(Delphi DLL) 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.

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

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

...

procedure TForm1.Button1Click(Sender: TObject);
var
cert: HCkCert;
success: Boolean;
pubKey: HCkPublicKey;
privKey: HCkPrivateKey;

begin
cert := CkCert_Create();

// Load a digital certificate.
success := CkCert_LoadFromFile(cert,'digitalCert.cer');
if (success <> True) then
  begin
    Memo1.Lines.Add(CkCert__lastErrorText(cert));
    Exit;
  end;

// A .cer file does not contain the private key.  It should contain
// the public key...
pubKey := CkCert_ExportPublicKey(cert);

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

// 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 := CkPrivateKey_Create();

success := CkPrivateKey_LoadAnyFormatFile(privKey,'PrivateKey.key','');
if (success <> True) then
  begin
    Memo1.Lines.Add(CkPrivateKey__lastErrorText(privKey));
    CkPublicKey_Dispose(pubKey);
    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(CkPrivateKey__getXml(privKey));

CkPublicKey_Dispose(pubKey);

CkCert_Dispose(cert);
CkPrivateKey_Dispose(privKey);

end;