199 lines
6.2 KiB
Markdown
199 lines
6.2 KiB
Markdown
# libemsha
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Version: 1.0.2
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Date: 2016-01-28
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## Table of Contents
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- Introduction
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- Getting and Building the Source
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- Library Overview
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- The Hash interface
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- The SHA256 class
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- The HMAC class
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- Miscellaneous functions
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- Test Programs
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- References
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## Introduction
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This library is an MIT-licensed compact HMAC-SHA-256 C++11 library
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designed for embedded systems. It is built following the JPL [Power of
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Ten](http://spinroot.com/gerard/pdf/P10.pdf) rules.
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This library came about as a result of a need for a standalone SHA-256
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library for an embedded system. The original goal was to implement a
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wrapper around the code extracted from [RFC
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6234](https://tools.ietf.org/html/rfc6234); instead a standalone
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implementation was decided on.
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Additional resources:
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- [Github page](https://github.com/kisom/libemsha)
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- [Travis CI status](https://travis-ci.org/kisom/libemsha/)
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- [Coverity Scan page](https://scan.coverity.com/projects/libemsha-52f2a5fd-e759-43c2-9073-cf6c2ed9abdb)
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## Getting and Building the Source
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The source code is available via
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[Github](https://github.com/kisom/libemsha/); each version should be git
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tagged. :
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git clone https://github.com/kisom/libemsha
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git clone git@github.com:kisom/libemsha
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The current release is
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[1.0.0](https://github.com/kisom/libemsha/archive/1.0.0.zip).
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The project is built using Autotools and `make`.
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When building from a git checkout, the [autobuild]{.title-ref} script
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will bootstrap the project from the autotools sources (e.g. via
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`autoreconf -i`), run `configurei` (by default to use clang), and
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attempt to build the library and run the unit tests.
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Once the autotools infrastructure has been bootstrapped, the following
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should work: :
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./configure && make && make check && make install
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There are three flags to `configure` that might be useful:
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- `--disable-hexstring` disables the provided `hexstring` function;
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while this might be useful in many cases, it also adds extra size to
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the code.
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- `--disable-hexlut` disables the larger lookup table used by
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`hexstring`, which can save around a kilobyte of program space. If
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the `hexstring` function is disabled, this option has no effect.
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- `--disable-selftest` disables the internal self-tests, which can
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reclaim some additional program space.
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## Library Overview
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The package provides a pair of classes, :cpp`SHA256`{.interpreted-text
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role="class"} and :cpp`HMAC`{.interpreted-text role="class"}, that both
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satisfy a common interface :cpp`Hash`{.interpreted-text role="class"}.
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All functionality provided by this library is found under the `emsha`
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namespace.
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### `EMSHAResult`
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The `EMSHAResult` enum is used to convey the result of an operation.
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The possible values are:
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// All operations have completed successfully so far.
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EMSHAResult::OK = 0,
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// A self test or unit test failed.
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EMSHAResult::TestFailure = 1,
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// A null pointer was passed in as a buffer where it
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// shouldn't have been.
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EMSHAResult::NullPointer = 2,
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// The Hash is in an invalid state.
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EMSHAResult::InvalidState = 3,
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// The input to SHA256::update is too large.
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EMSHAResult::InputTooLong = 4,
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// The self tests have been disabled, but a self test
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// function was called.
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EMSHAResult::SelfTestDisabled = 5
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As a convenience, the following `typedef` is also provided.
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> `typedef enum _EMSHA_RESULT_` :cpp`EMSHAResult`{.interpreted-text
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> role="type"}
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## The Hash interface
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In general, a [Hash]{.title-ref} is used along the lines of: :
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emsha::EMSHAResult
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hash_single_pass(uint8_t *m, uint32_t ml, uint8_t *digest)
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{
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// Depending on the implementation, the constructor may need
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// arguments.
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emsha::Hash h;
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emsha::EMSHAResult res;
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res = h.write(m, ml);
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if (emsha::EMSHAResult::OK != res) {
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return res;
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}
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// digest will contain the output of the Hash, and the
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// caller MUST ensure that there is enough space in
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// the buffer.
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return h.result(d);
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}
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### Methods
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## The SHA256 class
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## The HMAC class
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## Miscellaneous functions
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## Test Programs
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Running `make check` builds and runs the test programs. These are:
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- `emsha_core_test` runs the core tests.
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- `emsha_sha256_test` runs test vectors on the SHA-256 code.
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- `emsha_hmac_test` runs test vectors on the HMAC code.
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Additionally, the following test programs are built but not run. These
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programs do not link with the library as the above programs do; instead,
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they compile the object files in to avoid the libtool dance before the
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library is installed.
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- `emsha_mem_test` and `emsha_static_mem_test` are for memory
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profiling (e.g., with [Valgrind](http://valgrind.org/) during
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development.
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- `emsha_static_sha256_test` and `emsha_static_hmac_test` are used to
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facilitate testing and debugging the library. These programs run the
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same tests as the `emsha_sha256_test` and `emsha_hmac_test`
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programs.
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### Core Tests
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There are three tests run in the core tests: a hexstring test (if
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[support is built in](./building.html)) and the constant time check. The
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constant time test does not validate that the function is constant time,
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only that it correctly verifies that two byte arrays are equal.
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### SHA-256 Tests
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The SHA-256 checks take a number of test vectors from the Go standard
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library\'s SHA-256 library.
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### HMAC Tests
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The HMAC checks apply the [RFC 4231](http://tools.ietf.org/html/rfc4231)
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test vectors to the HMAC code.
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## References
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- [FIPS 180-4, the Secure Hash
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Standard](http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf)
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- [FIPS 198-1, The Keyed-Hash Message Authentication Code
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(HMAC)](http://csrc.nist.gov/publications/fips/fips198-1/FIPS-198-1_final.pdf)
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- [RFC 2014, HMAC: Keyed-Hashing for Message
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Authentication](https://tools.ietf.org/html/rfc2104)
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- [RFC 6234, US Secure Hash Algorithms (SHA and SHA-based HMAC and
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HKDF)](https://tools.ietf.org/html/rfc6234)[^1]
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- The behaviour of this package was cross-checked using the Go 1.5.1
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linux/amd64 standard library\'s
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[crypto/sha256](https://golang.org/src/crypto/sha256/) package.
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**Footnotes**
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[^1]:
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This library came about after extracting the relevant C code from
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RFC 6234, and needing a C++ version. It draws heavy inspiration from
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that code base.
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