With the inception of 6LoWPAN, it is possible to connect wireless sensor networks (WSN) and smart objects with the Internet using the IPv6 protocol, hence forming the IPv6-based Internet of Things (6IoT). Since the links in the 6IoT are lossy, UDP rather than TCP is mostly used for the communication between things. For the same reason, CoAP, a connection-less variant of HTTP, is being standardized as the web protocol for the 6IoT. Due to the sensitivity of the potential applications and presence of humans in the loop, End-to-End (E2E) security between constrained devices and hosts on Internet is one of the main requirements in the 6IoT. Secure CoAP (CoAPs) is used to provide end-to-end security in the CoAP-based 6IoT. Smartphones with sensing capabilities, direct human interaction, Internet connectivity, and relatively powerful processing and storage capabilities, are going to be an integral part of the 6IoT. In this paper we design, implement, and evaluate CoAPs for Android powered smartphones. We call our CoAPs INDIGO. To the best of our knowledge this is the first work that provides CoAPs support in smartphones. We implement and evaluate all cryptographic cipher suites proposed in the CoAP protocol, including the certificate-based authentication using Ecliptic Curve Cryptography (ECC). We also present novel application scenarios that are enabled by INDIGO on smartphones.