der: add Decode::from_der_partial (#1725)

Closes #1278

Author: dishmaker

der/src/decode.rs

@@ -7,7 +7,7 @@ use core::marker::PhantomData;
 use crate::{PemReader, pem::PemLabel};
 
 #[cfg(doc)]
-use crate::{Length, Tag};
+use crate::{ErrorKind, Length, Tag};
 
 #[cfg(feature = "alloc")]
 use alloc::boxed::Box;
@@ -34,11 +34,24 @@ pub trait Decode<'a>: Sized + 'a {
     }
 
     /// Parse `Self` from the provided DER-encoded byte slice.
+    ///
+    /// Returns [`ErrorKind::TrailingData`] if message is incomplete.
     fn from_der(bytes: &'a [u8]) -> Result<Self, Self::Error> {
         let mut reader = SliceReader::new(bytes)?;
         let result = Self::decode(&mut reader)?;
         Ok(reader.finish(result)?)
     }
+
+    /// Parse `Self` from the provided DER-encoded byte slice.
+    ///
+    /// Returns remaining byte slice, without checking for incomplete message.
+    fn from_der_partial(bytes: &'a [u8]) -> Result<(Self, &'a [u8]), Self::Error> {
+        let mut reader = SliceReader::new(bytes)?;
+        let result = Self::decode(&mut reader)?;
+
+        let remaining = reader.remaining()?;
+        Ok((result, remaining))
+    }
 }
 
 impl<'a, T> Decode<'a> for T

der/src/reader/slice.rs

@@ -56,7 +56,7 @@ impl<'a> SliceReader<'a> {
 
     /// Obtain the remaining bytes in this slice reader from the current cursor
     /// position.
-    fn remaining(&self) -> Result<&'a [u8], Error> {
+    pub(crate) fn remaining(&self) -> Result<&'a [u8], Error> {
         if self.is_failed() {
             Err(ErrorKind::Failed.at(self.position))
         } else {

der/tests/derive.rs

@@ -362,6 +362,35 @@ mod sequence {
         pub subject_public_key: &'a [u8],
     }
 
+    #[test]
+    fn decode_spki() {
+        let spki_bytes = hex!(
+        // first SPKI
+        "30 1A
+            30 0D 
+                06 09
+                    2A 86 48 86 F7 0D 01 01 01 
+                05 00
+            03 09
+                00 A0 A1 A2 A3 A4 A5 A6 A7"
+        // second SPKI
+        "30 1A
+            30 0D 
+                06 09
+                    2A 86 48 86 F7 0D 01 01 01 
+                05 00
+            03 09
+                00 B0 B1 B2 B3 B4 B5 B6 B7");
+
+        // decode first
+        let (spki, remaining) = SubjectPublicKeyInfo::from_der_partial(&spki_bytes).unwrap();
+        assert_eq!(spki.subject_public_key, hex!("A0 A1 A2 A3 A4 A5 A6 A7"));
+
+        // decode second
+        let (spki, _) = SubjectPublicKeyInfo::from_der_partial(remaining).unwrap();
+        assert_eq!(spki.subject_public_key, hex!("B0 B1 B2 B3 B4 B5 B6 B7"));
+    }
+
     /// PKCS#8v2 `OneAsymmetricKey`
     #[derive(Sequence)]
     pub struct OneAsymmetricKey<'a> {