(Delphi ActiveX) About RSA Public/Private Keys

This example provides some additional information for understanding public/private key pairs. In demonstrates how a private key is a superset of the public key. A public key contains the modulus and exponent. The matching private key also contains the modulus and exponent, but also contains the additional private key parts.

Note: This example requires Chilkat v11.0.0 or greater.

Chilkat for Delphi Downloads Chilkat ActiveX DLL for Delphi Chilkat non-ActiveX DLL for Delphi * The examples here use the ActiveX DLL.

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Chilkat_TLB;

...

procedure TForm1.Button1Click(Sender: TObject);
var
success: Integer;
cert: TChilkatCert;
pubKey: TPublicKey;
privKey: TPrivateKey;

begin
success := 0;

cert := TChilkatCert.Create(Self);

// Load a digital certificate.
success := cert.LoadFromFile('digitalCert.cer');
if (success = 0) then
  begin
    Memo1.Lines.Add(cert.LastErrorText);
    Exit;
  end;

// A .cer file does not contain the private key.  It should contain
// the public key...
pubKey := TPublicKey.Create(Self);
cert.GetPublicKey(pubKey.ControlInterface);

// Let's have a look at it (in XML format).
Memo1.Lines.Add('Public Key from Certificate:');
Memo1.Lines.Add(pubKey.GetXml());

// An RSA public key consists of a modulus and exponent.

// An RSA private key includes both the modulus and exponent,
// as well as other "big" numbers: P, Q, D, etc.

// Let's load an RSA private key from a DER-encoded file:
privKey := TPrivateKey.Create(Self);

success := privKey.LoadAnyFormatFile('PrivateKey.key','');
if (success <> 1) then
  begin
    Memo1.Lines.Add(privKey.LastErrorText);
    Exit;
  end;

// If this private key is the matching half to the public key from
// the certificate, then the modulus and exponent should
// be identical.  (Thus, a "private key" really contains both the public part as well as the private parts...).
Memo1.Lines.Add('Private Key from DER:');
Memo1.Lines.Add(privKey.GetXml());
end;