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(VBScript) 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 ActiveX Downloads

ActiveX for 32-bit and 64-bit Windows

Dim fso, outFile
Set fso = CreateObject("Scripting.FileSystemObject")
Set outFile = fso.CreateTextFile("output.txt", True)

set rsa = CreateObject("Chilkat_9_5_0.Rsa")

success = rsa.UnlockComponent("Anything for 30-day trial")
If (success <> 1) Then
    outFile.WriteLine(rsa.LastErrorText)
    WScript.Quit
End If

set privKey = CreateObject("Chilkat_9_5_0.PrivateKey")

success = privKey.LoadPemFile("myPrivateKey.pem")
If (success <> 1) Then
    outFile.WriteLine(privKey.LastErrorText)
    WScript.Quit
End If

success = rsa.ImportPrivateKeyObj(privKey)
If (success <> 1) Then
    outFile.WriteLine(rsa.LastErrorText)
    WScript.Quit
End If

'  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...
set bdEncrypted = CreateObject("Chilkat_9_5_0.BinData")
success = bdEncrypted.LoadFile("qa_data/passphrase.enc")
If (success <> 1) Then
    outFile.WriteLine("Failed to load file.")
    WScript.Quit
End If

'  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"
passphrase = rsa.DecryptStringENC(bdEncrypted.GetEncoded("base64"),1)
If (rsa.LastMethodSuccess <> 1) Then
    outFile.WriteLine(rsa.LastErrorText)
    WScript.Quit
End If

outFile.WriteLine("Decrypted passphrase: " & passphrase)

outFile.Close

 

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