(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.

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

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
success: Boolean;
cert: HCkCert;
pubKey: HCkPublicKey;
privKey: HCkPrivateKey;

begin
success := False;

cert := CkCert_Create();

// Load a digital certificate.
success := CkCert_LoadFromFile(cert,'digitalCert.cer');
if (success = False) 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 := CkPublicKey_Create();
CkCert_GetPublicKey(cert,pubKey);

// 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));
    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));

CkCert_Dispose(cert);
CkPublicKey_Dispose(pubKey);
CkPrivateKey_Dispose(privKey);

end;