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I'm attempting to build an Arduino-based device that will use NFC to know when my phone is close to it. My phone is a Nexus S, equipped with NFC. I'm looking for what I believe is called an NFC Transceiver, to be able to pair with my phone via NFC. I've found plenty of technical documents on this component, but can't seem to find anywhere that the general public can buy them in small quantities (namely 1 to 5).

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The term you're probably missing is 'mifare', which is the product name for the basic, unencrypted NFC tags. Sparkfun sell this transceiver, which works with this Arduino shield. I have one, and though I haven't tried using it with my Android in 'tag mode', it ought to work - you'd have to try it to see.

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Thanks! This definitely points me in the right direction. Question: My goal here is to build a system to unlock a door using within a certain proximity of my Android. Would it be unsafe to use unencrypted NFC for this purpose? Generally the way I see it: most homerobbers will kick down your door or break a window before hacking your computer system... Thoughts? –  giantcola Jul 16 '11 at 18:36
    
@giantcola 'Unsafe' in the sense of 'easily defeated by someone with the necessary skills and equipment'? Yes. Whether or not that's a significant threat is something only you can determine. –  Nick Johnson Jul 18 '11 at 1:42
    
Anyway, once you have written the application, it should be quite easy to implement a basic encryption to the messages... –  clabacchio Feb 16 '12 at 10:18
    
@clabacchio The problem is, encrypting messages on the card doesn't prevent your attacker reading the data off and doing an offline attack. A good system would decline to send the attacker the data in the first place. –  Nick Johnson Feb 20 '12 at 20:13
    
What I know is that for this kind of attacks there is a counter that prevents to accept two times the same message; now I don't know the level of security, but I don't think that a burglar would stay close to the door trying to decrypt messages... –  clabacchio Feb 20 '12 at 20:22
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I would suggest Adafruit's NXP PN532 breakout board: http://www.adafruit.com/products/364 It is a very reasonable price and the NXP chip allows for all three of the NFC modes: read tags, emulate tags and do peer-to-peer between NFC devices.

The only downside is that they're frequently out of stock and sell out very quickly when boards do arrive.

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Thanks for the link! Do you know if this board has any encryption options? –  giantcola Jul 16 '11 at 18:37
    
giantcola - check out the datasheet here: nxp.com/documents/user_manual/141520.pdf it has some hw encryption but I think just for MiFare Classic's broken CRYPTO1 scheme; to authenticate to sectors of the card, lock them down, etc. But most of those things are a function of the cards themselves too, like on the DESFires with strong (AES) encryption. –  nemik Jul 17 '11 at 7:40
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You need to realise that your Google Nexus S phone will have to run in tag (or Card Emulation mode). As of today, the official Android on the Nexus S does not provide the required APIs to enable Card Emulation mode. (Latest Android running on Nexus S is ICS 4.0.3).

You will need to hack the phone, and flash a custom ROM on the system.

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