Computer Security and Cryptography Reading Group
November 2004 List

Adrian Perrig

Leendert van Doorn

URL: http://www.ece.cmu.edu/~adrian/projects/swatt.pdf

We expect a future where we are surrounded by embedded devices, ranging from Java-enabled cell phones to sensor networks and smart appliances. An adversary can compromise our privacy and safety by maliciously modifying the memory contents of these embedded devices. In this paper, we propose a SoftWare-based ATTestation technique (SWATT) to verify the memory contents of embedded devices and establish the absence of malicious changes to the memory contents. SWATT does not need physical access to the device's memory, yet provides memory content attestation similar to TCG or NGSCB without requiring secure hardware. SWATT can detect any change in memory contents with high probability, thus detecting viruses, unexpected configuration settings, and Trojan Horses. To circumvent SWATT, we expect that an attacker needs to change the hardware to hide memory content changes. We present an implementation of SWATT in off-the-shelf sensor network devices, which enables us to verify the contents of the program memory even while the sensor node is running.

Reiner Sailer

Leendert van Doorn

URL: http://doi.acm.org/10.1145/1030083.1030125

Intranet access has become an essential function for corporate users. At the same time, corporation's security administrators have little ability to control access to corporate data once it is released to remote clients. At present, no confidentiality or integrity guarantees about the remote access clients are made, so it is possible that an attacker may have compromised a client process and is now downloading or modifying corporate data. Even though we have corporate-wideaccess control over remote users, the access control approach is currently insufficient to stop these malicious processes. We have designed and implemented a novel system that empowers corporations to verify client integrity properties and establish trust upon the clientpolicy enforcement before allowing clients (remote) access to corporate Intranet services. Client integrity is measured using a Trusted Platfor m Module (TPM), a new security technology that is becoming broadly available on client systems, and our system uses these measurementsfor access policy decisions enfor ced upon the client's processes. We have implemented a Linux 2.6 prototype system that utilizes the TPM measurement and attestation, existing Linux network control (Netfilter), and existing corporatepolicy management tools in the Tivoli Access Manager to control remote client access to corporate data. This prototype illustrates that our solution integrates seamlessly into scalable corporate policy management and introduces only a minor performance overhead.

Tadayoshi Kohno

URL: http://doi.acm.org/10.1145/1030083.1030095

WinZip is a popular compression utility for Microsoft Windows computers, the latest version of which is advertised as having "easy-to-use AES encryption to protect your sensitive data." We exhibit several attacks against WinZip's new encryption method, dubbed "AE-2" or "Advanced Encryption, version two." We then discuss secure alternatives. Since at a high level the underlying WinZip encryption method appears secure (the core is exactly Encrypt-then-Authenticate using AES-CTR and HMAC-SHA1), and since one of our attacks was made possible because of the way that WinZip Computing, Inc. decided to fix a different security problem with its previous encryption method AE-1, our attacks further underscore the subtlety of designing cryptographically secure software.

URL: http://www.springerlink.com/link.asp?id=gc8dp8f8v5kkjj7m

We study the question of how to generically compose symmetric encryption and authentication when building "secure channels" for the protection of communications over insecure networks. We show that any secure channels protocol designed to work with any combination of secure encryption (against chosen plaintext attacks) and secure MAC must use the encrypt-then-authenticate method. We demonstrate this by showing that the other common methods of composing encryption and authentication, including the authenticate-then-encrypt method used in SSL, are not generically secure. We show an example of an encryption function that provides (Shannon's) perfect secrecy but when combined with any MAC function under the authenticate-then-encrypt method yields a totally insecure protocol (for example, finding passwords or credit card numbers transmitted under the protection of such protocol becomes an easy task for an active attacker). The same applies to the encrypt-and-authenticate method used in SSH.

On the positive side we show that the authenticate-then-encrypt method is secure if the encryption method in use is either CBC mode (with an underlying secure block cipher) or a stream cipher (that xor the data with a random or pseudorandom pad). Thus, while we show the generic security of SSL to be broken, the current practical implementations of the protocol that use the above modes of encryption are safe.