Question

Calling a DLL from VB.net - Char arrays problem.

Asked by: Wierdy1024

I want to hash a piece a piece of text with MD4 to get a 16 byte hash.  To do this I got libeay32.dll (part of openssl) and used the MD4 function which looks like this:  (that is c++ example code)

unsigned char *MD4(const unsigned char *d, unsigned long n, unsigned char *md);

now when I change this to VB.net I made:

Declare Function MD4 Lib "libeay32" Alias "MD4" (ByRef d() As char, ByRef n As Long, ByRef md() As Char ) As Char

All seemed fine until I compiled it when I found that It couldn't marshall char byref's to unmanaged code.  I changed the "byref"s to "byval"s and all worked fine except that no output was made (the input variables are sposed to be changed to make the output ones).  I found that byvals can't make any response because the values are "copied" to the dll so it can't make any response.

So my final question is how to either:

1. Make the code work
2. Do MD4 hashing another way

Thanks in advance


PS.  Sorry for the vagueness above - I'm new to dealing with API's and DLL's - have escaped without them till now.

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Asked On
2005-02-16 at 11:16:10ID21317391
Tags

char

,

byref

Topic

Microsoft Visual Basic.Net

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Answers

 

by: AlexFMPosted on 2005-02-16 at 11:48:31ID: 13327883

unsigned char *MD4(const unsigned char *d, unsigned long n, unsigned char *md);

should be:

Declare Function MD4 Lib "libeay32" Alias "MD4" (ByVal d as IntPtr, ByVal n As Long, ByVal md as IntPtr) As IntPtr

Marshal class should be used to allocate, write and read unmanaged memory blocks for MD4 function. You need to provide more information about MD4 funcion: how does it use memory blocks d and md, what is input and output, what is their length and what is char* pointer returned by this function.

 

by: Wierdy1024Posted on 2005-02-16 at 12:30:23ID: 13328403

Thanks for the info.  I have found more information about MD4  on www.openssl.org:

MD4() computes the MD4 message digest of the n bytes at d and places it in md

I don't know what the "unsigned char" is at the beginning of the first line of code in the question - I guess some other output but I don't need it whatever it is.

The length of MD is always 16 bytes
the length of d is up to 14 bytes
and n therefore has a maximum of 14

Also, could you explain the Marshall class as I havn't used it before and don't know exactly how it works.

Thanks

 

by: AlexFMPosted on 2005-02-16 at 23:45:28ID: 13332397

Assuming that n = 14 (you can do required changes):

' Allocate unmanaged memory blocks
Dim d as IntPtr
d = Marshal.AllocCoTaskMem(14)
dim md as IntPtr
md = Marshal.AllocCoTaskMem(16)

' copy 14 bytes array to d, assuming that source is Byte() array having at least 14 elements
Marshal.Copy(source, 0, d, 14)

Dim result as IntPtr = MD4(d, 14, md)

' copy 16 bytes from md, assuming that dest is Byte() array having at least 16 elements
Marshal.Copy(md, dest, 0, 16)

' Release unmanaged memory
Marshal.FreeCoTaskMem(d)
Marshal.FreeCoTaskMem(md)

20120131-EE-VQP-002

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