LibCrypto+LibTLS: Implement Key Usage and Basic Constraints extensions

Root and intermediate CA certificates should have these extensions set
to indicate that they are allowed to sign other certificates. The values
reported in these extensions is now also checked by `verify_chain` to
make sure no non-CA certificates are used to sign another certificate.

The certificate parser now also aborts when a critical extension is
detected which is unsupported, as is required by the specification.

Userland/Libraries/LibTLS/Certificate.cpp

@@ -28,7 +28,9 @@ constexpr static Array<int, 7>
     rsa_sha512_encryption_oid { 1, 2, 840, 113549, 1, 1, 13 };
 
 constexpr static Array<int, 4>
-    subject_alternative_name_oid { 2, 5, 29, 17 };
+    key_usage_oid { 2, 5, 29, 15 },
+    subject_alternative_name_oid { 2, 5, 29, 17 },
+    basic_constraints_oid { 2, 5, 29, 19 };
 
 Optional<Certificate> Certificate::parse_asn1(ReadonlyBytes buffer, bool)
 {
@@ -457,6 +459,37 @@ Optional<Certificate> Certificate::parse_asn1(ReadonlyBytes buffer, bool)
                                     DROP_OBJECT_OR_FAIL("Certificate::TBSCertificate::Extensions::$::Extension::extension_value::SubjectAlternativeName::$::???");
                             }
                         }
+                    } else if (extension_id == key_usage_oid) {
+                        // RFC5280 section 4.2.1.3: The keyCertSign bit is asserted when the subject public key is used
+                        // for verifying signatures on public key certificates. If the keyCertSign bit is asserted,
+                        // then the cA bit in the basic constraints extension MUST also be asserted.
+                        Crypto::ASN1::Decoder decoder { extension_value.bytes() };
+                        READ_OBJECT_OR_FAIL(BitString, Crypto::ASN1::BitStringView, usage, "Certificate::TBSCertificate::Extensions::$::Extension::extension_value::KeyUsage");
+
+                        // keyCertSign (5)
+                        certificate.is_allowed_to_sign_certificate = usage.get(5);
+                    } else if (extension_id == basic_constraints_oid) {
+                        // RFC5280 section 4.2.1.9: The cA boolean indicates whether the certified public key may be
+                        // used to verify certificate signatures. If the cA boolean is not asserted, then the keyCertSign
+                        // bit in the key usage extension MUST NOT be asserted. If the basic constraints extension is
+                        // not present in a version 3 certificate, or the extension is present but the cA boolean is
+                        // not asserted, then the certified public key MUST NOT be used to verify certificate signatures.
+                        Crypto::ASN1::Decoder decoder { extension_value.bytes() };
+                        ENTER_SCOPE_OR_FAIL(Sequence, "Certificate::TBSCertificate::Extensions::$::Extension::extension_value::BasicConstraints");
+
+                        if (auto tag = decoder.peek(); !tag.is_error() && tag.value().kind == Crypto::ASN1::Kind::Boolean) {
+                            READ_OBJECT_OR_FAIL(Boolean, bool, is_certificate_authority, "Certificate::TBSCertificate::Extensions::$::Extension::extension_value::BasicConstraints::cA");
+                            certificate.is_certificate_authority = is_certificate_authority;
+
+                            if (auto tag = decoder.peek(); !tag.is_error() && tag.value().kind == Crypto::ASN1::Kind::Integer) {
+                                READ_OBJECT_OR_FAIL(Integer, Crypto::UnsignedBigInteger, path_length_constraint, "Certificate::TBSCertificate::Extensions::$::Extension::extension_value::BasicConstraints::pathLenConstraint");
+                                certificate.path_length_constraint = path_length_constraint.to_u64();
+                            }
+                        }
+                    } else {
+                        dbgln_if(TLS_DEBUG, "Certificate::TBSCertificate::Extensions::$::Extension::extension_id: unknown extension {} (critical: {})", extension_id, is_critical);
+                        if (is_critical)
+                            return {};
                     }
 
                     EXIT_SCOPE("Certificate::TBSCertificate::Extensions::$::Extension");

Userland/Libraries/LibTLS/Certificate.h

@@ -56,6 +56,9 @@ public:
     CertificateKeyAlgorithm signature_algorithm { CertificateKeyAlgorithm::Unsupported };
     ByteBuffer signature_value {};
     ByteBuffer original_asn1 {};
+    bool is_allowed_to_sign_certificate { false };
+    bool is_certificate_authority { false };
+    Optional<size_t> path_length_constraint {};
 
     static Optional<Certificate> parse_asn1(ReadonlyBytes, bool client_cert = false);
 

Userland/Libraries/LibTLS/TLSv12.cpp

@@ -297,6 +297,15 @@ bool Context::verify_chain(StringView host) const
                 return false;
             }
 
+            if (!(parent_certificate.is_allowed_to_sign_certificate && parent_certificate.is_certificate_authority)) {
+                dbgln("verify_chain: {} is not marked as certificate authority", issuer_string);
+                return false;
+            }
+            if (parent_certificate.path_length_constraint.has_value() && cert_index > parent_certificate.path_length_constraint.value()) {
+                dbgln("verify_chain: Path length for certificate exceeded");
+                return false;
+            }
+
             bool verification_correct = verify_certificate_pair(cert, parent_certificate);
             if (!verification_correct) {
                 dbgln("verify_chain: Signature inconsistent, {} was not signed by {}", subject_string, issuer_string);