Add public key into UEFI db using EDK2

[kowalski100]

Hi,

My PC is in User Mode. There is no option to put into Setup Mode. I have a private-public key pair which i want to use for secure boot verification. In UEFI/BIOS i can manually add public_key.cer with a GUID. I tried manually adding key and it works fine.
Now i want to do a little automation. What i want is to put public key (.esl format generated from .cer key) and use the following edk2 code to automatically insert the key but it always fails.
what it does is it reads the existing db and append my key and write back. but i get some invalid parameter error.
Can someone help me how to insert public key into uefi db using edk2 code?

.... read key from file 
.... some other processing.

  // Copy new certificate data
  CopyMem(NewVariableData + VariableSize, SignatureList, SignatureListSize);

  // Set the variable with combined data
  Status = gRT->SetVariable (
                  L"db",
                  &gEfiGlobalVariableGuid,
                  EFI_VARIABLE_NON_VOLATILE | EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS,
                  VariableSize + SignatureListSize,
                  NewVariableData
                  );