(MFC) Duplicate openssl rsautl -encrypt –in mytext.txt -out mytest.enc -inkey mycertificate.cer -certin –pkcs

Demonstrates how to duplicate this OpenSSL command:

openssl.exe  rsautl -encrypt –in mytext.txt -out mytest.enc -inkey mycertificate.cer -certin –pkcs

Important: The RSA encryption algorithm produces different results for each call, even when encrypting the same data with the same key. Decryption however, will produce the correct results. This example demonstrates.

The reason this occurs is that RSA encryption uses PKCS1 v1.5 padding, and this padding scheme uses random bytes. It is random bytes in the padding that causes the result to be different each time.

The mytext.txt file is a small file. (RSA can only be used to encrypt or sign small amounts of data.)

How much data can be encrypted with "openssl rsautl"? The maximum size (in bytes) is the key size minus the overhead for the padding. The overhead size depends on the padding. With OAEP padding, the overhead is 42 bytes. With the default PKCSv1.5 padding, the overhead is 11 bytes.

Given a 2048-bit RSA key (256 bytes) and using PKCSv1.5 padding, the max data size that can be signed is 256 - 11 = 245 bytes.

Chilkat C/C++ Library Downloads MS Visual C/C++ Libs

See Also: Using MFC CString in Chilkat

#include <CkCert.h>
#include <CkBinData.h>
#include <CkRsa.h>
#include <CkPublicKey.h>

void ChilkatSample(void)
    {
    CkString strOut;

    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    CkCert cert;

    // Load the certificate from a file.
    bool success = cert.LoadFromFile("mycertificate.cer");
    if (success != true) {
        strOut.append(cert.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    // Load the file to be encrypted.
    CkBinData bd;
    success = bd.LoadFile("mytext.txt");

    CkRsa rsa;

    // Get the cert's public key.
    CkPublicKey *pubkey = cert.ExportPublicKey();

    // Import the public key into the RSA component:
    success = rsa.ImportPublicKeyObj(*pubkey);
    if (success != true) {
        delete pubkey;
        strOut.append(rsa.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    delete pubkey;

    // OpenSSL uses big-endian.
    rsa.put_LittleEndian(false);

    // Encrypt the contents of bd, replacing it with the encrypted data.
    bool bUsePrivateKey = false;
    success = rsa.EncryptBd(bd,bUsePrivateKey);
    if (success != true) {
        strOut.append(rsa.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    // Save the RSA encrypted data to a file.
    success = bd.WriteFile("mytest.enc");
    if (success != true) {
        strOut.append("Failed to write RSA encrypted output file.");
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    strOut.append("Success.");
    strOut.append("\r\n");


    SetDlgItemText(IDC_EDIT1,strOut.getUnicode());

    }