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https://github.com/lkujaw/blake2s

The BLAKE2s hash function for SPARK83 [Ada (1987)]
https://github.com/lkujaw/blake2s

ada ada1987 blake2 blake2s hash mit-license spark-ada

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The BLAKE2s hash function for SPARK83 [Ada (1987)]

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# BLAKE2s for Ada



This is a SPARK83 implementation of the [BLAKE2s][1] hash function.

As SPARK83 is a strict subset of Ada 1987 (ISO-8652:1987), this

package should be usable with any compiler compliant with the

1987 (or later) edition of the ISO Ada standard.



## BLAKE2s Advantages



Like SHA-256, BLAKE2s operates on 32-bit words, but is not susceptible

to the length extension attacks of the former.  Although the FIPS hash

functions SHA-512/256 and SHA-3 mitigate the length extension

vulnerability of SHA-256, they require 64-bit words for decent

performance.  Thus, BLAKE2s fills a niche for resource-constrained

platforms. It also enjoys a high security margin, as each of the ten

BLAKE2s rounds is the equivalent of two ChaCha rounds.



## Build Instructions for GNAT



To compile the BLAKE2s library and executables, you will need to have

installed and accessible within your path copies of [AST NMAKE][2]

with Ada support and the GCC compiler built with the GNAT Ada

front-end enabled.



Once the prerequisites are available, simply run `cd gnat && nmake` .



## SPARK and Isabelle Verification



This project uses a combination of the original, annotation-based

SPARK tool set (search [AdaCore Libre][3] for the 2012 GPL release)

and the HOL-SPARK library bundled within [Isabelle][4] 2021 to prove

the absence of runtime errors.



To verify the proofs, check the file named `makefile.nmk` within the

project root first to ensure that you have the necessary programs

(including the `isabelle` command) installed within your path.



If everything is functioning as it should, all verification conditions

should be discharged.



## Ada 1987 Compatibility Note



This package utilizes the Ada 1995 pragma "Pure" in the following

package specifications:



* BLAKE2S (common/blake2s.ads)

* Octets (common/octets.ads)

* Octet_Arrays (common/octearra.ads)



Per section 2.8 of the Ada 1987 Language Reference Manual, "[a] pragma

that is not language-defined has no effect if its identifier is not

recognized by the (current) implementation."  However, as the pragma

is merely advisory to the compiler, it may be removed without adverse

effect from these files should it cause any issues.



## SPARK83 Rationale



For the 1987, 1995, and 2007 editions of the ISO Ada standard, the

corresponding SPARK language tools were based upon annotations.  The

placement of annotations within Ada comments has the advantage of not

requiring any special support for SPARK on the part of the compiler.



At the time of writing, the compiler support required by the post-2007

editions of the SPARK language is found only within the GNAT compiler.

Like the BLAKE2s reference source code, which targets the first

edition of the ISO C standard released in 1990, this project targets

the earliest release of the ISO Ada standard to maximize portability,

and for this same reason the annotation-based SPARK language tools are

ideal.



## License and Warranty



Licensing terms and important warranty information for the BLAKE2s

library may be found within the file named [license.txt][3].



Separate from the BLAKE2s library, the B2SSUM and B2STEST executables

are distributed under the terms of the GNU General Public License,

version 3.0; see the file [copying.txt][4] for licensing terms and

important warranty information.



`SPDX-License-Identifier: MIT-0`



## Acknowledgments



Thanks to Aumasson et al. for releasing the excellent BLAKE hash

functions into the public domain, and the [GNAT, SPARK][5],

[Isabelle][6], and [AdaControl][7] developers for publishing their

tremendously helpful [free software][8].



[1]: https://www.blake2.net/

[2]: https://sr.ht/~lev/ast/

[3]: license.txt

[4]: b2ssum/copying.txt

[5]: https://libre.adacore.com/

[6]: https://isabelle.in.tum.de/

[7]: https://www.adalog.fr/en/adacontrol.html

[8]: https://www.gnu.org/philosophy/free-sw.html