scep: set keyUsage on the self-signed enrollee certificate

[yosuke-wolfssl]

Problem

RFC 8894 §1 requires that a SCEP client with no existing certificate sign its
pkiMessage with a self-signed certificate carrying a keyUsage extension
asserting digitalSignature and keyEncipherment.
wolfcert_scep_self_signed_rsa() in src/scep/scep_msg.c built that
certificate without one: it set selfSigned, sigType and daysValid,
copied the CSR subject, then called wc_MakeSelfCert() directly. The
certificate goes on the wire in the SignedData certificates field via
wc_PKCS7_InitWithCert(), so every enrollment presented a non-conformant
signer identity to the CA. Reached from four call sites in
src/scep/scep_client.c — the blocking and non-blocking PKCSReq paths and
the GetCertInitial fallback signer — so it affected all SCEP enrollments,
and a CA that enforces the requirement rejects them. Standards conformance;
no memory-safety impact.

Fix (src/scep/scep_msg.c)

Set the usage before the certificate is signed:

    rc = wc_SetKeyUsage(cert, "digitalSignature,keyEncipherment");
    if (rc != 0) {
        wc_CertFree(cert);
        return WOLFCERT_ERR_WC(rc, "scep", "SetKeyUsage");
    }

Reuses the function's existing rc local and its wc_CertFree() +
early-return cleanup style. No #ifdef WOLFSSL_CERT_EXT guard is needed —
wolfcert/check_config.h already hard-errors when wolfSSL lacks it.

Closes f-9799.

Tests

test_signer_key_usage() in tests/unit/test_scep_msg.c builds a CSR,
generates the signer certificate, parses it, and requires extKeyUsageSet
plus both the KEYUSE_DIGITAL_SIG and KEYUSE_KEY_ENCIPHER bits.

Verification

• Build clean, no warnings.
• Negative control: the new test fails on the unpatched tree at the
  extKeyUsageSet assertion, passes after the fix.
• Full suite: 26/26 ctest pass, including scep_roundtrip,
  scep_poll_roundtrip, scep_async_roundtrip and parse_negative, which
  drive the real signer path end to end.

[Copilot]

Pull request overview

Updates SCEP self-signed enrollee certificates to include RFC 8894-required key usages.

Changes:

• Sets digitalSignature and keyEncipherment before signing.
• Adds unit coverage for both key-usage bits.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/unit/test_scep_msg.c	Tests the generated certificate’s key usages.
src/scep/scep_msg.c	Adds the required key-usage extension and error handling.

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[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #19

Scan targets checked: wolfcert-bugs, wolfcert-src

No new issues found in the changed files. ✅