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Near Field Communication (NFC), a short range wireless technology, has recently experienced a sharp rise in uptake because of its integration with cell phones. NFC-enabled cell phones interact with NFC tags to retrieve information in a single touch. Such tags can be used in variety of applications like smart posters, product identification, access control etc. The integrity of the data stored on these tags is assured by digital signatures. However, this does not guarantee the legitimacy of tags. They may be replaced with counterfeits. At present the NFC Forum does not provide any mechanism to detect duplicate tags. In an offline environment, when there is no shared secret between the tag and the reader, it is very challenging to differentiate between legitimate and counterfeit tags. This paper presents a protocol for the off-line authentication of NFC tags and provides a framework, based on NFC Forum specifications, to support the authentication. The proposal is based on a challenge-response protocol using public key cryptography and a PKI. In order to make the framework compatible with existing NFC Forum devices, a new Tag Authentication Record, designed according to the NFC Data Exchange Format (NDEF), is introduced. Our proposed framework successfully differentiates between legitimate and cloned tags which have sufficient resources to perform the required cryptography.