Malware often employs encryption to evade detection, and encrypted traffic and strings containing sensitive details like URLs and IP addresses are common tactics. Various encryption methods are used, including XOR, Rc4, AES, DES, and 3DES.
These methods scramble data to bypass security scans, including encryption fundamentals, classical ciphers, and their digital evolution.
It also explains bitwise operations, the XOR function, and XOR cipher detection with a practical example that demonstrates malware detection and decryption of XOR-encrypted C2 communication.
Fundamentals of Encryption:
Encryption employs an algorithm to transform readable plaintext into scrambled ciphertext using a key, which acts as a secret sauce for both encryption and decryption, ensuring only authorized parties can unlock the ciphertext back to its original form.
According to Any.Run, the algorithm essentially performs a series of substitutions and rearrangements on the data to render it unintelligible.
Working of Encryption:
Encryption involves substituting symbols or bytes according to predefined rules or keys. Simple substitution ciphers, like those used in ancient Egypt, replace words with arbitrary symbols.
Caesar Cipher, a more evolved version, shifts each letter by a fixed number of positions, but it’s still vulnerable due to predictable patterns as encryption always relies on some form of substitution to obfuscate the original message.
To enhance encryption strength, cryptographers introduced key-based mutation, which employs a key to control the transformation of each plaintext symbol or bit, as exemplified by the Vigenère cipher.
It uses a keyword to generate multiple substitution alphabets, applying a Caesar cipher with shifts determined by the corresponding key letter’s position.
Crucially, the key length should match the plaintext to avoid exploitable repetitions, a principle that holds true for modern symmetric ciphers like XOR as well.
Bitwise operations, such as XOR, manipulate data directly at the binary level, making them fundamental tools for encryption, whereas XOR stands out for its reversibility and ability to merge data with a key effectively. In encryption using XOR, plaintext and a key of equal length are converted to binary, and each corresponding bit is XORed.
The resulting ciphertext appears random and can be decrypted by applying XOR again with the same key, which is exemplified by the one-time pad (OTP) cipher, known for its unbreakable encryption achieved by using a random, single-use key.
How to Decrypt XOR:
The analysis of a malicious executable’s network traffic in ANY.RUN revealed a suspicious GET request for an .mp4 file containing encrypted content, where the pattern of repeated 5s and 3s in the HEX values suggested potential XOR encryption.
Further inspection of the executable’s source code in dnSpy confirmed the use of XOR with a key of 355, which can be used for the encrypted traffic, was successfully decrypted in CyberChef, revealing a DLL Windows Portable Executable.
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