Move additional_key.rs to serai-processor-view-keys

I don't love this. I wanted to simply add this function to `processor/key-gen`,
but then anyone who wants a view key needs to pull in Bulletproofs which is a
mess of code. They'd also be subject to an AGPL licensed library.

This is so small it should be a primitive elsewhere, yet there is no primitives
library eligible. Maybe serai-client since that has the code to make
transactions to Serai (and will have this as a dependency)? Except then the
processor has to import serai-client when this rewrite removed it as a
dependency.

processor/view-keys/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "serai-processor-view-keys"
+version = "0.1.0"
+description = "View keys for the Serai processor"
+license = "MIT"
+repository = "https://github.com/serai-dex/serai/tree/develop/processor/view-keys"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+keywords = []
+edition = "2021"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]
+
+[lints]
+workspace = true
+
+[dependencies]
+ciphersuite = { version = "0.4", path = "../../crypto/ciphersuite", default-features = false, features = ["std"] }

processor/view-keys/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2022-2024 Luke Parker
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

processor/view-keys/README.md

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+# Serai Processor View Keys
+
+View keys for the Serai processor.
+
+This is a MIT-licensed library made available for anyone to generate Serai's
+view keys, as necessary for auditing reasons and for sending coins to Serai.

processor/view-keys/src/lib.rs

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+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![doc = include_str!("../README.md")]
+#![deny(missing_docs)]
+
+use ciphersuite::Ciphersuite;
+
+/// Generate a view key for usage within Serai.
+///
+/// `k` is the index of the key to generate (enabling generating multiple view keys within a
+/// single context).
+pub fn view_key<C: Ciphersuite>(k: u64) -> C::F {
+  C::hash_to_F(b"Serai DEX View Key", &k.to_le_bytes())
+}