Military Communications and Information Technology: A Trusted ...

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in the IP header that are left unchanged by intermediate systems to hide a few, 28
in the named approach, bits in otherwise legitimate IP packets. Similar approaches
are conceivable for transport layer protocols, but we do not expect these to provide
a significantly larger count of bits per packet.
For IPv4 or TCP, the header length can be adjusted to allow adding additional
options. In principle, a bot herder could use the extra space obtainable by adjusting
the length field to increase the bandwidth of a steganographic approach. This
highlights however an issue which also appears, though with a different nature,
in the approach described in the previous paragraph. The use of additional options
is very rare for both TCP and IPv4, i.e. while the attacker is free to choose
an unsuspicious payload, using oversized headers may attract even more attention
than simply transferring the steganographic payload in the application layer section
of the packet. For the approaches that do not inflate the size of the header, the minuscule
steganographic payload will require that a significant number of packets
is transferred for any significant botnet protocol payload. Thus, differences in communication
patterns may again be similarly or even more striking than without
such an attempt to obfuscate the C 2 channel using these techniques.
C. Use of cryptography
Cryptographic protocols are designed to provide three core properties:
• Confidentiality
• Integrity
• Authenticity
Confidentiality ensures that messages cannot be read in transit. Integrity allows
a peer to verify that messages have not been modified in transit and authenticity
provides proof of identification or approval by a verified entity.
The Storm botnet employed a custom encryption algorithm, its supposed
successor, the Waledac botnet, the standardised AES algorithm but both implementations
employed static keys [16]. When a static key is used, a network intrusion
detection system may either decrypt an observed payload with the known
key and then apply its pattern matching algorithm or sometimes it may even be
enough to match on the encrypted message.
Duqu could be considered an example for correct use of cryptography
in that it can connect to its C 2 server through a legitimate HTTPS connection,
however the C 2 server uses a frequently replaced self-signed certificate, rendering
the connection subject to man-in-the-middle attacks. This appears to be
acceptable from the malware author’s point of view since the actual payload
is encrypted with a symmetric key stored in its binary, illustrated also by that
a second method for establishing a C 2 channel exchanges the same data but
using plain HTTP [17].