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As far as I understand, Taproot enhances privacy by allowing someone to only reveal a specific key path, which in my understanding means that it's fairly flexible - is it possible to construct a P2TR address that'd be spendable by someone with (knowledge about the possibility and) "access" to an existing P2PKH address?

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    – Murch
    Sep 21 at 19:54
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Short answer: no, at least not via the key path (and using a script path would be less efficient than using P2WPKH instead)

Longer answer: no, and even if it were possible, this would be a terrible idea. The address a receiver gives you is how they expect you to pay them. Giving them money by paying in a different way is not something you should expect them to honor. Specifically, their wallet software may not support taproot, making the money unspendable for them. Even worse, their wallet could include a hardware security module or other security mechanism that can't just easily be upgraded to support taproot.

If they want you to send to a taproot address, they'll give you one.

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  • Aren't you supposed to be able use an arbitrary redeem script? Surely there's a way to P2TR wrap P2SH-like script (which itself can wrap P2PKH).
    – monke
    Sep 17 at 7:21
  • Yes, but that'd be very inefficient. You can't construct a P2TR address that uses the key path (as you need the X coordinate of the public key, not just its 160-bit hash) given just a P2(W)PKH address. Sep 17 at 12:54
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    Well, yes, a receiver could craft an arbitrary script and communicate that script's address representation in order to get paid. The sender unilaterally making unsupported assumptions how they may change the instructions to pay the receiver will certainly lead to conflict, if not outright lost funds.--Imagine you're running a restaurant in the US and when you bring the bill, the customer pays in Euros instead. Would you accept that?
    – Murch
    Sep 17 at 12:59
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    The analogy I've used in the past (when this question came up about converting P2PKH to P2WPKH) was that it's like someone "paying" you by at night coming to dig a hole in your lawn and bury an enveloppe with money there. Sure, the money is there, on the receiver's property. But the receiver might not even notice it's there unless you tell them. Similarly, with such converted addresses, the receiver likely won't even recognize it as a payment to them. Sep 17 at 13:08

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