Abstract
The QC-MDPC code-based KEM Bit Flipping Key Encapsulation (BIKE) is one of the Round-2 candidates of the NIST PQC standardization project. It has a variant that is proved to be IND-CCA secure. The proof models the KEM with some black-box (“ideal”) primitives. Specifically, the decapsulation invokes an ideal primitive called “decoder”, required to deliver its output with a negligible Decoding Failure Rate (DFR). The concrete instantiation of BIKE substitutes this ideal primitive with a new decoding algorithm called “Backflip”, that is shown to have the required negligible DFR. However, it runs in a variable number of steps and this number depends on the input and on the key. This paper proposes a decoder that has a negligible DFR and also runs in a fixed (and small) number of steps. We propose that the instantiation of BIKE uses this decoder with our recommended parameters. We study the decoder’s DFR as a function of the scheme’s parameters to obtain a favorable balance between the communication bandwidth and the number of steps that the decoder runs. In addition, we build a constant-time software implementation of the proposed instantiation, and show that its performance characteristics are quite close to the IND-CPA variant. Finally, we discuss a subtle gap that needs to be resolved for every IND-CCA secure KEM (BIKE included) where the decapsulation has nonzero failure probability: the difference between average DFR and “worst-case” failure probability per key and ciphertext.
Original language | English |
---|---|
Title of host publication | Code-Based Cryptography - 8th International Workshop, CBCrypto 2020, Revised Selected Papers |
Editors | Marco Baldi, Paolo Santini, Edoardo Persichetti |
Publisher | Springer |
Pages | 50-79 |
Number of pages | 30 |
ISBN (Print) | 9783030540739 |
DOIs | |
State | Published - 2020 |
Event | 8th International Workshop on Code-Based Cryptography, CBCrypto 2020 - Zagreb, Croatia Duration: 9 May 2020 → 10 May 2020 |
Publication series
Name | Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) |
---|---|
Volume | 12087 LNCS |
ISSN (Print) | 0302-9743 |
ISSN (Electronic) | 1611-3349 |
Conference
Conference | 8th International Workshop on Code-Based Cryptography, CBCrypto 2020 |
---|---|
Country/Territory | Croatia |
City | Zagreb |
Period | 9/05/20 → 10/05/20 |
Bibliographical note
Publisher Copyright:© Springer Nature Switzerland AG 2020.
Keywords
- BIKE
- Constant-time algorithm
- Constant-time implementation
- IND-CCA
- QC-MDPC codes
ASJC Scopus subject areas
- Theoretical Computer Science
- General Computer Science