(C# UWP/WinRT) Duplicate openssl rsautl -sign -in small.dat -inkey private.pem -out rsautl.sig

Demonstrates how to duplicate this OpenSSL command:

openssl rsautl -sign -in small.dat -inkey private.pem -out rsautl.sig

How much data can be signed 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 Universal Windows Platform (UWP) / WinRT Downloads Chilkat for the Universal Windows Platform (UWP)

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

Chilkat.PrivateKey pkey = new Chilkat.PrivateKey();

// Load the private key from an PEM file:
bool success = pkey.LoadPemFile("private.pem");
if (success != true) {
    Debug.WriteLine(pkey.LastErrorText);
    return;
}

// Load the file to be signed.
Chilkat.BinData bd = new Chilkat.BinData();
success = bd.LoadFile("small.dat");

Chilkat.Rsa rsa = new Chilkat.Rsa();

// Import the private key into the RSA component:
success = rsa.ImportPrivateKeyObj(pkey);
if (success != true) {
    Debug.WriteLine(rsa.LastErrorText);
    return;
}

// OpenSSL uses big-endian.
rsa.LittleEndian = false;

// Sign bd.  If successful, bd will contain the RSA signature that contains the original data.
success = rsa.OpenSslSignBd(bd);
if (success != true) {
    Debug.WriteLine(rsa.LastErrorText);
    return;
}

// Save the binary signature to a file.
success = bd.WriteFile("rsautl.sig");
if (success != true) {
    Debug.WriteLine("Failed to save output signature file.");
    return;
}

Debug.WriteLine("Success.");