VBA AES CBC encryption

Question

I have referred encryption in https://github.com/susam/aes.vbs and below is the code i endup with

Function Min(a, b)
    Min = a
    If b < a Then Min = b
End Function

Function B64Encode(bytes)
    Dim result As String
    Dim b64Block() As Byte
    Dim b64Enc As Object
    Dim utf8 As Object
    Dim Offset, Length, BlockSize As Integer
    
    Set b64Enc = CreateObject("System.Security.Cryptography.ToBase64Transform")
    Set utf8 = CreateObject("System.Text.UTF8Encoding")
    BlockSize = b64Enc.InputBlockSize
    For Offset = 0 To LenB(bytes) - 1 Step BlockSize
        Length = Min(BlockSize, UBound(bytes) - Offset)
        b64Block = b64Enc.TransformFinalBlock((bytes), Offset, Length)
        result = result & utf8.GetString((b64Block))
    Next
    B64Encode = result
End Function

Function B64Decode(b64Str)
    Dim utf8 As Object
    Dim bytes() As Byte
    Dim b64Dec As Object
    
    Set utf8 = CreateObject("System.Text.UTF8Encoding")
    Set b64Dec = CreateObject("System.Security.Cryptography.FromBase64Transform")
    bytes = utf8.GetBytes_4(b64Str)
    B64Decode = b64Dec.TransformFinalBlock((bytes), 0, UBound(bytes))
End Function

Function Encrypt(plaintext, aesKey)
    Dim cipherBytes, aesKeyBytes, ivKeyBytes, plainBytes() As Byte
    
    Dim utf8, AES, aesEnc As Object
    Dim aesIV() As Byte
    Set AES = CreateObject("System.Security.Cryptography.RijndaelManaged")
    Set utf8 = CreateObject("System.Text.UTF8Encoding")
    'Set cipherMode = GetObject("System.Security.Cryptography.CipherMode")
    AES.KeySize = 256
    AES.Mode = 1
    AES.Key = CreateObject("System.Text.UTF8Encoding").GetBytes_4("V$ry300DP3r$0NM3")
    AES.IV = CreateObject("System.Text.UTF8Encoding").GetBytes_4("HR$2pIjHR$2pIjPa")
    plainBytes = utf8.GetBytes_4(plaintext)
    'Set aesEnc = AES.CreateEncryptor_2((aesKeyBytes), (ivKeyBytes))
    cipherBytes = AES.TransformFinalBlock((plainBytes), 0, UBound(plainBytes))
        
    Encrypt = B64Encode(cipherBytes)
End Function

Now the above code is saying me "specified initialization vector (iv) does not match the block size for this algorithm"

Cant we set IV dynamic?

I think you're looking for `UBound(bytes)` instead of `Len(bytes)`. — BigBen, Sep 03 '20 at 20:28

score 2 · Accepted Answer · answered Sep 09 '20 at 20:16

Function Min(a, b)
    Min = a
    If b < a Then Min = b
End Function

Function B64Encode(bytes)
    Dim result As String
    Dim b64Block() As Byte
    Dim b64Enc As Object
    Dim utf8 As Object
    Dim Offset, Length, BlockSize As Integer
    
    Set b64Enc = CreateObject("System.Security.Cryptography.ToBase64Transform")
    Set utf8 = CreateObject("System.Text.UTF8Encoding")
    BlockSize = b64Enc.InputBlockSize
    For Offset = 0 To LenB(bytes) - 1 Step BlockSize
        Length = Min(BlockSize, UBound(bytes) - Offset)
        b64Block = b64Enc.TransformFinalBlock((bytes), Offset, Length)
        result = result & utf8.GetString((b64Block))
    Next
    B64Encode = result
End Function

Function B64Decode(b64Str)
    Dim utf8 As Object
    Dim bytes() As Byte
    Dim b64Dec As Object
    
    Set utf8 = CreateObject("System.Text.UTF8Encoding")
    Set b64Dec = CreateObject("System.Security.Cryptography.FromBase64Transform")
    bytes = utf8.GetBytes_4(b64Str)
    B64Decode = b64Dec.TransformFinalBlock((bytes), 0, UBound(bytes))
End Function


Function Encrypt(plaintext, aesKey)
    Dim cipherBytes, aesKeyBytes, ivKeyBytes, plainBytes() As Byte
    
    Dim utf8, AES, aesEnc, cipherMode As Object
    Dim aesIV() As Byte
        
    Set AES = CreateObject("System.Security.Cryptography.RijndaelManaged")
    Set utf8 = CreateObject("System.Text.UTF8Encoding")
    'Set cipherMode = CreateObject("System.Security.Cryptography.CipherMode")
    
    AES.KeySize = 256
    AES.BlockSize = 256
    'CipherMode.CBC
    AES.Mode = 1
    'PaddingMode.PKCS7
    AES.Padding = 2
    AES.Key = CreateObject("System.Text.UTF8Encoding").GetBytes_4("ThirtyTwoBytes3$ThirtyTwoBytes3$")
    AES.IV = CreateObject("System.Text.UTF8Encoding").GetBytes_4("3$ThreeTwoBytes3$ThreeTwoBytes3$")
    'plainBytes = utf8.GetBytes_4(plaintext)
    plainBytes = B64Decode(plaintext)
    'Set aesEnc = AES.CreateEncryptor_2((aesKeyBytes), (ivKeyBytes))
    cipherBytes = AES.CreateEncryptor().TransformFinalBlock((plainBytes), 0, UBound(plainBytes))
        
    Encrypt = B64Encode(cipherBytes)
End Function

Sub encrypt_hell()
Debug.Print Encrypt("Hello", "PattamuthuArumug")
End Sub

I am using AES CBC 256 bits so 32 bytes Key and IV has to be used

TransformFinalBlock API is (byte[] inputBuffer, int inputOffset, int inputCount). Ubound() gives you the Index e.g. 31, but you need to provide 32 so +1. But you only notice this when you do a decrypt afterwards and something is missing. You could leave the B64 decode out there if you don't need it. — Stejin, Apr 28 '23 at 09:38
Fails with Invalid Character in Base64 String when I debug.print "Hello Nicholas, I'm writing to make you aware that, the quick brown fox jumps over the lazy dog. This happens quite frequently now and it's becoming a real problem" Crashes on last line of B64decode routine. Works for me on lots of other short strings — Tin Bum, Jul 16 '23 at 11:09

VBA AES CBC encryption

1 Answers1

Linked