pkcs8: eagerly decode PEM labels

This should give better errors for invalid PEM type labels even if there
are subsequent Base64 processing errors, which might occur if e.g.
Base64 is wrapped at a nonstandard width.

Notably such errors will include the expected PEM type label.

pem-rfc7468/src/error.rs

@@ -45,7 +45,7 @@ pub enum Error {
 
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
+        match *self {
             Error::Base64(err) => write!(f, "PEM Base64 error: {}", err),
             Error::CharacterEncoding => f.write_str("PEM character encoding error"),
             Error::EncapsulatedText => f.write_str("PEM error in encapsulated text"),
@@ -60,7 +60,11 @@ impl fmt::Display for Error {
                 f.write_str("PEM error in post-encapsulation boundary")
             }
             Error::UnexpectedTypeLabel { expected } => {
-                write!(f, "unexpected PEM type label: expecting \"{}\"", expected)
+                write!(
+                    f,
+                    "unexpected PEM type label: expecting \"BEGIN {}\"",
+                    expected
+                )
             }
         }
     }

pkcs8/src/traits.rs

@@ -12,10 +12,14 @@ use {
 };
 
 #[cfg(feature = "pem")]
-use {crate::LineEnding, alloc::string::String, der::zeroize::Zeroizing};
-
-#[cfg(feature = "pem")]
-use der::pem::PemLabel;
+use {
+    crate::LineEnding,
+    alloc::string::String,
+    der::{
+        pem::{self, PemLabel},
+        zeroize::Zeroizing,
+    },
+};
 
 #[cfg(feature = "std")]
 use std::path::Path;
@@ -43,8 +47,11 @@ pub trait DecodePrivateKey: Sized {
     /// ```
     #[cfg(feature = "pem")]
     fn from_pkcs8_pem(s: &str) -> Result<Self> {
-        let (label, doc) = SecretDocument::from_pem(s)?;
+        // Validate PEM label
+        let label = pem::decode_label(s.as_bytes())?;
         PrivateKeyInfo::validate_pem_label(label)?;
+
+        let doc = SecretDocument::from_pem(s)?.1;
         Self::from_pkcs8_der(doc.as_bytes())
     }