(PHP Extension) RSA Decrypt Binary

When something is RSA encrypted, the output is always equal in size to the RSA key. For example, if "Hello World" is encrypted with a 1024-bit RSA key, the output is 128 bytes (because 1024 bits / 8 bits per byte = 128 bytes). You can see that the size of the RSA key imposes a limit on how many bytes can be encrypted. Thus RSA encryption is for small amounts of data. A typical use for RSA encryption is to encrypt a bulk (symmetric) encryption key.

In this example, we have a binary file containing the result of RSA encryption. The size of the file tells us the RSA key size. For example, if the file is exactly 256 bytes, we know that the RSA key required to decrypt must be a 2048-bit key.

Chilkat PHP Downloads PHP Extension for Windows, Linux, MacOS, Alpine Linux, Solaris

<?php

// The version number (9_5_0) should match version of the Chilkat extension used, omitting the micro-version number.
// For example, if using Chilkat v9.5.0.48, then include as shown here:
include("chilkat_9_5_0.php");

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

$rsa = new CkRsa();

$privKey = new CkPrivateKey();

$success = $privKey->LoadPemFile('myPrivateKey.pem');
if ($success != true) {
    print $privKey->lastErrorText() . "\n";
    exit;
}

$success = $rsa->ImportPrivateKeyObj($privKey);
if ($success != true) {
    print $rsa->lastErrorText() . "\n";
    exit;
}

// Load the encrypted bytes.
// This will typically be a file that is 128, 256, etc. bytes in length.
// For example, maybe it is a file containing an encrypted passphrase...
$bdEncrypted = new CkBinData();
$success = $bdEncrypted->LoadFile('qa_data/passphrase.enc');
if ($success != true) {
    print 'Failed to load file.' . "\n";
    exit;
}

// In this case, we know that it was a string that was encrypted,
// so decryption should result in a string.
// To make things easy, we'll pass the RSA encrypted data as a Base64 string 
// to the decryptor.
$rsa->put_EncodingMode('base64');
$passphrase = $rsa->decryptStringENC($bdEncrypted->getEncoded('base64'),true);
if ($rsa->get_LastMethodSuccess() != true) {
    print $rsa->lastErrorText() . "\n";
    exit;
}

print 'Decrypted passphrase: ' . $passphrase . "\n";

?>