Matthäus Wander

SoNet -- Privacy and Replication in Federated Online Social Networks

In this paper we propose a federated online social network (OSN) which focuses on user privacy and data availability. All user content is encrypted and decrypted on end-user devices, hiding the content from the OSN providers. The social graph is hidden from the OSN provider by employing a novel aliasing approach and using secure algorithms for mutual friendship establishment. Usernames are mapped to friend-specific aliases, which reduces the amount of information a provider can gather from analyzing these identifiers. Users authenticate to each other without revealing their identities to a potential attacker. The proposed system allows for user interactions between independent OSN providers. To improve data availability we use a replication scheme which does not jeopardize the obfuscation of the social graph. Our approach differs from existing works mainly by the social graph obfuscation in combination with replication.

Privacy Preservation in Decentralized Online Social Networks

The housing of personal data on large online social networks arouses concerns from privacy advocates and leery users. Researchers have proposed decentralized architectures to create online social networks with technically imposed privacy preservation. This survey of several approaches discusses their privacy benefits, architecture, and suitability for mobile devices.

A New Protocol to Determine the NAT Characteristics of a Host

The shortage of IPv4 addresses and the very slow transition to IPv6 leads to well-established pragmatic solutions in the Internet: today many hosts are still using IPv4 and are connected to the Internet over a Network Address Translation router. For many applications, which need inbound connections, like e.g. voice-over-IP or peer-to-peer-based systems it is necessary to determine the characteristics of the surrounding network environment, i.e. the behavior of the used router. In most cases this information is required to successfully establish inbound connections. Therefore, we present in this paper a new protocol to determine the characteristics of the used router. Our protocol is backward compatible to the well known STUN protocol while providing more detailed results. Furthermore, our protocol can be used in a fully decentralized way, i.e. without any centralized servers, making it suitable for pure peer-to-peer-based systems. We complete the presentation of our new protocol with an evaluation through a field experiment.

Measurement of Globally Visible DNS Injection

Domain Name System (DNS) injection is a censorship method for blocking access to blacklisted domain names. The method uses deep packet inspection on all DNS queries passing through the network and injects spoofed responses. Compared with other blocking mechanisms, DNS injection impacts uninvolved third-parties if their traffic is routed through a censored network. In this paper, we look for large deployments of DNS injection, measured from vantage points outside of the censored networks. DNS injection is known to be used in China since it leaked unintentionally into foreign networks. We find that DNS injection is also used in Iran and can be observed by sending DNS queries to Iranian networks. In mid 2013, the Iranian DNS filter was temporarily suspended for some names, which correlated with media coverage of political debates in Iran about blocking social media. Spoofed responses from China and Iran can be detected passively by the IP address returned. We propose an algorithm to obtain these addresses remotely. After testing 255002 open resolvers outside of China, we determined that 6% are potentially affected by Chinese DNS injection when querying top-level domains outside of China. This is essentially the result of one top-level domain name server for which an anycast instance is hosted in China.

Towards peer-to-peer-based cryptanalysis

Modern cryptanalytic algorithms require a large amount of computational power. An approach to cope with this requirement is to distribute these algorithms among many computers and to perform the computation massively parallel. However, existing approaches for distributing cryptanalytic algorithms are based on a client/server or a grid architecture. In this paper we propose the usage of peer-to-peer (P2P) technology for distributed cryptanalytic calculations. Our contribution in this paper is three-fold: We first identify the challenges resulting from this approach and provide a classification of algorithms suited for P2P-based computation. Secondly, we discuss and classify some specific cryptanalytic algorithms and their suitability for such an approach. Finally we provide a new, fully decentralized approach for distributing such computationally intensive jobs. Our design takes special care about scalability and the possible untrustworthy nature of the participating peers.

SYNI - TCP Hole Punching Based on SYN Injection

The shortage of IPv4 addresses and the very slow transition to IPv6 leads to pragmatic solutions in the Internet: today many hosts are still using IPv4 and are connected to the Internet over a Network Address Translation (NAT) router. However, there are many applications, which need inbound connections, like e.g. peer-to-peer-based systems or voice-over-IP. For such NATed hosts inbound connections usually pose a problem, since without additional measures the router filters the incoming connection attempts. These additional measures are usually referred to as NAT traversal mechanisms and hole punching is one of those techniques. In this paper we propose a new protocol for a TCP-based hole punching mechanism based on self-injecting SYN-packets in the local network stack.

GPU-Based NSEC3 Hash Breaking

When a client queries for a non-existent name in the Domain Name System (DNS), the server responds with a negative answer. With the DNS Security Extensions (DNSSEC), the server can either use NSEC or NSEC3 for authenticated negative answers. NSEC3 claims to protect DNSSEC servers against domain enumeration, but incurs significant CPU and bandwidth overhead. Thus, DNSSEC server admins must choose between more efficiency (NSEC) or privacy (NSEC3). We present a GPU-based attack on NSEC3 that revealed 64% of all DNSSEC names in the com domain in 4.5 days. This attack shows that the NSEC3 privacy promises are weak and thus DNSSEC server admins must carefully decide whether the limited privacy is worth the overhead. Furthermore, we show that an increase of the cryptographic strength of NSEC3 puts attackers at an advantage, since the cost of an attack does not rise faster than the costs incurred on the DNSSEC server.

Detecting Opportunistic Cheaters in Volunteer Computing

For computationally expensive but parallelizable search problems distributed computing approaches based on volunteer computing can be used. Volunteering users spend their computation time to gain some sort of credit or for the sake of appearing in a ranking. Some of the users may try to gain reward without investing their computation time, i.e. they cheat. Hence, a cheat detection mechanism against such opportunistic cheaters is needed. The simplest approach is the recalculation of all results by multiple users followed by a voting. This simple approach is inefficient since it increases the computational complexity by the factor of the executed recalculations. In this paper we propose a new and efficient approach for cheat detection in search problems using a combination of sample testing and result aggregation. Our approach provides a high probability of detecting a cheating user while reducing the computational complexity using sample testing and the required bandwidth using result aggregation. In a limited range, one can compensate a small available bandwidth with more computations, thus providing a trade-off between bandwidth and computational complexity.

Measuring occurrence of DNSSEC validation

DNSSEC is a security extension that adds public-key signatures to the Domain Name System for the purpose of data authenticity and integrity. While DNSSEC signatures are being deployed on an increasing number of name servers, little is known about the deployment advancements of client-side DNSSEC validation. In this paper we present a methodology to determine whether a client is protected by DNSSEC validation. We applied our methodology over a period of 7 months collecting results from different data sources. After data cleaning, we gathered 131,320 results from 98,179 distinct IP addresses, out of which 4.8% had validation enabled. The ratio varies significantly per country, with Sweden, the Czech Republic and the United States having the largest ratios of validating clients in the field.

Measurement survey of server-side DNSSEC adoption

This paper answers the question how far DNSSEC signing has found adoption in practice. By applying zone enumeration techniques on all top-level domains we gather the number of 6.4 million signed second-level domains. This figure is a complete snapshot of the DNSSEC ecosystem as of January 2017. The adoption concentrates among a small number of top-level domains, some of them having half of their domains signed with DNSSEC, while most top-level domains have adoption ratios of 1%, or less. The majority of top-level domains use NSEC3 hashing to thwart zone enumeration, but GPU-based zone enumeration allows us to recover 79% of cleartext domain names. Most second-level domains use RSA as signing algorithm with a combination of 2048-bit and 1024-bit keys, but 512-bit keys are also common despite being demonstrably insecure. ECDSA adoption has grown to 8% within the last two years. 0.45% of domains are not signed correctly and fail to validate. However, there are fewer domains failing due to DNSSEC errors than due to other misconfigurations or network problems.