Crypto plug-in

Author: GAG (talk, contrib)

Links

cryptoplg11.zip (43 KB) (plugin dll + readme + examples)
hashcalc.zip (30 KB) (Calculates the hash for Strings or specified files)

Forum Thread

Introduction

Version: 1.1.

This plugin provides you cryptographic interface using CryptoAPI.

Plugin DLL size: 3 660 bytes (not packed), 2 886 bytes (upx packed)

Hash Algorithms (Supported by Crypto Plugin)

Using this plugin you can get common cryptographic hashes.

Here is a list of algorithms currently supported by the Crypto Plugin
MD5, SHA-1, MD2, MD4, SSL3, MAC, HMAC

Hash Algorithms (Supported by CryptoAPI)

This section shows you all of the possibly supported Hash Algorithms that can be supported by the Crypto Plugin if they are not already.

MD2
MD5
Secure Hash Algorithms (SHA, SHA-1, SHA-256, SHA-384, and SHA-512 only)
MAC
HMAC
SSL3_SHAMD5 - need to find out more information (Crypto Plugin supports this but simply calls it SSL3)

Sources

Method used for the hash

How to use

String Hash

Crypto::HashData "MD5" "String to be hashed"
Pop $0

Supported algorithms: MD5|SHA1|MD2|MD4

File Hash

Crypto::HashFile "MD5" "$WINDIR\notepad.exe"
Pop $0

Supported algorithms: MD5|SHA1|MD2|MD4

Implemented in software

Quick Example

Calculate the hash of the file you want checked using the Hash Calculator (hashcalc.zip). From there I was able to use Crypto to ensure that notepad hasn't changed:

Crypto::HashFile "MD5" "$WINDIR\notepad.exe"
Pop $0
StrCmp $0 "FF7F14FDA901090E337488A1900E3660" +3
MessageBox MB_OK|MB_TOPMOST|MB_SETFOREGROUND|MB_ICONSTOP "ERROR: notepad.exe has been changed!!!"
Quit

Detailed Explanation: (of the code above)
Crypto::HashFile "MD5" "$WINDIR\notepad.exe"
This generates the hash of notepad.exe and then pushes that hash into a stack (which is in memory reserved by NSIS).
Pop $0
Then the returned hash is "popped" off of the stack and then thrown into the user defined variable $0.
StrCmp $0 "FF7F14FDA901090E337488A1900E3660" +3
The user defined variable $0 and "FF7F14FDA901090E337488A1900E3660" (which is the hash generated by hashcalc.zip) are then compared. If they are equal then +3 means to go plus three commands down. Therefore MessageBox and Quit commands are skipped. If they aren't equal then it continues to the next command (or +1)
MessageBox MB_OK|MB_TOPMOST|MB_SETFOREGROUND|MB_ICONSTOP "ERROR: notepad.exe has been changed!!!"
Quit
Since the two checksums don't match, show the user an error and then exit the program.

Software Example

File: uharc sfx examples.zip (95 KB)
Short Description: This is a real world implementation of the Crypto plugin. It does an MD5 on the extractor right before extracting the archive, this ensures that the extractor isn't tampered with and the user isn't tricked into giving their password away.

How can a file be tampered with and why?

Lets say someone really wants that password. Well they (or malware) can replace the extractor (uharc.exe) with a dummy exe file with the same name that takes all the arguments given to it and puts them into a text file to be read by the evil doer. Well since the password is one of the arguments passed off to extract the archive then you can see where this would be a problem. The advantage to having an MD5 checksum of a file is to ensure that it is not changed (or replaced). Whether it be changed for malicious purpose or simply a corrupted file.

Another use for checksums

If you are managing your software then you can make your update process even faster. If you create an MD5 checksum of all the files that are currently installed on the system, then you can compare those checksums with the checksums of all the files that are in your patch. Anything that doesn't match will be installed (or reinstalled) and all the checksums that match will simply be skipped. This way your installer doesn't have to take the time to install all the files, only the ones that need to be updated.