From 0b5bda2266c1d7e226b3b24233a9c1e4bd614473 Mon Sep 17 00:00:00 2001
From: Daniel James <dwhjames@gmail.com>
Date: Tue, 14 Sep 2021 12:45:23 -0700
Subject: [PATCH] Add pem_rfc7468::decode_label

Decode the encapsulation boundaries of a PEM document, returning the label.

The motivation is to capture the document label in the case of an error
(by repeating the decoding only to extract the label)
---
 pem-rfc7468/src/decoder.rs  | 6 ++++++
 pem-rfc7468/src/lib.rs      | 2 +-
 pem-rfc7468/tests/decode.rs | 4 ++++
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/pem-rfc7468/src/decoder.rs b/pem-rfc7468/src/decoder.rs
index d0bf5d64e..7d0c84cc8 100644
--- a/pem-rfc7468/src/decoder.rs
+++ b/pem-rfc7468/src/decoder.rs
@@ -44,6 +44,12 @@ fn decode_encapsulated_text<'i, 'o>(
     }
     Ok(())
 }
+/// Decode the encapsulation boundaries of a PEM document according to RFC 7468's "Strict" grammar.
+///
+/// On success, returning the decoded label.
+pub fn decode_label(pem: &[u8]) -> Result<&str> {
+    Ok(Encapsulation::try_from(pem)?.label())
+}
 
 /// Decode a PEM document according to RFC 7468's "Strict" grammar.
 ///
diff --git a/pem-rfc7468/src/lib.rs b/pem-rfc7468/src/lib.rs
index defd0a805..b58fee369 100644
--- a/pem-rfc7468/src/lib.rs
+++ b/pem-rfc7468/src/lib.rs
@@ -112,7 +112,7 @@ mod error;
 mod grammar;
 
 pub use crate::{
-    decoder::decode,
+    decoder::{decode, decode_label},
     encoder::{encode, encoded_len, LineEnding},
     error::{Error, Result},
 };
diff --git a/pem-rfc7468/tests/decode.rs b/pem-rfc7468/tests/decode.rs
index 20e4b09df..6b2c571b1 100644
--- a/pem-rfc7468/tests/decode.rs
+++ b/pem-rfc7468/tests/decode.rs
@@ -17,6 +17,8 @@ fn pkcs1_enc_example() {
         Err(pem_rfc7468::Error::HeaderDetected) => (),
         _ => panic!("Expected HeaderDetected error"),
     }
+    let label = pem_rfc7468::decode_label(pem).unwrap();
+    assert_eq!(label, "RSA PRIVATE KEY");
 }
 
 #[test]
@@ -37,6 +39,8 @@ fn header_of_length_64() {
         Err(pem_rfc7468::Error::HeaderDetected) => (),
         _ => panic!("Expected HeaderDetected error"),
     }
+    let label = pem_rfc7468::decode_label(pem).unwrap();
+    assert_eq!(label, "RSA PRIVATE KEY");
 }
 
 #[test]