Ascon_SPARK

Introduction

This is an Ada 2012 / SPARK 2014 project that implements the Ascon Authenticated Encryption with Additional Data Algorithm, the NIST Lightweight Cryptography standard. It was also selected as a finalist in the CAESAR competition. Ascon was designed by Christoph Dobraunig, Maria Eichlseder, Florian Mendel and Martin Schläffer.

This project implements both of the recommended Ascon variants in v1.1 and v1.2 of the specification. A single generic package can be instantiated with the relevant parameters. There are a few additional requirements on the target system which should not be a problem - for example it must support operations on the Interfaces.Unsigned_64 type.

This project is free software (using the ISC permissive licence) and is provided with no warranties, as set out in the file LICENSE.

Overview of the packages

The main generic package is Ascon which implements the high-level API. This consists of just two procedures, AEADEnc and AEADDec. The former procedure takes in a key K, a 'nonce' N, optional associated data A (not encrypted) and the optional message to be encrypted M and returns the encrypted cipher-text C and the authentication tag T. The latter procedure performs the decryption and returns the decoded message and a Boolean that indicates whether the message was valid. If either of the A or M parameters are not used, the constant Null_Storage_Array can be passed to the routines.

Packages Ascon128v11 and Ascon128av11 are instantiations of the generic package with the parameters recommended in version 1.1 of the specification. Packages Ascon128v12 and Ascon128av12 are instantiations of the generic package with the parameters recommended in version 1.2 of the specification.

Ascon_Definitions defines some constrained types used in the generic formal parameters and Ascon.Load_Store contains functions that convert between standard word sizes and Storage_Array in Little-Endian format.

Ascon.Access_Internals allows access to the lower-level API which allows you to follow the internal state of the cipher as it processes some data. Ascon.Utils contains useful helper functions for printing out Storage_Array and State types.

Examples

Three example programs are included. ascon128_demo is a simple example of using the high-level API and demonstrates successful encryption and decryption, and also unsuccessful decryption if the tag is altered.

ascon_test_vectors uses the lower-level API to print the trace of a sample encryption for both of the suggested variants of Ascon128. These can be compared with the output of the reference C code provided by the Ascon designers. In order to get a more detailed trace from the reference C code, the line //#define PRINTSTATE in ascon.c should be un-commented.

ascon_check_padding checks that authenticated encryption and decryption works correctly when the lengths of the associated data and message vary. This is primarily to check for any bugs in the implementation of the padding.

Status of SPARK proof

As the code is written in the SPARK 2014 subset of Ada 2012, it is possible to formally verify properties of the code and eliminate the possibility of run-time exceptions.

The GPL SPARK prover gnatprove shipped with GNAT Community 2021 from AdaCore is used for this project. It is also able to prove the absence of all potential sources of run-time exceptions without manual intervention. This includes checking for the non-initialization of arrays, which was not possible before GNAT Community 2020. It also proves that AEADDec will not return any decrypted data if the tag verification failed.

Project files

The project has been updated to work withe Alire package manager, so it can be built with the usual command alr build. The tests directory contains a sub-project that includes tests of the code.

Alternatively, the gprbuild project files can be used directly. These have the usual flags available, but with one custom flag:

• ascon_spark_load_store can only be set to explicit (the default). This setting is reserved for possible future accelerated big-/little-endian conversions.

Note that the file ascon-compare_tags.adb is given special treatment, as it is never optimized regardless of the optimization flags selected. This is deliberate, in an attempt to prevent the compiler from optimizing the tag check to 'short-circuit' - that is, to prevent it from re-writing the code to return early if two tags differ at an early position. This optimization can produce a timing small side-channel which can, in certain situations, leak information to opponents. This is also the reason why the comparison of the two tag arrays is not written in the most obvious way.

Using GNATprove for verification

To verify the code, GNATprove can be invoked via the GnatStudio IDE. Alternatively the following command line can be used:

• SPARK from GNAT Community 2021

  gnatprove -P ascon_spark.gpr -j0 -Xload_store=explicit -Xmode=debug --level=0

Add --report=all if you want to see the checks that are proved as well.