Това би трябвало да ти свърши работа:

procedure TfrmMain.Button1Click(Sender: TObject);

Const

 CAPICOM_MY_STORE = 'My';

var

  Store: CAPICOM_TLB.IStore;

  sd: CAPICOM_TLB.ISignedData;

  signer:CAPICOM_TLB.ISigner;

  signer2:ISigner2;

  signedData:WideString;

begin

    Store:=CoStore.Create;

    Store.Open(CAPICOM_CURRENT_USER_STORE, CAPICOM_MY_STORE, CAPICOM_STORE_OPEN_READ_ONLY);

    sd := CoSignedData.Create;

    sd.Content:='Koko Marinkin';

    signer:=CoSigner.Create;

    signer2:=signer as ISigner2;

    signedData := sd.Sign(signer2, False,CAPICOM_ENCODE_BASE64);

    ShowMessage(signedData);

end;