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A tr() internal key, or a pk() tapscript leaf key, can resolve to a 33-byte compressed point when it is derived from an extended key or written as a raw compressed key. Taproot uses x-only public keys, so such a key is reduced to its 32-byte x-coordinate before it reaches taproot_output_script, dropping the parity byte as Bitcoin Core does. This is correct for either parity, since taproot keys are lifted to even-y when tweaked. A key already in x-only form is left unchanged.
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A key in a
tr()descriptor can resolve to a 33-byte compressed point. This happens when it is derived from an extended key, or written as a raw compressed key. Taproot needs a 32-byte x-only key, and Electrum does not reduce it. The two places where this happens fail in different ways:TRDescriptor.expandpasses the 33 bytes totaproot_output_script, andassert len(pubkey32) == 32fails. Sotr(<xpub>/<path>), the usual single-key taproot descriptor, cannot be expanded.pk()leaf key. Expansion succeeds, but the leaf script pushes a 33-byte key. BIP-0342 treats a key that is neither 0 nor 32 bytes as an unknown public key type, and signature validation for it succeeds. So under consensus anyone can spend through that leaf with any non-empty signature. Only relay policy makes such a spend non-standard.Bitcoin Core builds the x-only key by dropping the parity byte (
XOnlyPubKey(CPubKey)). This PR does the same. A small helper reduces a 33-byte key to its 32-byte x-only form. It runs on the internal key and onpk()leaf keys, both raw and derived from an extended key.pk()is the only leaf function Electrum's parser accepts. A 32-byte key is unchanged. Signing is not touched.The test checks the internal key, raw and derived from an extended key, and a raw compressed
pk()leaf key. The leaf case must give the BIP-0386 x-only vector.Made with my usual tools: a computer, the Internet and an LLM. The mistakes, as usual, are all mine.