PAD_BytesBlock

Creates an input block suitably padded for encryption by a block cipher in ECB or CBC mode.

VB6/VBA Syntax

nRet = PAD_BytesBlock(abOutput(0), nOutputLen, abInput(0), nInputLen, nBlockLen, 0)

Parameters

abOutput

[out] Byte array to be filled with padded encryption block.

nOutputLen

[in] Long specifying the size of the output array in bytes.

abInput

[in] Byte array containing the plaintext bytes to be padded.

nInputLen

[in] Long specifying the length of the plaintext in bytes.

nBlockLen

[in] Long specifying the cipher block length in bytes (8 or 16).

nOptions

[in] Long option flags: not used in this release. Specify zero.

C/C++ Syntax

Returns (VB6/C)

Long: If successful, the return value is the number of bytes in the output; otherwise it returns a negative error code.

.NET Equivalent

Use the method associated with the relevant encryption algorithm class:

Aes128.Pad Method (Byte[])
Blowfish.Pad Method (Byte[])
Des.Pad Method (Byte[])
Tdea.Pad Method (Byte[])

Remarks

The output will be padded according to the convention in PKCS#5, PKCS#7 and CMS. If abOutput is set to vbNull or nOutputLen set to zero, then the required number of bytes will be returned. The output is always longer than the input. Only block lengths of 8 or 16 bytes are supported.

VB6/VBA users: Note the '(0)' in abOutput(0) and abInput(0).

Example

Dim abInput() As Byte
Dim abOutput() As Byte
Dim nOutputLen As Long
Dim nInputLen As Long
Dim nBlockLen As Long
Dim i As Long

' Prepare test input 5 bytes long
nInputLen = 5
ReDim abInput(nInputLen - 1)
For i = 0 To nInputLen - 1
    abInput(i) = &HFF
Next
Debug.Print "Input data=0x" & cnvHexStrFromBytes(abInput)

' Find out the required length
nBlockLen = 8
nOutputLen = PAD_BytesBlock(vbNull, 0, abInput(0), nInputLen, nBlockLen, 0)
Debug.Print "Required length is " & nOutputLen & " bytes"
' Check for error
If (nOutputLen <= 0) Then Exit Function

' Pre-dimension output
ReDim abOutput(nOutputLen - 1)

nOutputLen = PAD_BytesBlock(abOutput(0), nOutputLen, abInput(0), nInputLen, nBlockLen, 0)
Debug.Print "Padded data=0x" & cnvHexStrFromBytes(abOutput)

' Now set input as padded output and remove padding
abInput = abOutput
nInputLen = nOutputLen

' Remove padding...
' No need to query for length because we know the output will be shorter than input
' so make sure output is as long as the input
nOutputLen = nInputLen
ReDim abOutput(nOutputLen - 1)
nOutputLen = PAD_UnpadBytes(abOutput(0), nOutputLen, abInput(0), nInputLen, nBlockLen, 0)
Debug.Print "Unpadded length is " & nOutputLen & " bytes"
' Check for error
If (nOutputLen <= 0) Then Exit Function

' Re-dimension the output to the correct length
ReDim Preserve abOutput(nOutputLen - 1)
Debug.Print "Unpadded data=0x" & cnvHexStrFromBytes(abOutput)

This should result in output as follows:

Input data=0xFFFFFFFFFF
Required length is 8 bytes
Padded data=0xFFFFFFFFFF030303
Unpadded length is 5 bytes
Unpadded data=0xFFFFFFFFFF

See Also

PAD_UnpadBytes PAD_HexBlock PAD_UnpadHex

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