code: func Get(c context.Context) io.Reader {
  	if r, _ := c.Value(&key).(io.Reader); r != nil {
  		return r
  	}
  	return crypto.Reader
  }
docs: // Get returns an io.Reader that emits random stream of bytes.
  //
  // Usually this returns crypto/rand.Reader, but unit tests may replace it with
  // a mock by using 'MockForTest' function.