Simple XOR Obfuscation
Bitwise XOR (exclusive OR) is among the simplest methods to transform data and hide its original form. It’s a fundamental operation not only in cryptography, but in computing in general, due to its symmetric (self-inverting) properties, which we’ll explore below.,
How XOR Obfuscation Works
The XOR operation works on individual bits. It returns 1 if the two input bits are different, and 0 if they are the same:
0 XOR 0 = 00 XOR 1 = 11 XOR 0 = 11 XOR 1 = 0
When applied to bytes (which are just sequences of 8 bits), the XOR operation is performed bitwise on each corresponding pair of bits.
The magic of XOR for obfuscation lies in its symmetry and self-inverting nature. If you have plaintext data (P) and a key (K), you can generate ciphertext (C) using XOR:
C = P XOR K
To get the original plaintext back from the ciphertext, you simply apply the exact same operation with the exact same key:
P = C XOR K
So applying the key twice essentially cancels out the effect, revealing the original data.
In practice, to obfuscate a block of data (like our DLL payload), you would typically choose a key (which could be a single byte or a sequence of bytes). You then XOR each byte of the plaintext data with a corresponding byte from the key. If the key is shorter than the data, the key is usually repeated cyclically.
Simple Example
K = 10101010 (0xAA)
P = 01010101 (0x55)
Then to encrypt C = P XOR K:
C = 11111111 (0xFF)
This is easy to confirm visually - every bit position in K and P differ, so every result will be 1.
Now to decrypt P = C XOR K:
P = 01010101 (0x55)
Properties
XOR has 2 main properties that stand out - simplicity, and relatedly, speed. As we’ve just seen, XOR is extremely easy to understand. It can be implemented in any language with little effort, one would not even require a library. Further, since the operation itself is “native” to our CPUs, its one of the fastest operations a CPU can perform. This means that XOR-based obfuscation is computationally very cheap for both encrypting and decrypting, even for large payloads.
This is important since high levels of sudden resource usage by a new process (if for example decrypting large amount of data that were encrypted using a complex algo) can be a dead giveaway that something foul is afoot.
Weaknesses
Now despite its simplicity and speed, basic XOR obfuscation (especially with simple key handling) has significant weaknesses that make it unsuitable for strong security on its own.
It is extremely vulnerable to Known-Plaintext Attacks - if an analyst possesses any portion of the original plaintext (P) and the corresponding ciphertext (C) generated using XOR, they can trivially recover the key (K) used for that portion by XORing the known plaintext and ciphertext: K = P XOR C. For executable files like DLLs, known plaintext is often easy to find (e.g., standard PE headers like the “MZ” or “PE” signatures, common function prologues, standard library strings). Recovering even part of the key can help in decrypting other sections or guessing the rest of the key.
It is also vulnerable to Frequency Analysis (with Key Reuse): If a short key is reused cyclically across a large plaintext (like applying a 4-byte key repeatedly to a multi-kilobyte DLL), patterns in the plaintext can leak into the ciphertext. Certain byte values or sequences appear more frequently than others in typical executable code. Analyzing the frequency of bytes in the ciphertext can reveal information about the key length and potentially the key itself, especially when combined with known-plaintext attacks. A single-byte key is trivially broken by finding the most common ciphertext byte and guessing it corresponds to the most common plaintext byte (often 0x00 or space).
Because of these two weaknesses the entire security of XOR relies on the secrecy and strength of the key. If the key is hardcoded directly within the loader application, it can often be easily extracted through static analysis (finding the key constant) or dynamic analysis (observing the key being used during the decryption routine). If the key is derived using a weak or predictable method, it can also be compromised.
Conclusion
We’ll now move ahead with a basic, and therefor pretty vulnerable, implementation of XOR in the following two labs. This does however set us up nicely to employ more sophisticated techniques to mitigate these weaknesses by adding more robust layers on top of the basic XOR concept in Module 7.