(Objective-C) 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.

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#import <CkoRsa.h>
#import <CkoPrivateKey.h>
#import <CkoBinData.h>
#import <NSString.h>

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

CkoRsa *rsa = [[CkoRsa alloc] init];

CkoPrivateKey *privKey = [[CkoPrivateKey alloc] init];

BOOL success = [privKey LoadPemFile: @"myPrivateKey.pem"];
if (success != YES) {
    NSLog(@"%@",privKey.LastErrorText);
    return;
}

success = [rsa ImportPrivateKeyObj: privKey];
if (success != YES) {
    NSLog(@"%@",rsa.LastErrorText);
    return;
}

// 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...
CkoBinData *bdEncrypted = [[CkoBinData alloc] init];
success = [bdEncrypted LoadFile: @"qa_data/passphrase.enc"];
if (success != YES) {
    NSLog(@"%@",@"Failed to load file.");
    return;
}

// 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.EncodingMode = @"base64";
NSString *passphrase = [rsa DecryptStringENC: [bdEncrypted GetEncoded: @"base64"] bUsePrivateKey: YES];
if (rsa.LastMethodSuccess != YES) {
    NSLog(@"%@",rsa.LastErrorText);
    return;
}

NSLog(@"%@%@",@"Decrypted passphrase: ",passphrase);