libmceliece: Security

libmceliece

Cryptosystem security model: Classic McEliece is a KEM designed for IND-CCA2 security, upgrading from the original McEliece design goal of OW-CPA security. Some applications need other security notions; typically these can be provided by generic wrappers on top of IND-CCA2.

Cryptosystem security review: The McEliece cryptosystem has a stable security track record. Extensive cryptanalysis over four decades has produced only minor changes in the McEliece security level, and, in particular, zero change in the asymptotic security exponent. The QROM IND-CCA2 security level of Classic McEliece is provably close to the OW-CPA security level of the original McEliece system. Classic McEliece addresses the risk of IND-CCA2 attacks beyond QROM IND-CCA2 attacks by choosing a well-studied, high-security, "unstructured" hash function, namely SHAKE256.

Security level: The 6688128 and 6960119 parameter sets are recommended for long-term security and provide a large security margin against known attacks, including quantum attacks.

Software verification: Covered in a separate file.

Timing attacks: libmceliece is designed to avoid all data flow from secret data to memory addresses and branch conditions. Fully protecting the user against timing attacks requires addressing more issues, such as the following:

- Other CPU instructions can take variable time. For example, there are some embedded CPUs with variable-time multipliers; libmceliece avoids data flow from secrets to multipliers, but many other cryptographic libraries rely on secret multiplications, and there could be other variable-time instructions that the compiler ends up using for libmceliece.

- Many CPUs include dynamic frequency-selection mechanisms such as Turbo Boost, exposing power information via timing information. Fortunately, these CPUs are normally shipped with simple options to disable Turbo Boost etc., closing this leak; unfortunately, Turbo Boost is enabled by default on CPUs that support it.

- Cryptographic keys are normally handled by cryptographic software, but other user secrets are handled by many different pieces of software.

See https://timing.attacks.cr.yp.to for a timing-attack survey and many references.

Speculative-execution attacks: Some countermeasures against speculative-execution attacks are planned but are not included in the current version of libmceliece. Full protection again requires addressing issues at other system layers.

Further side-channel attacks: Even if all legitimate user sensors are successfully kept isolated from attackers, attackers can set up their own power sensors, electromagnetic sensors, acoustic sensors, etc. Keeping cryptographic operations physically separated from sensors tends to make such attacks much more expensive but is often infeasible. "Masking" cryptographic computations seems to help and can be affordable, although the security of masking is difficult to evaluate and there are many broken masked implementations. Currently libmceliece does not include any masked implementations, so presumably it is easily breakable by power attacks in environments where attackers can see power consumption.

Further attacks: Presumably libmceliece is easily breakable by fault attacks in environments where attackers can trigger faults. Beyond attacker-triggered faults, natural DRAM faults occur surprisingly often. Implicit rejection fails to provide IND-CCA2 security when DRAM faults corrupt the secret key used for implicit rejection. On the defense side, error correction is recommended (and often not provided by hardware), and there is a general-purpose libsecded library that applies error correction to any array, although this does not catch errors that occur during computations.

Version: This is version 2023.02.18 of the "Security" web page.