Reject CONTEXT-SPECIFIC tag class in serial number parser

[afldl]

Problem

parse_serial() checks that the tag number equals Tag::Integer but does not verify the tag class. A CONTEXT-SPECIFIC [2] tag byte (0x82) has the same tag number as UNIVERSAL INTEGER (0x02) but a different class per X.690 §8.1.2. This makes the parser accept certificates with mismatched tag classes.

13 of 14 tested X.509 implementations reject this mutation. Only x509-parser and mbedTLS accept. See #248.

Fix

Check any.class() == Class::Universal before comparing the tag number.

     let (rem, any) = Any::parse_der(input).map_err(|_| X509Error::InvalidSerial)?;
+    if any.class() != Class::Universal {
+        return Err(Err::Error(X509Error::InvalidSerial));
+    }
     any.tag()
         .assert_eq(Tag::Integer)

Test

Single-byte PoC: offset 0x0d of a standard RSA-2048 self-signed cert, tag byte changed from 0x02 to 0x82.

Test added: test_serial_rejects_context_specific_tag. All 66 existing tests pass.

[cpu]

You can ignore the clippy & audit failings in CI. I believe both are going to be fixed by #223 Please just keep the commits related to the issue/fix.

[chifflier]

Hi,
Thank you for the report and PR.
The parser is indeed a bit permissive, and the proposed fix is correct.
I just suggested a few changes - can you update the PR?

[chifflier]

This change seems unrelated to the problem (and cargo fmt does not give the same result, FYI). Please remove this from the patch

[chifflier]

Ditto (unrelated)

[chifflier]

To stay consistent with the next test, this should be changed to use .class().assert_eq() (see suggestion)

Suggested change

	if any.class() != Class::Universal {
	return Err(Err::Error(X509Error::InvalidSerial));
	}
	any.class()
	.assert_eq(Class::Universal)
	.map_err(|_| X509Error::InvalidSerial)?;