der: use read_nested to check length of IMPLICIT types (#1739)

Author: dishmaker

der/src/asn1/context_specific.rs

@@ -63,8 +63,14 @@ impl<T> ContextSpecific<T> {
         T: DecodeValue<'a> + Tagged,
     {
         Self::decode_with::<_, _, T::Error>(reader, tag_number, |reader| {
+            // Decode IMPLICIT header
             let header = Header::decode(reader)?;
-            let value = T::decode_value(reader, header)?;
+
+            // read_nested checks if header matches decoded length
+            let value = reader.read_nested(header.length, |reader| {
+                // Decode inner IMPLICIT value
+                T::decode_value(reader, header)
+            })?;
 
             if header.tag.is_constructed() != value.tag().is_constructed() {
                 return Err(header.tag.non_canonical_error().into());
@@ -119,6 +125,7 @@ where
     type Error = T::Error;
 
     fn decode<R: Reader<'a>>(reader: &mut R) -> Result<Self, Self::Error> {
+        // Decode EXPLICIT header
         let header = Header::decode(reader)?;
 
         match header.tag {
@@ -128,7 +135,10 @@ where
             } => Ok(Self {
                 tag_number: number,
                 tag_mode: TagMode::default(),
-                value: reader.read_nested(header.length, |reader| T::decode(reader))?,
+                value: reader.read_nested(header.length, |reader| {
+                    // Decode inner tag-length-value of EXPLICIT
+                    T::decode(reader)
+                })?,
             }),
             tag => Err(tag.unexpected_error(None).into()),
         }

x509-cert/tests/pkix_extensions.rs

@@ -843,8 +843,6 @@ fn decode_cert() {
 
 #[test]
 fn decode_idp() {
-    use der::TagNumber;
-
     // IDP from 04A8739769B3C090A11DCDFABA3CF33F4BEF21F3.crl in PKITS 2048 in ficam-scvp-testing repo
     let idp = IssuingDistributionPoint::from_der(&hex!("30038201FF")).unwrap();
     assert_eq!(idp.only_contains_ca_certs, true);
@@ -1112,7 +1110,11 @@ fn decode_idp() {
             panic!("Expected FullName")
         }
     }
+}
 
+#[test]
+fn decode_idp_negative() {
+    use der::TagNumber;
     //---------------------------------
     // Negative tests
     //---------------------------------
@@ -1143,7 +1145,7 @@ fn decode_idp() {
         "3067A060A05EA45C305A310B3009060355040613025553311F301D060355040A131654657374204365727469666963617465732032303137311C301A060355040B13136F6E6C79536F6D65526561736F6E7320434133310C300A0603550403130343524C8304079F80"
     ));
     let err = idp.err().unwrap();
-    assert_eq!(err.position().unwrap(), 103u8.into());
+    assert_eq!(err.position().unwrap(), 105u8.into());
     assert_eq!(
         ErrorKind::Incomplete {
             expected_len: 106u8.into(),
@@ -1197,7 +1199,13 @@ fn decode_idp() {
         "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"
     ));
     let err = idp.err().unwrap();
-    assert_eq!(ErrorKind::Length { tag: Tag::Boolean }, err.kind());
+    assert_eq!(
+        ErrorKind::Incomplete {
+            expected_len: Length::new(365),
+            actual_len: Length::new(364)
+        },
+        err.kind()
+    );
 
     // Boolean value is neither 0x00 nor 0xFF
     let idp = IssuingDistributionPoint::from_der(&hex!(