0002: Parse the CMS in-package, by declared length

Status: accepted, implemented, and the decision changed during implementation. Read the outcome.

Context

There is no clean native replacement for reading a detached PDF signature: openssl_pkcs7_verify() needs the signed message reassembled, which is fragile. v1 sidestepped the problem entirely: it extracted the PKCS#7 blob with a ByteRange regex, ran three preg_replace calls over the openssl text output, and reported a document as validated when the parsed subject happened to contain a CN or OU field. A tampered document still has those.

Two options were weighed:

1. Keep the CLI, encapsulated, but parse certificates with openssl_x509_parse() instead of regex. No new dependency, immediate robustness gain. Recommended for 2.0 at the time.
2. Add phpseclib/phpseclib ^3 and decode the CMS through ASN.1, which removes the CLI entirely and opens the way to actually verifying the signature. New dependency, more work. Pencilled in for 2.1.

Decision

Neither, as written. The package reads ASN.1 itself, in Validation\DerReader and Validation\Pkcs7Reader, and verifies cryptographically, without taking phpseclib as a dependency.

Validation\OpenSslCliSignatureVerifier remains a deliberate shell-out: verifying the CMS against the covered bytes is the one operation where reimplementing was not worth it.

Consequences

• SignatureReport::isValid() means the CMS actually verifies against the bytes each signature covers. It is not a metadata check.
• Every structure is read by its declared length. Trimming trailing 0 bytes cuts legitimate DER. See the invariants.
• DocTimeStamps are classified separately (isTimestamp) and excluded from isValid(): they are timestamps over the file, not signatures by a signer.
• All signatures in a document are reported, not only the first.

Outcome

The honest note the plan carried, "the current method does not verify the signature cryptographically, and the v2 naming must reflect that limitation until option (ii) exists", is obsolete. Verification landed in 2.0, ahead of the schedule the plan set, and without the dependency the plan expected to pay for it.