Phone NFC signal?

I have spent the morning searching to no avail, so I figure I will ask here.

I am in the process of making my KBR reader personalized, and I happened to set it on my phone when my phone was locked. It started reading numerous outputs into my computer.

Does anyone know why? I thought when phones locked, the NFC shut off also? It gives a different number when it reads it each time also.

This isn’t important really, just curious.

I have a Galaxy s9.

What does the output look like?

I found the same thing with my note 9 and my s10 5g. They both even when locked with the screen off respond to a arduino nfc reader with varying uid’s (never repeated in almost 1000 tests with the s10)

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it just keeps going.

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Hmm, they’re all 8 bytes. Maybe it’s just noise, maybe it’s tossing out random keys to pair with something. I dunno, I’ll look into it. What’s the interval?

EDIT: yeah, it’s random but the last byte is always 8. It must be random UIDs, I was thrown off because they’re usually 7 bytes, but that last one must be a command or filler. I bet it’s for active card emulation mode. You can probably turn it on and off

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As fast as I can scan it. I don’t have Amy equipment for my computer yet, as I am not sure what I would buy.

It just got me thinking because I know I cannot unlock my phone with NFC, so I am just unsure why it sits there with a readable a signal.

Android phones have HCE or Host Card Emulation and depending on the reader chip used inside, it may be on by default. Basically when you present your phone to a reader, the chip inside will realize that the phone has been presented to a reader instead of an NFC tag and switch to active-active mode. In this mode, if the reader begins interrogations then the Android phone will present a random ISO14443A compliant UID so the reader can establish a session with the phone… then the phone is expecting the reader to basically be a payment terminal and waits for a common payment AID to be selected. When it doesn’t, or if the session is broken, it cycles and the next read gets a new random UID.