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AES-256-CBC Encrypt and Decrypt Functions in VBScript

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AES-256-CBC in VBScript
=======================

This project provides VBScript functions to perform encryption and
decryption with AES-256-CBC.

[![View Source][Source SVG]][src]
[![MIT License][License SVG]][L]

[Source SVG]: https://img.shields.io/badge/view-source-brightgreen.svg
[License SVG]: https://img.shields.io/badge/license-MIT-blue.svg
[src]: aes.vbs

While performing encryption/decryption, it also computes/verifies a
message authentication code (MAC) using HMAC-SHA-256 to maintain
integrity and authenticity of initialization vector (IV) and ciphertext.

Note: This project does not implement the cryptographic primitives from
scratch. It is a wrapper around Microsoft's Common Object Runtime
Library (`mscorlib.dll`).

Contents
--------

* [Features](#features)
* [Why?](#why)
* [Demo Script](#demo-script)
* [Demo Output](#demo-output)
* [Example Ciphertexts](#example-ciphertexts)
* [Crypto Properties](#crypto-properties)
* [OpenSSL CLI Examples](#openssl-cli-examples)
* [Questions and Answers](#questions-and-answers)
* [References](#references)
* [License](#license)
* [Support](#support)

Features
--------

Here are some of the features of this project:

- Works with Base64 encoded keys.

- Exposes two simple functions named `Encrypt()` and `Decrypt()` that
    perform AES-256-CBC encryption and decryption along with computing
    and verifying MAC using HMAC-SHA-256 to ensure integrity and
    authenticity of IV and ciphertext.

- Initialization vector (IV) is automatically generated. Caller does
    not need to worry about it.

- Message authentication code (MAC) is computed by `Encrypt()` and
    verified by `Decrypt()` to ensure the integrity and authenticity of
    IV and ciphertext. HMAC-SHA-256 is used to generate MAC.

- `Encrypt()` returns the MAC, IV, and ciphertext concatenated
    together as a single string which can then be fed directly to
    `Decrypt()`. The three fields are joined by colons in the
    concatenated string. No need to worry about maintaining the MAC and
    IV yourself.

- Can be used with Classic ASP. Just put the entire [source code][src]
    within the ASP `<%` and `%>` delimiters in a file named `aes.inc`
    and include it in ASP files with an `#include` directive like this:

```asp
    
    ```

Note: This is not a crypto library. You can use it like one if all you
want to use is AES-256-CBC with HMAC-SHA-256. It does not support
anything else. If you want a different key size, cipher mode, or MAC
algorithm, you'll have to dive into the [source code][src] and modify it
as per your needs. If you do so, you might find one or more of the
documentation links provided in the [References](#references) section
useful.

Why?
----

Why not?

Okay, the real story behind writing this project involved a legacy
application written in Classic ASP and VBScript. It used an outdated
cipher that needed to be updated. That's what prompted me to create a
wrapper for AES-256-CBC with HMAC-SHA-256 in VBScript. After writing it,
I thought it would be good to share it on the Internet in case someone
else is looking for something like this.

Demo Script
-----------

On a Windows system, enter the following command to see a quick demo:

```
cscript aesdemo.wsf
```

The Windows Script File (WSF) named [`aesdemo.wsf`][aesdemo] loads
[`aes.vbs`][src] and executes the functions defined in it.

[aesdemo]: aesdemo.wsf

Demo Output
-----------

The output from the demo script looks like this:

```
demoAESKey: CKkPfmeHzhuGf2WYY2CIo5C6aGCyM5JR8gTaaI0IRJg=
demoMACKey: wDF4W9XQ6wy2DmI/7+ONF+mwCEr9tVgWGLGHUYnguh4=
encrypted1: Ru7CEo3KJBMT9ati55ASO0xJOVw5+7crhL4RxQSVu1s=:dHTHWCy5sGu9z7gUAa0tpA==:sBbmFDtzkPU7kD4T1OSbvw==
decrypted1: hello
encrypted2: 7BnQ5trOLDk8cecEnVayfSW9Q2fA38FvFkDlwHxbAKA=:M1ipFnh884qcXYlX9NPjwA==:ANF8P0PfaUQwvcS2jiIpdQ==
decrypted2: hello

aes.BlockSize: 128
aes.FeedbackSize: 128
aes.KeySize: 256
aes.Mode: 1
aes.Padding: 2
mac.HashName: SHA256
mac.HashSize: 256
aesEnc.InputBlockSize: 16
aesEnc.OutputBlockSize: 16
aesDec.InputBlockSize: 16
aesDec.OutputBlockSize: 16
b64Enc.InputBlockSize: 3
b64Enc.OutputBlockSize: 4
b64Dec.InputBlockSize: 1
b64Dec.OutputBlockSize: 3
```

Only the third line of output (`encrypted1`) changes on every run
because it depends on a dynamically generated initialization vector
(IV) which is different each time it is generated. This is, in fact,
an important security requirement for achieving semantic security.

For example, if a database contains encrypted secrets from various users
and if the same plaintext always encrypts to the same ciphertext (which
would happen if the key and IV remain constant between encryptions),
then we can tell that two users have the same plaintext secrets if their
ciphertexts are equal in the database. Being able to do so violates the
requirements of semantic security which requires that an adversary must
not be able to compute any information about a plaintext from its
ciphertext. This is why the ciphertext needs to be different for the
same plaintext in different encryptions and this is why we need a random
IV for each encryption.

There are two blocks of output shown above. The first block shows
example keys and ciphertexts along with the plaintexts they decrypt to.
The second block shows the default properties of the cryptography
objects used in the VBScript code. Both blocks of output are explained
in detail in the sections below.

Example Ciphertexts
-------------------

The first block of output in the demo script looks like this:

The third line of output (`encrypted1`) changes on every run because it
depends on a dynamically generated initialization vector (IV) used in
the encryption. The fifth line of output (`encrypted2`) remains the
same because it is hardcoded in the demo script.

Each encrypted value in the output is actually a concatenation of the
message authentication code (MAC), colon (`:`), initialization vector
(IV), colon (`:`), and ciphertext.

The ciphertext is generated with a function call like this:

```vbs
demoPlaintext = "hello"
demoAESKey = "CKkPfmeHzhuGf2WYY2CIo5C6aGCyM5JR8gTaaI0IRJg="
demoMACKey = "wDF4W9XQ6wy2DmI/7+ONF+mwCEr9tVgWGLGHUYnguh4="
encrypted1 = Encrypt(demoPlaintext, demoAESKey, demoMACKey)
```

The encrypted value (i.e., concatenated MAC, colon, IV, colon, and
ciphertext) can be decrypted back to plaintext with a function call like
this:

```vbs
decrypted1 = Decrypt(encrypted1, demoAESKey, demoMACKey)
```

The `Encrypt` and `Decrypt` functions are defined in [`aes.vbs`][src].

Crypto Properties
-----------------

The second block of output from the demo script looks like this:

```
aes.BlockSize: 128
aes.FeedbackSize: 128
aes.KeySize: 256
aes.Mode: 1
aes.Padding: 2
mac.HashName: SHA256
mac.HashSize: 256
aesEnc.InputBlockSize: 16
aesEnc.OutputBlockSize: 16
aesDec.InputBlockSize: 16
aesDec.OutputBlockSize: 16
b64Enc.InputBlockSize: 3
b64Enc.OutputBlockSize: 4
b64Dec.InputBlockSize: 1
b64Dec.OutputBlockSize: 3
```

These are the values of the properties of various cryptography objects
used in [`aes.vbs`][src]. The output shows the following details:

- The `aes` object (of class `RijndaelManaged`) has a default key size
    of 256 bits.
  - The block size is 128 bits. The mode is 1, i.e., CBC. See
    [`RijndaelManaged.Mode`] and [`CipherMode`] documentation to confirm
    that the default mode is indeed CBC.
  - The padding mode is 2, i.e., PKCS #7. See
    [`RijndaelManaged.Padding`] and [`PaddingMode`] documentation to
    confirm that the default padding mode is indeed PKCS #7.

We do not change these defaults in [`aes.vbs`][src] and we supply a
256-bit encryption key to `Encrypt` and `Decrypt` functions to ensure
that we use AES-256-CBC for encryption.

[`RijndaelManaged.Mode`]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.rijndaelmanaged.mode
[`CipherMode`]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.ciphermode
[`RijndaelManaged.Padding`]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.rijndaelmanaged.padding
[`PaddingMode`]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.paddingmode

OpenSSL CLI Examples
--------------------

For troubleshooting purpose, there are two shell scripts named
[`encrypt`](encrypt) and [`decrypt`](decrypt) present in the current
directory. Here is the synopsis of these scripts:

```shell
plaintext=PLAINTEXT aes_key=AES_KEY aes_iv=AES_IV mac_key=MAC_KEY sh encrypt
ciphertext=CIPHERTEXT aes_key=AES_KEY mac_key=MAC_KEY sh decrypt
```

These scripts are merely wrappers around OpenSSL. They accept Base64 key
and Base64 IV and convert them to hexadecimal for OpenSSL command line
tool to consume.

To see example usage of the script we will use these three values from
the demo output:

```
demoAESKey: CKkPfmeHzhuGf2WYY2CIo5C6aGCyM5JR8gTaaI0IRJg=
demoMACKey: wDF4W9XQ6wy2DmI/7+ONF+mwCEr9tVgWGLGHUYnguh4=
encrypted2: 7BnQ5trOLDk8cecEnVayfSW9Q2fA38FvFkDlwHxbAKA=:M1ipFnh884qcXYlX9NPjwA==:ANF8P0PfaUQwvcS2jiIpdQ==
decrypted2: hello
```

Here is how to use the [`encrypt`](encrypt) and [`decrypt`](decrypt)
scripts:

1. Encrypt the plaintext `hello` with the demo AES key and IV, and
    compute MAC of the result with the demo MAC key:

```shell
    plaintext=hello \
    aes_key=CKkPfmeHzhuGf2WYY2CIo5C6aGCyM5JR8gTaaI0IRJg= \
    aes_iv=M1ipFnh884qcXYlX9NPjwA== \
    mac_key=wDF4W9XQ6wy2DmI/7+ONF+mwCEr9tVgWGLGHUYnguh4= \
    sh encrypt
    ```

The output should match the string in the `encrypted2` value.

2. Verify MAC and decrypt the ciphertext to obtain the plaintext
    AES keys and IV.

```shell
    ciphertext=7BnQ5trOLDk8cecEnVayfSW9Q2fA38FvFkDlwHxbAKA=:M1ipFnh884qcXYlX9NPjwA==:ANF8P0PfaUQwvcS2jiIpdQ== \
    aes_key=CKkPfmeHzhuGf2WYY2CIo5C6aGCyM5JR8gTaaI0IRJg= \
    mac_key=wDF4W9XQ6wy2DmI/7+ONF+mwCEr9tVgWGLGHUYnguh4= \
    sh decrypt
    ```

The output should match the plaintext in the `decrypted2` value.

Questions and Answers
---------------------

1. Can we not use [`AesManaged`][AesManaged] instead of
    [`RijndaelManaged`][RijndaelManaged] in [`aes.vbs`][src]?

No, we cannot use [`AesManaged`][AesManaged] in VBScript. We can use
    only those classes that are defined in `mscorlib.dll`.
    [`RijndaelManaged`][RijndaelManaged] exists in `mscorlib.dll` but
    [`AesManaged`][AesManaged] does not.

As a result, we get an error for this VBScript code:

```vbs
    Set aes = CreateObject("System.Security.Cryptography.AesManaged")
    ```

Here is the error that occurs:

```
    ActiveX component can't create object: 'System.Security.Cryptography.AesManaged'
    ```

Here is a command that shows the DLLs in which the two classes are
    present:

```cmd
    C:\>powershell -Command [System.Reflection.Assembly]::GetAssembly('System.Security.Cryptography.RijndaelManaged')

GAC    Version        Location
    ---    -------        --------
    True   v4.0.30319     C:\Windows\Microsoft.NET\Framework64\v4.0.30319\mscorlib.dll

C:\>powershell -Command [System.Reflection.Assembly]::GetAssembly('System.Security.Cryptography.AesManaged')

GAC    Version        Location
    ---    -------        --------
    True   v4.0.30319     C:\windows\Microsoft.Net\assembly\GAC_MSIL\System.Core\v4.0_4.0.0.0__b77a5c561934e089\System.Core.dll
    ```

This is why we use [`RijndaelManaged`][RijndaelManaged] in our
    VBScript code.

2. Is [`RijndaelManaged`][RijndaelManaged] suitable as AES cipher?

Yes, it is.

The [`RijndaelManaged`][RijndaelManaged] class allows setting its
    `KeySize` and `BlockSize` properties independently to `128`, `160`,
    `192`, `224`, or `256`. Further, the key size need not match the
    block size.

But the [`AesManaged`][AesManaged] class allows setting its
    `KeySize` to `128`, `192`, or `256` only. The `BlockSize` property
    must be `128` only. These constraints match the key sizes and the
    block size defined in the AES standard (see section 5 of [FIPS 197]).

Therefore, [`AesManaged`][AesManaged] class is the preferred class
    to use because it is not possible to use it in a non-conformant
    manner. But since this class cannot be used in VBScript as explained
    in the previous point and since we stick to the default key size of
    256 bits and the default block size of 128 bits while using
    [`RijndaelManaged`][RijndaelManaged] as explained in section
    [Crypto Properties](#crypto-properties), it is a suitable substitute
    for [`AesManaged`][AesManaged].

Further, the section [OpenSSL CLI examples](#openssl-cli-examples)
    shows that the default properties of this cipher is compatible with
    `openssl aes-256-cbc`.

3. Instead of using a Base64 encoded 256-bit key, can we not use a
    password and derive a 256-bit key from it using a key derivative
    function (KDF)?

This is not easily possible in VBScript because there are only two
    key derivative functions available in `mscorlib.dll`:

- [`PasswordDeriveBytes`][PasswordDeriveBytes]: This is an
        implementation of an extension of the PBKDF1 algorithm.
      - [`Rfc2898DeriveBytes`][Rfc2898DeriveBytes]: This is an
        implementation of the PBKDF2 algorithm.

However, they are not registered in Windows registry by default.
    Therefore, like [`AesManaged`][AesManaged] class, these two classes
    too cannot be used in VBScript code without modifying the registry.

If we consider brute-force search of the key, then in case of a key
    derived from a password using a KDF, an attacker could either search
    the password which would be inefficient due to the use of KDF or an
    attacker could search the encryption key directly. Brute-force
    search of the key can be slower than brute-search of the password if
    the password is small and small number of iterations is used in the
    KDF. Therefore, using a 256-bit key directly is never worse than
    using a key derived from a password.

References
----------

- [Windows Scripting][winscript]
- [VBScript User's Guide][vbsguide]
- [Using Windows Script Files (.wsf)][wsfusage]
- [`cscript`][cscript]
- [`System.Text.UTF8Encoding`][UTF8Encoding]
- [`System.Security.Cryptography.FromBase64Transform`][FromBase64Transform]
- [`System.Security.Cryptography.ToBase64Transform`][ToBase64Transform]
- [`System.Security.Cryptography.RijndaelManaged`][RijndaelManaged]
- [`System.Security.Cryptography.HMACSHA256`][HMACSHA256]
- [FIPS 197]

[winscript]: https://docs.microsoft.com/en-us/previous-versions/ms950396(v=msdn.10)
[vbsguide]: https://docs.microsoft.com/en-us/previous-versions//sx7b3k7y(v=vs.85)
[wsfusage]: https://docs.microsoft.com/en-us/previous-versions//15x4407c%28v%3dvs.85%29
[cscript]: https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/cscript
[UTF8Encoding]: https://docs.microsoft.com/en-us/dotnet/api/system.text.utf8encoding
[FromBase64Transform]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.frombase64transform
[ToBase64Transform]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.tobase64transform
[RijndaelManaged]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.rijndaelmanaged
[AesManaged]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.aesmanaged
[HMACSHA256]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.hmacsha256
[PasswordDeriveBytes]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.passwordderivebytes
[Rfc2898DeriveBytes]: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.rfc2898derivebytes
[FIPS 197]: https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.197.pdf

License
-------

This is free and open source software. You can use, copy, modify,
merge, publish, distribute, sublicense, and/or sell copies of it,
under the terms of the MIT License. See [LICENSE.md][L] for details.

This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
express or implied. See [LICENSE.md][L] for details.

[L]: LICENSE.md

Support
-------

To report bugs, suggest improvements, or ask questions, please create a
new issue at <http://github.com/susam/aes.vbs/issues>.

aes.vbs

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