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LibCrypto+LibTLS: Implement Key Usage and Basic Constraints extensions

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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.
This commit is contained in:
Michiel Visser 2022-02-23 17:35:05 +01:00 • committed by Ali Mohammad Pur
parent a6e465fba2
commit 804af863b4
3 changed files with 46 additions and 1 deletions
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35
Userland/Libraries/LibTLS/Certificate.cpp
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@ -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");