Nytro Posted June 14, 2014 Report Posted June 14, 2014 An Introduction to Recognizing and Decoding RC4 Encryption in Malware There is something that we come across almost daily when we analyze malware in the VRT: RC4. We recently came across CVE-2014-1776 and like many malware samples and exploits we analyze, RC4 is used to obfuscate or encrypt what it is really doing. There are many ways to implement RC4 and it is a very simple, small algorithm. This makes it very common in the wild and in various standard applications. Open-source C implementations can be found on several websites such as Apple.com and OpenSSL.org. What is RC4? RC4 was designed by Ron Rivest of RSA Security in 1987. RC4 is a fast and simple stream cipher that uses a pseudo-random number generation algorithm to generate a key stream. This key stream can be used in an XOR operation with plaintext to generate ciphertext. The same key stream can then be used in an XOR operation against the ciphertext to generate the original plaintext. While it is still common in malware, RC4 has been legitimately implemented in a number of areas where speed and privacy are of concern. In the past, both WEP and TLS both used RC4 to protect data sent across the wire. However, last Fall, Microsoft recommended that customers disable RC4 by enabling TLS1.2 and AES-GCM. For more information including a detailed history of RC4, check out the Wikipedia article. Why is it used in malware? Increasingly, we find that RC4 is used to encode data that is sent to a remote server to be decrypted on the other side using a pre-shared key. This makes detection a bit trickier (but not impossible) and also makes it harder to determine exactly what is being sent across the wire. What we will usually do when we think we’ve come across some sort of encryption is determine the source of it and whether the data being sent is static (for matching purposes) and what exactly that data is.Articol complet: VRT: An Introduction to Recognizing and Decoding RC4 Encryption in Malware Quote
