Add functions to create private keys to the secp256k1 module

Author: alcuadrado

packages/ethereum-cryptography/README.md

@@ -345,27 +345,39 @@ The `secp256k1` submodule provides a library for elliptic curve operations on
 the curve Secp256k1.
 
 It has the exact same API than the version `4.x` of the [`secp256k1`](https://github.com/cryptocoinjs/secp256k1-node)
-module from cryptocoinjs.
+module from cryptocoinjs, with two added function to create private keys.
+
+### Creating private keys
+
+Secp256k1 private keys need to be cryptographycally secure random numbers with
+certain caracteristics. If this is not the case, the security of Secp256k1 is
+compromissed.
+
+We strongly recommend to use this module to create new private keys.
 
 ### Function types
 
-Go to [`secp256k1`'s documentation](https://github.com/cryptocoinjs/secp256k1-node)
-to learn how to use it.
+Functions to create private keys:
+
+```ts
+function createPrivateKey(): Promise<Uint8Array>;
+
+function function createPrivateKeySync(): Uint8Array;
+```
+
+For the rest of the functions, pleasse read [`secp256k1`'s documentation](https://github.com/cryptocoinjs/secp256k1-node).
 
 ### Example usage
 
 ```js
-const { sign } = require("ethereum-cryptography/secp256k1");
+const { sign, createPrivateKeySync } = require("ethereum-cryptography/secp256k1");
 
 const msgHash = Buffer.from(
   "82ff40c0a986c6a5cfad4ddf4c3aa6996f1a7837f9c398e17e5de5cbd5a12b28",
   "hex"
 );
 
-const privateKey = Buffer.from(
-  "3c9229289a6125f7fdf1885a77bb12c37a8d3b4962d936f7e3084dece32a3ca1",
-  "hex"
-);
+const privateKey = createPrivateKeySync();
 
 console.log(Buffer.from(sign(msgHash, privateKey)).signature.toString("hex"));
 ```

packages/ethereum-cryptography/src/secp256k1.ts

@@ -1 +1,24 @@
+import { privateKeyVerify } from "secp256k1";
+import { getRandomBytes, getRandomBytesSync } from "./random";
+
+const SECP256K1_PRIVATE_KEY_SIZE = 32;
+
+export async function createPrivateKey(): Promise<Uint8Array> {
+  while (true) {
+    const pk = await getRandomBytes(SECP256K1_PRIVATE_KEY_SIZE);
+    if (privateKeyVerify(pk)) {
+      return pk;
+    }
+  }
+}
+
+export function createPrivateKeySync(): Uint8Array {
+  while (true) {
+    const pk = getRandomBytesSync(SECP256K1_PRIVATE_KEY_SIZE);
+    if (privateKeyVerify(pk)) {
+      return pk;
+    }
+  }
+}
+
 export * from "secp256k1";

packages/ethereum-cryptography/test/node/secp256k1.ts

@@ -1,4 +1,18 @@
-import { ecdsaRecover, ecdsaSign, publicKeyConvert } from "../../src/secp256k1";
+import {
+  createPrivateKey,
+  createPrivateKeySync,
+  ecdsaRecover,
+  ecdsaSign,
+  privateKeyVerify,
+  publicKeyConvert
+} from "../../src/secp256k1";
 import { createTests } from "../test-vectors/secp256k1";
 
-createTests(ecdsaSign, ecdsaRecover, publicKeyConvert);
+createTests(
+  ecdsaSign,
+  ecdsaRecover,
+  publicKeyConvert,
+  createPrivateKey,
+  createPrivateKeySync,
+  privateKeyVerify
+);

packages/ethereum-cryptography/test/pure/secp256k1.ts

@@ -1,4 +1,18 @@
-import { ecdsaRecover, ecdsaSign, publicKeyConvert } from "../../src/secp256k1";
+import {
+  createPrivateKey,
+  createPrivateKeySync,
+  ecdsaRecover,
+  ecdsaSign,
+  privateKeyVerify,
+  publicKeyConvert
+} from "../../src/secp256k1";
 import { createTests } from "../test-vectors/secp256k1";
 
-createTests(ecdsaSign, ecdsaRecover, publicKeyConvert);
+createTests(
+  ecdsaSign,
+  ecdsaRecover,
+  publicKeyConvert,
+  createPrivateKey,
+  createPrivateKeySync,
+  privateKeyVerify
+);

packages/ethereum-cryptography/test/test-vectors/secp256k1.ts

@@ -11,9 +11,20 @@ export function createTests(
     signature: Uint8Array,
     compressed?: boolean
   ) => Uint8Array,
-  publicKeyConvert: (publicKey: Uint8Array, compressed?: boolean) => Uint8Array
+  publicKeyConvert: (publicKey: Uint8Array, compressed?: boolean) => Uint8Array,
+  createPrivakeKey: () => Promise<Uint8Array>,
+  createPrivakeKeySync: () => Uint8Array,
+  privateKeyVerify: (pk: Uint8Array) => boolean
 ) {
   describe("secp256k1", function() {
+    it("should create valid private keys", async function() {
+      const asyncPk = await createPrivakeKey();
+      const syncPk = createPrivakeKeySync();
+
+      assert.isTrue(privateKeyVerify(asyncPk));
+      assert.isTrue(privateKeyVerify(syncPk));
+    });
+
     it("Should sign correctly", function() {
       // This test has been adapted from ethereumjs-util
       // https://github.com/ethereumjs/ethereumjs-util/blob/3b1085059194b02354177d334f89cd82a5187883/test/index.js#L531