Verify that generated keypair correctly signs and verifies message

package.json

@@ -43,7 +43,7 @@
     "compile-with-source-maps": "babel --optional runtime -s -t -d distrib/npm/ src/",
     "prepublish": "npm test && npm run lint && npm run compile",
     "test": "istanbul test _mocha",
-     "codecov": "cat ./coverage/coverage.json | ./node_modules/codecov.io/bin/codecov.io.js",
+    "codecov": "cat ./coverage/coverage.json | ./node_modules/codecov.io/bin/codecov.io.js",
     "lint": "if ! [ -f eslintrc ]; then curl -o eslintrc 'https://raw.githubusercontent.com/ripple/javascript-style-guide/es6/eslintrc'; echo 'parser: babel-eslint' >> eslintrc; fi; eslint -c eslintrc src/*.js test/*.js"
   },
   "repository": {

src/index.js

@@ -73,7 +73,14 @@ function select(algorithm) {
 function deriveKeypair(seed, options) {
   const decoded = addressCodec.decodeSeed(seed)
   const algorithm = decoded.type === 'ed25519' ? 'ed25519' : 'ecdsa-secp256k1'
-  return select(algorithm).deriveKeypair(decoded.bytes, options)
+  const method = select(algorithm)
+  const keypair = method.deriveKeypair(decoded.bytes, options)
+  const messageToVerify = hash('This test message should verify.')
+  const signature = method.sign(messageToVerify, keypair.privateKey)
+  if (method.verify(messageToVerify, signature, keypair.publicKey) !== true) {
+    throw new Error('derived keypair did not generate verifiable signature');
+  }
+  return keypair
 }
 
 function getAlgorithmFromKey(key) {