Merge bitcoin-core/secp256k1#1431: Add CONTRIBUTING.md

0e5ea62 CONTRIBUTING: add some coding and style conventions (Jonas Nick)
1a432cb README: update first sentence (Jonas Nick)
0922a04 docs: move coverage report instructions to CONTRIBUTING (Jonas Nick)
76880e4 Add CONTRIBUTING.md including scope and guidelines for new code (Jonas Nick)

Pull request description:

  Following offline discussions, this PR documents the scope of the library and the requirements for adding new modules. I think this fixes most of bitcoin#997. It also updates the README very slightly.

  In addition, I added some coding conventions that I remembered explaining to new contributors in the past year. Even though it's far from exhaustive, I think this is an easy improvement to the CONTRIBUTING.md. Feel free to suggest more conventions.

ACKs for top commit:
  sipa:
    ACK 0e5ea62
  real-or-random:
    ACK 0e5ea62

Tree-SHA512: ffdbab22982fd632de92e81bd135f141ac86e24cc0dcfc0e1ae12b0d2a2e4f91377ab2c0cc440cb919889eaed8bfc1447b880fa1430fd771b956f2af0fe3766e

CONTRIBUTING.md

@@ -0,0 +1,107 @@
+# Contributing to libsecp256k1
+
+## Scope
+
+libsecp256k1 is a library for elliptic curve cryptography on the curve secp256k1, not a general-purpose cryptography library.
+The library primarily serves the needs of the Bitcoin Core project but provides additional functionality for the benefit of the wider Bitcoin ecosystem.
+
+## Adding new functionality or modules
+
+The libsecp256k1 project welcomes contributions in the form of new functionality or modules, provided they are within the project's scope.
+
+It is the responsibility of the contributors to convince the maintainers that the proposed functionality is within the project's scope, high-quality and maintainable.
+Contributors are recommended to provide the following in addition to the new code:
+
+* **Specification:**
+    A specification can help significantly in reviewing the new code as it provides documentation and context.
+    It may justify various design decisions, give a motivation and outline security goals.
+    If the specification contains pseudocode, a reference implementation or test vectors, these can be used to compare with the proposed libsecp256k1 code.
+* **Security Arguments:**
+    In addition to a defining the security goals, it should be argued that the new functionality meets these goals.
+    Depending on the nature of the new functionality, a wide range of security arguments are acceptable, ranging from being "obviously secure" to rigorous proofs of security.
+* **Relevance Arguments:**
+    The relevance of the new functionality for the Bitcoin ecosystem should be argued by outlining clear use cases.
+
+These are not the only factors taken into account when considering to add new functionality.
+The proposed new libsecp256k1 code must be of high quality, including API documentation and tests, as well as featuring a misuse-resistant API design.
+
+We recommend reaching out to other contributors (see [Communication Channels](#communication-channels)) and get feedback before implementing new functionality.
+
+## Communication channels
+
+Most communication about libsecp256k1 occurs on the GitHub repository: in issues, pull request or on the discussion board.
+
+Additionally, there is an IRC channel dedicated to libsecp256k1, with biweekly meetings (see channel topic).
+The channel is `#secp256k1` on Libera Chat.
+The easiest way to participate on IRC is with the web client, [web.libera.chat](https://web.libera.chat/#secp256k1).
+Chat history logs can be found at https://gnusha.org/secp256k1/.
+
+## Contributor workflow & peer review
+
+The Contributor Workflow & Peer Review in libsecp256k1 are similar to Bitcoin Core's workflow and review processes described in its [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md).
+
+### Coding conventions
+
+In addition, libsecp256k1 tries to maintain the following coding conventions:
+
+* No runtime heap allocation (e.g., no `malloc`) unless explicitly requested by the caller (via `secp256k1_context_create` or `secp256k1_scratch_space_create`, for example). Morever, it should be possible to use the library without any heap allocations.
+* The tests should cover all lines and branches of the library (see [Test coverage](#coverage)).
+* Operations involving secret data should be tested for being constant time with respect to the secrets (see [src/ctime_tests.c](src/ctime_tests.c)).
+* Local variables containing secret data should be cleared explicitly to try to delete secrets from memory.
+* Use `secp256k1_memcmp_var` instead of `memcmp` (see [#823](https://github.com/bitcoin-core/secp256k1/issues/823)).
+
+#### Style conventions
+
+* Commits should be atomic and diffs should be easy to read. For this reason, do not mix any formatting fixes or code moves with actual code changes. Make sure each individual commit is hygienic: that it builds successfully on its own without warnings, errors, regressions, or test failures.
+* New code should adhere to the style of existing, in particular surrounding, code. Other than that, we do not enforce strict rules for code formatting.
+* The code conforms to C89. Most notably, that means that only `/* ... */` comments are allowed (no `//` line comments). Moreover, any declarations in a `{ ... }` block (e.g., a function) must appear at the beginning of the block before any statements. When you would like to declare a variable in the middle of a block, you can open a new block:
+    ```C
+    void secp256k_foo(void) {
+        unsigned int x;              /* declaration */
+        int y = 2*x;                 /* declaration */
+        x = 17;                      /* statement */
+        {
+            int a, b;                /* declaration */
+            a = x + y;               /* statement */
+            secp256k_bar(x, &b);     /* statement */
+        }
+    }
+    ```
+* Use `unsigned int` instead of just `unsigned`.
+* Use `void *ptr` instead of `void* ptr`.
+* Arguments of the publicly-facing API must have a specific order defined in [include/secp256k1.h](include/secp256k1.h).
+* User-facing comment lines in headers should be limited to 80 chars if possible.
+* All identifiers in file scope should start with `secp256k1_`.
+* Avoid trailing whitespace.
+
+### Tests
+
+#### Coverage
+
+This library aims to have full coverage of reachable lines and branches.
+
+To create a test coverage report, configure with `--enable-coverage` (use of GCC is necessary):
+
+    $ ./configure --enable-coverage
+
+Run the tests:
+
+    $ make check
+
+To create a report, `gcovr` is recommended, as it includes branch coverage reporting:
+
+    $ gcovr --exclude 'src/bench*' --print-summary
+
+To create a HTML report with coloured and annotated source code:
+
+    $ mkdir -p coverage
+    $ gcovr --exclude 'src/bench*' --html --html-details -o coverage/coverage.html
+
+#### Exhaustive tests
+
+There are tests of several functions in which a small group replaces secp256k1.
+These tests are *exhaustive* since they provide all elements and scalars of the small group as input arguments (see [src/tests_exhaustive.c](src/tests_exhaustive.c)).
+
+### Benchmarks
+
+See `src/bench*.c` for examples of benchmarks.

README.md

@@ -4,7 +4,7 @@ libsecp256k1
 ![Dependencies: None](https://img.shields.io/badge/dependencies-none-success)
 [![irc.libera.chat #secp256k1](https://img.shields.io/badge/irc.libera.chat-%23secp256k1-success)](https://web.libera.chat/#secp256k1)
 
-Optimized C library for ECDSA signatures and secret/public key operations on curve secp256k1.
+High-performance high-assurance C library for digital signatures and other cryptographic primitives on the secp256k1 elliptic curve.
 
 This library is intended to be the highest quality publicly available library for cryptography on the secp256k1 curve. However, the primary focus of its development has been for usage in the Bitcoin system and usage unlike Bitcoin's may be less well tested, verified, or suffer from a less well thought out interface. Correct usage requires some care and consideration that the library is fit for your application's purpose.
 
@@ -116,28 +116,6 @@ Usage examples can be found in the [examples](examples) directory. To compile th
 
 To compile the Schnorr signature and ECDH examples, you also need to configure with `--enable-module-schnorrsig` and `--enable-module-ecdh`.
 
-Test coverage
------------
-
-This library aims to have full coverage of the reachable lines and branches.
-
-To create a test coverage report, configure with `--enable-coverage` (use of GCC is necessary):
-
-    $ ./configure --enable-coverage
-
-Run the tests:
-
-    $ make check
-
-To create a report, `gcovr` is recommended, as it includes branch coverage reporting:
-
-    $ gcovr --exclude 'src/bench*' --print-summary
-
-To create a HTML report with coloured and annotated source code:
-
-    $ mkdir -p coverage
-    $ gcovr --exclude 'src/bench*' --html --html-details -o coverage/coverage.html
-
 Benchmark
 ------------
 If configured with `--enable-benchmark` (which is the default), binaries for benchmarking the libsecp256k1 functions will be present in the root directory after the build.
@@ -154,3 +132,8 @@ Reporting a vulnerability
 ------------
 
 See [SECURITY.md](SECURITY.md)
+
+Contributing to libsecp256k1
+------------
+
+See [CONTRIBUTING.md](CONTRIBUTING.md)