Generate tokens and ids with cryptoUtils module.

Move object ID, token, and random string generation into their own
module, cryptoUtils.

Remove hat dependency, which was used to generate session and some other
tokens, because it  used non-cryptographic random number generator.
Replace it with the cryptographically secure one. The result has the
same format (32-character hex string, 128 bits of entropy).

Remove randomstring dependency, as we already have this functionality.

Add tests.

spec/cryptoUtils.spec.js

@@ -0,0 +1,83 @@
+var cryptoUtils = require('../src/cryptoUtils');
+
+function givesUniqueResults(fn, iterations) {
+  var results = {};
+  for (var i = 0; i < iterations; i++) {
+    var s = fn();
+    if (results[s]) {
+      return false;
+    }
+    results[s] = true;
+  }
+  return true;
+}
+
+describe('randomString', () => {
+  it('returns a string', () => {
+    expect(typeof cryptoUtils.randomString(10)).toBe('string');
+  });
+
+  it('returns result of the given length', () => {
+    expect(cryptoUtils.randomString(11).length).toBe(11);
+    expect(cryptoUtils.randomString(25).length).toBe(25);
+  });
+
+  it('throws if requested length is zero', () => {
+    expect(() => cryptoUtils.randomString(0)).toThrow();
+  });
+
+  it('returns unique results', () => {
+    expect(givesUniqueResults(() => cryptoUtils.randomString(10), 100)).toBe(true);
+  });
+});
+
+describe('randomHexString', () => {
+  it('returns a string', () => {
+    expect(typeof cryptoUtils.randomHexString(10)).toBe('string');
+  });
+
+  it('returns result of the given length', () => {
+    expect(cryptoUtils.randomHexString(10).length).toBe(10);
+    expect(cryptoUtils.randomHexString(32).length).toBe(32);
+  });
+
+  it('throws if requested length is zero', () => {
+    expect(() => cryptoUtils.randomHexString(0)).toThrow();
+  });
+
+  it('throws if requested length is not even', () => {
+    expect(() => cryptoUtils.randomHexString(11)).toThrow();
+  });
+
+  it('returns unique results', () => {
+    expect(givesUniqueResults(() => cryptoUtils.randomHexString(20), 100)).toBe(true);
+  });
+});
+
+describe('newObjectId', () => {
+  it('returns a string', () => {
+    expect(typeof cryptoUtils.newObjectId()).toBe('string');
+  });
+
+  it('returns result with at least 10 characters', () => {
+    expect(cryptoUtils.newObjectId().length).toBeGreaterThan(9);
+  });
+
+  it('returns unique results', () => {
+    expect(givesUniqueResults(() => cryptoUtils.newObjectId(), 100)).toBe(true);
+  });
+});
+
+describe('newToken', () => {
+  it('returns a string', () => {
+    expect(typeof cryptoUtils.newToken()).toBe('string');
+  });
+
+  it('returns result with at least 32 characters', () => {
+    expect(cryptoUtils.newToken().length).toBeGreaterThan(31);
+  });
+
+  it('returns unique results', () => {
+    expect(givesUniqueResults(() => cryptoUtils.newToken(), 100)).toBe(true);
+  });
+});