Cleanly check validity rules for DescriptorKeys

Each context has slightly different rules on what Pks are allowed in descriptors

Legacy/Bare does not allow x_only keys
SegwitV0 does not allow uncompressed keys and x_only keys
Tapscript does not allow uncompressed keys

Note that String types are allowed everywhere

Author: sanket1729

src/miniscript/context.rs

@@ -220,6 +220,12 @@ where
         Ok(())
     }
 
+    /// Each context has slightly different rules on what Pks are allowed in descriptors
+    /// Legacy/Bare does not allow x_only keys
+    /// Segwit does not allow uncompressed keys and x_only keys
+    /// Tapscript does not allow uncompressed keys
+    fn check_pk<Pk: MiniscriptKey>(pk: &Pk) -> Result<(), ScriptContextError>;
+
     /// Depending on script context, the size of a satifaction witness may slightly differ.
     fn max_satisfaction_size<Pk: MiniscriptKey>(ms: &Miniscript<Pk, Self>) -> Option<usize>;
     /// Depending on script Context, some of the Terminals might not
@@ -367,6 +373,18 @@ impl ScriptContext for Legacy {
         }
     }
 
+    // Only compressed and uncompressed public keys are allowed in Legacy context
+    fn check_pk<Pk: MiniscriptKey>(pk: &Pk) -> Result<(), ScriptContextError> {
+        if pk.is_x_only_key() {
+            Err(ScriptContextError::XOnlyKeysNotAllowed(
+                pk.to_string(),
+                Self::name_str(),
+            ))
+        } else {
+            Ok(())
+        }
+    }
+
     fn check_witness<Pk: MiniscriptKey>(witness: &[Vec<u8>]) -> Result<(), ScriptContextError> {
         // In future, we could avoid by having a function to count only
         // len of script instead of converting it.
@@ -384,31 +402,21 @@ impl ScriptContext for Legacy {
         }
 
         match ms.node {
-            Terminal::PkK(ref key) if key.is_x_only_key() => {
-                return Err(ScriptContextError::XOnlyKeysNotAllowed(
-                    key.to_string(),
-                    Self::name_str(),
-                ))
-            }
+            Terminal::PkK(ref pk) => Self::check_pk(pk),
             Terminal::Multi(_k, ref pks) => {
                 if pks.len() > MAX_PUBKEYS_PER_MULTISIG {
                     return Err(ScriptContextError::CheckMultiSigLimitExceeded);
                 }
                 for pk in pks.iter() {
-                    if pk.is_x_only_key() {
-                        return Err(ScriptContextError::XOnlyKeysNotAllowed(
-                            pk.to_string(),
-                            Self::name_str(),
-                        ));
-                    }
+                    Self::check_pk(pk)?;
                 }
+                Ok(())
             }
             Terminal::MultiA(..) => {
                 return Err(ScriptContextError::MultiANotAllowed);
             }
-            _ => {}
+            _ => Ok(()),
         }
-        Ok(())
     }
 
     fn check_local_consensus_validity<Pk: MiniscriptKey>(
@@ -472,6 +480,20 @@ impl ScriptContext for Segwitv0 {
         Ok(())
     }
 
+    // No x-only keys or uncompressed keys in Segwitv0 context
+    fn check_pk<Pk: MiniscriptKey>(pk: &Pk) -> Result<(), ScriptContextError> {
+        if pk.is_uncompressed() {
+            Err(ScriptContextError::UncompressedKeysNotAllowed)
+        } else if pk.is_x_only_key() {
+            Err(ScriptContextError::XOnlyKeysNotAllowed(
+                pk.to_string(),
+                Self::name_str(),
+            ))
+        } else {
+            Ok(())
+        }
+    }
+
     fn check_witness<Pk: MiniscriptKey>(witness: &[Vec<u8>]) -> Result<(), ScriptContextError> {
         if witness.len() > MAX_STANDARD_P2WSH_STACK_ITEMS {
             return Err(ScriptContextError::MaxWitnessItemssExceeded {
@@ -490,30 +512,13 @@ impl ScriptContext for Segwitv0 {
         }
 
         match ms.node {
-            Terminal::PkK(ref pk) => {
-                if pk.is_uncompressed() {
-                    return Err(ScriptContextError::CompressedOnly(pk.to_string()));
-                } else if pk.is_x_only_key() {
-                    return Err(ScriptContextError::XOnlyKeysNotAllowed(
-                        pk.to_string(),
-                        Self::name_str(),
-                    ));
-                }
-                Ok(())
-            }
+            Terminal::PkK(ref pk) => Self::check_pk(pk),
             Terminal::Multi(_k, ref pks) => {
                 if pks.len() > MAX_PUBKEYS_PER_MULTISIG {
                     return Err(ScriptContextError::CheckMultiSigLimitExceeded);
                 }
                 for pk in pks.iter() {
-                    if pk.is_uncompressed() {
-                        return Err(ScriptContextError::CompressedOnly(pk.to_string()));
-                    } else if pk.is_x_only_key() {
-                        return Err(ScriptContextError::XOnlyKeysNotAllowed(
-                            pk.to_string(),
-                            Self::name_str(),
-                        ));
-                    }
+                    Self::check_pk(pk)?;
                 }
                 Ok(())
             }
@@ -594,6 +599,15 @@ impl ScriptContext for Tap {
         Ok(())
     }
 
+    // No uncompressed keys in Tap context
+    fn check_pk<Pk: MiniscriptKey>(pk: &Pk) -> Result<(), ScriptContextError> {
+        if pk.is_uncompressed() {
+            Err(ScriptContextError::UncompressedKeysNotAllowed)
+        } else {
+            Ok(())
+        }
+    }
+
     fn check_witness<Pk: MiniscriptKey>(witness: &[Vec<u8>]) -> Result<(), ScriptContextError> {
         // Note that tapscript has a 1000 limit compared to 100 of segwitv0
         if witness.len() > MAX_STACK_SIZE {
@@ -618,9 +632,10 @@ impl ScriptContext for Tap {
         }
 
         match ms.node {
-            Terminal::PkK(ref pk) => {
-                if pk.is_uncompressed() {
-                    return Err(ScriptContextError::UncompressedKeysNotAllowed);
+            Terminal::PkK(ref pk) => Self::check_pk(pk),
+            Terminal::MultiA(_, ref keys) => {
+                for pk in keys.iter() {
+                    Self::check_pk(pk)?;
                 }
                 Ok(())
             }
@@ -705,30 +720,32 @@ impl ScriptContext for BareCtx {
         Ok(())
     }
 
+    // No x-only keys in Bare context
+    fn check_pk<Pk: MiniscriptKey>(pk: &Pk) -> Result<(), ScriptContextError> {
+        if pk.is_x_only_key() {
+            Err(ScriptContextError::XOnlyKeysNotAllowed(
+                pk.to_string(),
+                Self::name_str(),
+            ))
+        } else {
+            Ok(())
+        }
+    }
+
     fn check_global_consensus_validity<Pk: MiniscriptKey>(
         ms: &Miniscript<Pk, Self>,
     ) -> Result<(), ScriptContextError> {
         if ms.ext.pk_cost > MAX_SCRIPT_SIZE {
             return Err(ScriptContextError::MaxWitnessScriptSizeExceeded);
         }
         match ms.node {
-            Terminal::PkK(ref key) if key.is_x_only_key() => {
-                return Err(ScriptContextError::XOnlyKeysNotAllowed(
-                    key.to_string(),
-                    Self::name_str(),
-                ))
-            }
+            Terminal::PkK(ref key) => Self::check_pk(key),
             Terminal::Multi(_k, ref pks) => {
                 if pks.len() > MAX_PUBKEYS_PER_MULTISIG {
                     return Err(ScriptContextError::CheckMultiSigLimitExceeded);
                 }
                 for pk in pks.iter() {
-                    if pk.is_x_only_key() {
-                        return Err(ScriptContextError::XOnlyKeysNotAllowed(
-                            pk.to_string(),
-                            Self::name_str(),
-                        ));
-                    }
+                    Self::check_pk(pk)?;
                 }
                 Ok(())
             }
@@ -799,6 +816,11 @@ impl ScriptContext for NoChecks {
         Ok(())
     }
 
+    // No checks in NoChecks
+    fn check_pk<Pk: MiniscriptKey>(_pk: &Pk) -> Result<(), ScriptContextError> {
+        Ok(())
+    }
+
     fn check_global_policy_validity<Pk: MiniscriptKey>(
         _ms: &Miniscript<Pk, Self>,
     ) -> Result<(), ScriptContextError> {