Small device to transmit a code to smart phones (IOS, Android, ...) without the need of manual pairing

I need a detailed advice on how to solve the following requirements: We need to transmit a unique code that can be received by smart phones (major OS platforms, best case: all platforms) No manual pairing should be necessary by the owner of the receiving smart phone Ideally the transmission should be possible from the smart phone of the person that transmits the code There should be no security risks to receiving and transmitting smart phone The transmission range should be as a large as possible, but must be minimum of approximately 25 meters If a seperate device is required to transmit the unique code; - the device must be as small as possible - battery life should be as long as possible, best case several months, worst case 24 hours (if rechargeable) or a approx a week with batteries - if battery life is shorter than 3 months, the device should be rechargeable or use batteries - the device should be cheap to produce - the device should be able to pair with the smart phone of the owner to transmit the unique code into the device Please describe the solution with regard to the requirements mentioned above. Also possible altnernatives, which technology is to be used, compatibilty to platforms, reliabilty, risks and cost. Please indicate if you use off-the-shelve products and describe their profile with regard to the above