Centralize password md5 hashing logic

Centralize logic for md5 hashing of passwords for authentication. Adds
a new function postgresMd5PasswordHash(user, password, salt) to utils
and updates client.js and tests to use it.

Author: sehrope

lib/client.js

@@ -106,10 +106,7 @@ Client.prototype.connect = function (callback) {
 
   // password request handling
   con.on('authenticationMD5Password', checkPgPass(function (msg) {
-    var inner = utils.md5(self.password + self.user)
-    var outer = utils.md5(Buffer.concat([Buffer.from(inner), msg.salt]))
-    var md5password = 'md5' + outer
-    con.password(md5password)
+    con.password(utils.postgresMd5PasswordHash(self.user, self.password, msg.salt))
   }))
 
   con.once('backendKeyData', function (msg) {

lib/utils.js

@@ -138,12 +138,20 @@ const md5 = function (string) {
   return crypto.createHash('md5').update(string, 'utf-8').digest('hex')
 }
 
+// See AuthenticationMD5Password at https://www.postgresql.org/docs/current/static/protocol-flow.html
+const postgresMd5PasswordHash = function (user, password, salt) {
+  var inner = md5(password + user)
+  var outer = md5(Buffer.concat([Buffer.from(inner), salt]))
+  return 'md5' + outer
+}
+
 module.exports = {
   prepareValue: function prepareValueWrapper (value) {
     // this ensures that extra arguments do not get passed into prepareValue
     // by accident, eg: from calling values.map(utils.prepareValue)
     return prepareValue(value)
   },
   normalizeQueryConfig: normalizeQueryConfig,
+  postgresMd5PasswordHash: postgresMd5PasswordHash,
   md5: md5
 }

test/integration/connection/test-helper.js

@@ -21,9 +21,7 @@ var connect = function (callback) {
       con.password(helper.args.password)
     })
     con.once('authenticationMD5Password', function (msg) {
-      var inner = utils.md5(helper.args.password + helper.args.user)
-      var outer = utils.md5(Buffer.concat([Buffer.from(inner), msg.salt]))
-      con.password('md5' + outer)
+      con.password(utils.postgresMd5PasswordHash(helper.args.user, helper.args.password, msg.salt));
     })
     con.once('readyForQuery', function () {
       con.query('create temp table ids(id integer)')

test/unit/client/md5-password-tests.js

@@ -11,9 +11,7 @@ test('md5 authentication', function () {
   test('responds', function () {
     assert.lengthIs(client.connection.stream.packets, 1)
     test('should have correct encrypted data', function () {
-      var encrypted = utils.md5(client.password + client.user)
-      encrypted = utils.md5(encrypted + salt.toString('binary'))
-      var password = 'md5' + encrypted
+      var password = utils.postgresMd5PasswordHash(client.user, client.password, salt)
       // how do we want to test this?
       assert.equalBuffers(client.connection.stream.packets[0], new BufferList()
                         .addCString(password).join(true, 'p'))