add std error

Cargo.toml

@@ -57,6 +57,7 @@ ed25519 = [ "sha2", "dep:ed25519-dalek" ]
 x509 = [ "dep:x509-cert", "dep:ocsp-x509" ]
 pem = [ "x509", "der/pem", "x509-cert?/pem" ]
 strict = []
+std = []
 
 [package.metadata.docs.rs]
 all-features = true

src/error.rs

@@ -1,5 +1,6 @@
 //! Verification Errors
 
+use alloc::fmt;
 use core::convert::Infallible;
 use der::asn1::ObjectIdentifier;
 
@@ -25,6 +26,22 @@ pub enum Error {
     Encode,
 }
 
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Error::Verification => write!(f, "Verification failure"),
+            Error::InvalidKey => write!(f, "Invalid key"),
+            Error::InvalidSignature => write!(f, "Invalid signature"),
+            Error::UnknownOid(oid) => write!(f, "Unknown OID: {}", oid),
+            Error::Decode => write!(f, "Decode failure"),
+            Error::Encode => write!(f, "Encode failure"),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for Error {}
+
 impl From<Infallible> for Error {
     fn from(_: Infallible) -> Self {
         unreachable!()

src/lib.rs

@@ -13,6 +13,9 @@
 
 extern crate alloc;
 
+#[cfg(feature = "std")]
+extern crate std;
+
 mod error;
 pub use error::Error;