C> {
     fn drop(&mut self) {
         unsafe { ffi::secp256k1_context_destroy(self.ctx); }
     }
 }
 
-impl Secp256k1 {
+impl Secp256k1<None> {
-    /// Creates a new Secp256k1 context
-    #[inline]
-    pub fn new() -> Secp256k1 {
-        Secp256k1::with_caps(ContextFlag::Full)
-    }
-
-    /// Creates a new Secp256k1 context with the specified capabilities
-    pub fn with_caps(caps: ContextFlag) -> Secp256k1 {
-        let flag = match caps {
-            ContextFlag::None => ffi::SECP256K1_START_NONE,
-            ContextFlag::SignOnly => ffi::SECP256K1_START_SIGN,
-            ContextFlag::VerifyOnly => ffi::SECP256K1_START_VERIFY,
-            ContextFlag::Full => ffi::SECP256K1_START_SIGN | ffi::SECP256K1_START_VERIFY
-        };
-        Secp256k1 { ctx: unsafe { ffi::secp256k1_context_create(flag) }, caps: caps }
-    }
-
     /// Creates a new Secp256k1 context with no capabilities (just de/serialization)
-    pub fn without_caps() -> Secp256k1 {
+    pub fn without_caps() -> Secp256k1<None> {
-        Secp256k1::with_caps(ContextFlag::None)
+        Secp256k1 { ctx: unsafe { ffi::secp256k1_context_create(ffi::SECP256K1_START_NONE) }, phantom: PhantomData }
     }
+}
+
+impl Secp256k1<All> {
+    /// Creates a new Secp256k1 context
+    pub fn new() -> Secp256k1<All> {
+        Secp256k1 { ctx: unsafe { ffi::secp256k1_context_create(ffi::SECP256K1_START_SIGN | ffi::SECP256K1_START_VERIFY) }, phantom: PhantomData }
+    }
+}
+
+impl Secp256k1<SignOnly> {
+
+    pub fn signing_only() -> Secp256k1<SignOnly> {
+        Secp256k1 { ctx: unsafe { ffi::secp256k1_context_create(ffi::SECP256K1_START_SIGN) }, phantom: PhantomData }
+    }
+}
+
+impl Secp256k1<VerifyOnly> {
+
+    pub fn verification_only() -> Secp256k1<VerifyOnly> {
+        Secp256k1 { ctx: unsafe { ffi::secp256k1_context_create(ffi::SECP256K1_START_VERIFY) }, phantom: PhantomData }
+    }
+}
+
+impl<C> Secp256k1<C> {
 
     /// (Re)randomizes the Secp256k1 context for cheap sidechannel resistence;
     /// see comment in libsecp256k1 commit d2275795f by Gregory Maxwell
@@ -476,25 +493,14 @@ impl Secp256k1 {
         }
     }
 
-    /// Generates a random keypair. Convenience function for `key::SecretKey::new`
-    /// and `key::PublicKey::from_secret_key`; call those functions directly for
-    /// batch key generation. Requires a signing-capable context.
-    #[inline]
-    #[cfg(any(test, feature = "rand"))]
-    pub fn generate_keypair<R: Rng>(&self, rng: &mut R)
-                                   -> Result<(key::SecretKey, key::PublicKey), Error> {
-        let sk = key::SecretKey::new(self, rng);
-        let pk = try!(key::PublicKey::from_secret_key(self, &sk));
-        Ok((sk, pk))
 }
 
+impl<C: Signing> Secp256k1<C> {
+
     /// Constructs a signature for `msg` using the secret key `sk` and RFC6979 nonce
     /// Requires a signing-capable context.
     pub fn sign(&self, msg: &Message, sk: &key::SecretKey)
                 -> Result<Signature, Error> {
-        if self.caps == ContextFlag::VerifyOnly || self.caps == ContextFlag::None {
-            return Err(Error::IncapableContext);
-        }
 
         let mut ret = unsafe { ffi::Signature::blank() };
         unsafe {
@@ -511,9 +517,6 @@ impl Secp256k1 {
     /// Requires a signing-capable context.
     pub fn sign_recoverable(&self, msg: &Message, sk: &key::SecretKey)
                             -> Result<RecoverableSignature, Error> {
-        if self.caps == ContextFlag::VerifyOnly || self.caps == ContextFlag::None {
-            return Err(Error::IncapableContext);
-        }
 
         let mut ret = unsafe { ffi::RecoverableSignature::blank() };
         unsafe {
@@ -526,13 +529,25 @@ impl Secp256k1 {
         Ok(RecoverableSignature::from(ret))
     }
 
+    /// Generates a random keypair. Convenience function for `key::SecretKey::new`
+    /// and `key::PublicKey::from_secret_key`; call those functions directly for
+    /// batch key generation. Requires a signing-capable context.
+    #[inline]
+    #[cfg(any(test, feature = "rand"))]
+    pub fn generate_keypair<R: Rng>(&self, rng: &mut R)
+                                    -> Result<(key::SecretKey, key::PublicKey), Error> {
+        let sk = key::SecretKey::new(self, rng);
+        let pk = try!(key::PublicKey::from_secret_key(self, &sk));
+        Ok((sk, pk))
+    }
+}
+
+impl<C: Verification> Secp256k1<C> {
+
     /// Determines the public key for which `sig` is a valid signature for
     /// `msg`. Requires a verify-capable context.
     pub fn recover(&self, msg: &Message, sig: &RecoverableSignature)
                    -> Result<key::PublicKey, Error> {
-        if self.caps == ContextFlag::SignOnly || self.caps == ContextFlag::None {
-            return Err(Error::IncapableContext);
-        }
 
         let mut pk = unsafe { ffi::PublicKey::blank() };
 
@@ -552,9 +567,6 @@ impl Secp256k1 {
     /// verify-capable context.
     #[inline]
     pub fn verify(&self, msg: &Message, sig: &Signature, pk: &key::PublicKey) -> Result<(), Error> {
-        if self.caps == ContextFlag::SignOnly || self.caps == ContextFlag::None {
-            return Err(Error::IncapableContext);
-        }
 
         if !pk.is_valid() {
             Err(Error::InvalidPublicKey)
@@ -567,14 +579,13 @@ impl Secp256k1 {
     }
 }
 
-
 #[cfg(test)]
 mod tests {
     use rand::{Rng, thread_rng};
 
     use key::{SecretKey, PublicKey};
     use super::constants;
-    use super::{Secp256k1, Signature, RecoverableSignature, Message, RecoveryId, ContextFlag};
+    use super::{Secp256k1, Signature, RecoverableSignature, Message, RecoveryId};
     use super::Error::{InvalidMessage, InvalidPublicKey, IncorrectSignature, InvalidSignature,
                        IncapableContext};
 
@@ -603,29 +614,23 @@ mod tests {
 
     #[test]
     fn capabilities() {
-        let none = Secp256k1::with_caps(ContextFlag::None);
+        let none = Secp256k1::without_caps();
-        let sign = Secp256k1::with_caps(ContextFlag::SignOnly);
+        let sign = Secp256k1::signing_only();
-        let vrfy = Secp256k1::with_caps(ContextFlag::VerifyOnly);
+        let vrfy = Secp256k1::verification_only();
-        let full = Secp256k1::with_caps(ContextFlag::Full);
+        let full = Secp256k1::new();
 
         let mut msg = [0u8; 32];
         thread_rng().fill_bytes(&mut msg);
         let msg = Message::from_slice(&msg).unwrap();
 
         // Try key generation
-        assert_eq!(none.generate_keypair(&mut thread_rng()), Err(IncapableContext));
-        assert_eq!(vrfy.generate_keypair(&mut thread_rng()), Err(IncapableContext));
         assert!(sign.generate_keypair(&mut thread_rng()).is_ok());
         assert!(full.generate_keypair(&mut thread_rng()).is_ok());
         let (sk, pk) = full.generate_keypair(&mut thread_rng()).unwrap();
 
         // Try signing
-        assert_eq!(none.sign(&msg, &sk), Err(IncapableContext));
-        assert_eq!(vrfy.sign(&msg, &sk), Err(IncapableContext));
         assert!(sign.sign(&msg, &sk).is_ok());
         assert!(full.sign(&msg, &sk).is_ok());
-        assert_eq!(none.sign_recoverable(&msg, &sk), Err(IncapableContext));
-        assert_eq!(vrfy.sign_recoverable(&msg, &sk), Err(IncapableContext));
         assert!(sign.sign_recoverable(&msg, &sk).is_ok());
         assert!(full.sign_recoverable(&msg, &sk).is_ok());
         assert_eq!(sign.sign(&msg, &sk), full.sign(&msg, &sk));
@@ -634,14 +639,10 @@ mod tests {
         let sigr = full.sign_recoverable(&msg, &sk).unwrap();
 
         // Try verifying
-        assert_eq!(none.verify(&msg, &sig, &pk), Err(IncapableContext));
-        assert_eq!(sign.verify(&msg, &sig, &pk), Err(IncapableContext));
         assert!(vrfy.verify(&msg, &sig, &pk).is_ok());
         assert!(full.verify(&msg, &sig, &pk).is_ok());
 
         // Try pk recovery
-        assert_eq!(none.recover(&msg, &sigr), Err(IncapableContext));
-        assert_eq!(sign.recover(&msg, &sigr), Err(IncapableContext));
         assert!(vrfy.recover(&msg, &sigr).is_ok());
         assert!(full.recover(&msg, &sigr).is_ok());
 

src/schnorr.rs

@@ -19,13 +19,15 @@ use ContextFlag;
 use Error;
 use Message;
 use Secp256k1;
+use Signing;
 
 use constants;
 use ffi;
-use key::{SecretKey, PublicKey};
+use key::{PublicKey, SecretKey};
 
-use std::{mem, ptr};
+use Verification;
 use std::convert::From;
+use std::{mem, ptr};
 
 /// A Schnorr signature.
 pub struct Signature([u8; constants::SCHNORR_SIGNATURE_SIZE]);
@@ -47,35 +49,41 @@ impl Signature {
     }
 }
 
-impl Secp256k1 {
+impl<C: Signing> Secp256k1<C> {
     /// Create a Schnorr signature
     pub fn sign_schnorr(&self, msg: &Message, sk: &SecretKey) -> Result<Signature, Error> {
-        if self.caps == ContextFlag::VerifyOnly || self.caps == ContextFlag::None {
-            return Err(Error::IncapableContext);
-        }
-
         let mut ret: Signature = unsafe { mem::uninitialized() };
         unsafe {
             // We can assume the return value because it's not possible to construct
             // an invalid signature from a valid `Message` and `SecretKey`
-            let err = ffi::secp256k1_schnorr_sign(self.ctx, ret.as_mut_ptr(), msg.as_ptr(),
+            let err = ffi::secp256k1_schnorr_sign(
-                                                  sk.as_ptr(), ffi::secp256k1_nonce_function_rfc6979,
+                self.ctx,
-                                                  ptr::null());
+                ret.as_mut_ptr(),
+                msg.as_ptr(),
+                sk.as_ptr(),
+                ffi::secp256k1_nonce_function_rfc6979,
+                ptr::null(),
+            );
             debug_assert_eq!(err, 1);
         }
         Ok(ret)
     }
-
-    /// Verify a Schnorr signature
-    pub fn verify_schnorr(&self, msg: &Message, sig: &Signature, pk: &PublicKey) -> Result<(), Error> {
-        if self.caps == ContextFlag::SignOnly || self.caps == ContextFlag::None {
-            return Err(Error::IncapableContext);
 }
 
+impl<C: Verification> Secp256k1<C> {
+    /// Verify a Schnorr signature
+    pub fn verify_schnorr(
+        &self,
+        msg: &Message,
+        sig: &Signature,
+        pk: &PublicKey,
+    ) -> Result<(), Error> {
         if !pk.is_valid() {
             Err(Error::InvalidPublicKey)
-        } else if unsafe { ffi::secp256k1_schnorr_verify(self.ctx, sig.as_ptr(), msg.as_ptr(),
+        } else if unsafe {
-                                                         pk.as_ptr()) } == 0 {
+            ffi::secp256k1_schnorr_verify(self.ctx, sig.as_ptr(), msg.as_ptr(), pk.as_ptr())
+        } == 0
+        {
             Err(Error::IncorrectSignature)
         } else {
             Ok(())
@@ -84,16 +92,10 @@ impl Secp256k1 {
 
     /// Retrieves the public key for which `sig` is a valid signature for `msg`.
     /// Requires a verify-capable context.
-    pub fn recover_schnorr(&self, msg: &Message, sig: &Signature)
+    pub fn recover_schnorr(&self, msg: &Message, sig: &Signature) -> Result<PublicKey, Error> {
-                           -> Result<PublicKey, Error> {
-        if self.caps == ContextFlag::SignOnly || self.caps == ContextFlag::None {
-            return Err(Error::IncapableContext);
-        }
-
         let mut pk = unsafe { ffi::PublicKey::blank() };
         unsafe {
-            if ffi::secp256k1_schnorr_recover(self.ctx, &mut pk,
+            if ffi::secp256k1_schnorr_recover(self.ctx, &mut pk, sig.as_ptr(), msg.as_ptr()) != 1 {
-                                              sig.as_ptr(), msg.as_ptr()) != 1 {
                 return Err(Error::InvalidSignature);
             }
         };