This thesis studies efficiency and security problems of implementations of code-based
cryptosystems. These cryptosystems, though not currently used in the field, are of great
scientific interest, since no quantum algorithm is known that breaks them essentially
faster than any known classical algorithm. This qualifies them as cryptographic schemes
for the quantum-computer era, where the currently used cryptographic schemes are
rendered insecure.
Concerning the efficiency of these schemes, we propose a solution for the handling of
the public keys, which are, compared to the currently used schemes, of an enormous size.
Here, the focus lies on resource-constrained devices, which are not capable of storing a
code-based public key of communication partner in their volatile memory. Furthermore,
we show a solution for the decryption without the parity check matrix with a passable
speed penalty. This is also of great importance, since this matrix is of a size that is
comparable to that of the public key. Thus, the employment of this matrix on memory-constrained devices
is not possible or incurs a large cost.
Subsequently, we present an analysis of improvements to the generally most
time-consuming part of the decryption operation, which is the determination of the roots of
the error locator polynomial. We compare a number of known algorithmic variants and
new combinations thereof in terms of running time and memory demands. Though the
speed of pure software implementations must be seen as one of the strong sides of code-based schemes,
the optimisation of their running time on resource-constrained devices
and servers is of great relevance.
The second essential part of the thesis studies the side channel security of these
schemes. A side channel vulnerability is given when an attacker is able to retrieve
information about the secrets involved in a cryptographic operation by measuring physical
quantities such as the running time or the power consumption during that operation.
Specifically, we consider attacks on the decryption operation, which either target the
message or the secret key. In most cases, concrete countermeasures are proposed and
evaluated. In this context, we show a number of timing vulnerabilities that are linked to
the algorithmic variants for the root-finding of the error locator polynomial mentioned
above. Furthermore, we show a timing attack against a vulnerability in the Extended
Euclidean Algorithm that is used to solve the so-called key equation during the decryption
operation, which aims at the recovery of the message. We also present a related
practical power analysis attack. Concluding, we present a practical timing attack that
targets the secret key, which is based on the combination of three vulnerabilities, located
within the syndrome inversion, a further suboperation of the decryption, and the already
mentioned solving of the key equation.
We compare the attacks that aim at the recovery of the message with the analogous
attacks against the RSA cryptosystem and derive a general methodology for the discovery
of the underlying vulnerabilities in cryptosystems with specific properties.
Furthermore, we present two implementations of the code-based McEliece cryptosystem:
a smart card implementation and flexible implementation, which is based on a
previous open-source implementation. The previously existing open-source implementation
was extended to be platform independent and optimised for resource-constrained
devices. In addition, we added all algorithmic variants presented in this thesis, and
we present all relevant performance data such as running time, code size and memory
consumption for these variants on an embedded platform. Moreover, we implemented
all side channel countermeasures developed in this work.
Concluding, we present open research questions, which will become relevant once
efficient and secure implementations of code-based cryptosystems are evaluated by the
industry for an actual application
