(Visual Basic 6.0) 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 ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

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

Dim pkey As New PrivateKey

' Load the private key from an PEM file:
Dim success As Long
success = pkey.LoadPemFile("private.pem")
If (success <> 1) Then
    Debug.Print pkey.LastErrorText
    Exit Sub
End If

' Load the file to be signed.
Dim bd As New ChilkatBinData
success = bd.LoadFile("small.dat")

Dim rsa As New ChilkatRsa

' Import the private key into the RSA component:
success = rsa.ImportPrivateKeyObj(pkey)
If (success <> 1) Then
    Debug.Print rsa.LastErrorText
    Exit Sub
End If

' OpenSSL uses big-endian.
rsa.LittleEndian = 0

' Sign bd.  If successful, bd will contain the RSA signature that contains the original data.
success = rsa.OpenSslSignBd(bd)
If (success <> 1) Then
    Debug.Print rsa.LastErrorText
    Exit Sub
End If

' Save the binary signature to a file.
success = bd.WriteFile("rsautl.sig")
If (success <> 1) Then
    Debug.Print "Failed to save output signature file."
    Exit Sub
End If

Debug.Print "Success."