Protecting Your Software

Reverse-engineering protection (built-in and active)

Protecting Your Software

All programs involved in security present vulnerabilities; it is up

to the programmers to either eliminate them or provide detection. By

reverse-engineering a program, an attacker can make it do or not do

anything he pleases. Button captions and string constants can be

modified to trick the user into compromising security, and a simple

swapping of pointers could result in sensitive information being

leaked. The simplest counter-measure to these types of attacks is

ensuring no program files have been modified. A more versatile

tactic involves digital signatures and a program loader.

Attackers will usually try cracking the program before breaking the

cryptosystem; most programs are surprisingly weak against


A program is a series of instructions stored locally; if the user

can run the program, then he can reverse- engineer it, and change it

to his will. With a good debugger, an attacker can trace the program

through its interface messages and procedure calls. It is important

to understand that all programs can be reverse-engineered, but the

task can be made very difficult. It has been common practice for

years to: camouflage security-related functions, sprinkle decoy

function calls, and implement anti-debugging procedures. These

practices do not make programs tamper-proof in any way, but they

certainly make them tamper-resistant. There is no simple solution to

this problem, but the true goal remains to prevent the program from

being modified.

Strings contained within a program have tremendous influence, and

can be a security weakness. They are used to communicate with the

user; if strings are replaced with misleading information, an

attacker could remove "security compromised" messages, and prevent

the user from detecting intrusion. These attacks only require a

simple hex editor, and are very easy to achieve. There are many ways

to make his attack difficult. If the strings are encrypted properly,

an attacker would have to: reverse-engineer the program, find the

key, extract the decryption functions, solve the encryption

functions (they aren't in the program!), encrypt the fake strings,

and swap the strings. With clever usage of pointers, the key can be

sprinkled throughout the source code, and recombined without making

any function calls that would attract an attacker's attention.

Strings must be hidden to ensure users are not passed false

messages, and thus compromising security.

Pointers offer abundant opportunities to reek data leaking havoc.

Imagine a situation where a pointer to a record of the user's

program settings was swapped with a pointer to the user's key, and

the key was written to the registry, out in the open for the

attacker to grab. These situations aren't heard of very often

because programs are unprotected, and simpler means to obtaining

data are usually available to attackers. If you name the key "key"

nothing is left to the attacker's imagination. It is very easy to


"#define key btnOkClicked"

at the beginning of the source code to rename all references to

"key" while keeping the source code clear and consistent.

"btnOkClicked" is very misleading and will be ignored by attackers

for a very long time unless the procedure you call it from is called

"getKey". It is important that the naming scheme does not reveal

which pointers, variables, and procedures are security related. It

is simple enough to make spoofed version of the source code that

will be compiled with ambiguous names for the release version so

that reverse-engineering reveals no significant information from the

naming scheme. Names give attackers significant clues about a

pointer's use; use this as an advantage to confuse and misdirect


Developers often run checksums or hash functions on the executable

and compare it with a value hard-coded into the program. There are a

few problems that emerge from this counter-measure. An attacker only

has to change a conditional statement from "if = then" to "if <>

then" to follow the same procedure regardless of whether the

checksums matched or not. Of course you can camouflage this

conditional statement but dodging these counter-measures is no

problem for an experienced cracker. Simply verifying if a file has

changed is not enough; ideally modifying the executable would render

it unworkable.

Digital signatures offer a very sound and comforting solution. By

signing the executable you ensure 2 things: anyone can run the

program, and only you can create or modify it. For an attacker to

modify the program he would have to break the digital signature

algorithm or the private key. You can either use an unsigned portion

of the program to decipher the rest of the program or use an

external program; this is referred to as a program loader. In either

case the attacker will only know the public key. If an attacker

modifies the executable he will not be able to sign it properly,

making detection simple. Another variety of this protection involves

getting the public key from a server to prevent the attacker from

modifying the public key hard-coded into the program.

It is recommended to combine counter-measures. For example, digital

signatures can be used in conjunction with a checksum. This method

is behind Microsoft's Authenticode signatures; part of the CryptoAPI

Tools. Certified programs are guarantied to come from the specified

source, and to be unmodified. The problem with Authenticode is the

program is only protected as online content, and once on your

hard-drive, there is no guarantee. Because of the complexity in

designing programs which employ digital signatures natively, very

few implementations are used, but it is likely some variety of them

will be built into most applications in the next decade as security

concerns increase amongst the consumers' priorities.

(C)Copyright DrMungkee 2000


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