Create a X.509 Certificate with Authority Key Identifier extension in C# using CERTEnroll.dll

Question

for a legacy .NET Framework application I need to create a self-signed certificate using CERTENROLLLib (i.e. CERTEnroll.dll from the Windows SDK). A basic certificate can be successfully created using this method.

As a new feature this certicicate must contain an Authority Key Identifier extension. This is added to the working certificate request:

var dn = new CX500DistinguishedName();
dn.Encode("CN=" + certificate.SubjectName);

var rng = RandomNumberGenerator.Create();
var serialNumberBytes = new byte[SerialNumberLengthMin];
rng.GetBytes(serialNumberBytes);

// Only positive numbers are allowed
serialNumberBytes[serialNumberBytes.Length - 1] &= 0x7f;

// Convert it to a hex string in little endian
var serialNumber = GetHexString(serialNumberBytes, true);

var certificateRequest = new CX509CertificateRequestCertificate();

CX509ExtensionAuthorityKeyIdentifier aki = new CX509ExtensionAuthorityKeyIdentifierClass();
aki.InitializeEncode(EncodingType.XCN_CRYPT_STRING_HEX, CreateAuthorityKeyIdentifier(dn.Name, serialNumberBytes));
certificateRequest.X509Extensions.Add((CX509Extension)aki);

The method CreateAuthorityKeyIdentifier uses AsnWriter(AsnEncodingRules.DER) to generate a string that is converted to a big endian hex string.

private static string CreateAuthorityKeyIdentifier(string issuer, byte[] serialNumber)
{
  var keyId = "2.5.29.35";
  
  var writer = new AsnWriter(AsnEncodingRules.DER);
  writer.PushSequence();

  var keyIdTag = new Asn1Tag(TagClass.ContextSpecific, 0);
  writer.WriteOctetString(Encoding.ASCII.GetBytes(keyId), keyIdTag);
 
  var issuerNameTag = new Asn1Tag(TagClass.ContextSpecific, 1);
  writer.PushSequence(issuerNameTag);
  var directoryNameTag = new Asn1Tag(TagClass.ContextSpecific, 4, true);
  writer.PushSetOf(directoryNameTag);
  writer.WriteEncodedValue((new X500DistinguishedName(issuer)).RawData);
  writer.PopSetOf(directoryNameTag);
  writer.PopSequence(issuerNameTag);

  var issuerSerialTag = new Asn1Tag(TagClass.ContextSpecific, 2);
  var issuerSerial = new BigInteger(serialNumber);
  writer.WriteInteger(issuerSerial, issuerSerialTag);

  writer.PopSequence();

  return GetHexString(writer.Encode());
}

After creating an enrollment and calling CX509Enrollment.CreatePFX this throws an exception with error code CertEnroll::CX509Enrollment::CreatePFX: Invalid Signature. 0x80090006 (-2146893818 NTE_BAD_SIGNATURE). The CX509Enrollment.InstallResponse is called before and does not throw an exception.

I'm at a loss solving the cause of the exception.

The certificate is correctly created with other extensions. Sadly the authority key identifier is needed by the used library to accept this certificate.