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From: Alex Nagy <optimiz3@hotmail.com>
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Thread-Topic: P2WPKH Scripts, P2PKH Addresses, and Uncompressed Public Keys
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Subject: [bitcoin-dev] P2WPKH Scripts, P2PKH Addresses,
	and Uncompressed Public Keys
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Let's say Alice has a P2PKH address derived from an uncompressed public key=
, 1MsHWS1BnwMc3tLE8G35UXsS58fKipzB7a (from https://bitcoin.stackexchange.co=
m/questions/3059/what-is-a-compressed-bitcoin-key).

If Alice gives Bob 1MsHWS1BnwMc3tLE8G35UXsS58fKipzB7a, is there any way Bob=
 can safely issue Native P2WPKH outputs to Alice?

BIPs 141 and 143 make it very clear that P2WPKH scripts may only derive fro=
m compressed public-keys.  Given this restriction, assuming all you have is=
 a P2PKH address - is there any way for Bob to safely issue spendable Nativ=
e P2WPKH outputs to Alice?

The problem is Bob as no idea whether Alice's P2PKH address represents a co=
mpressed or uncompressed public-key, so Bob cannot safely issue a Native P2=
WPKH output.

AFAICT all code is supposed to assume P2WPHK outputs are compressed public-=
key derived.  The conclusion would be that the existing P2PKH address forma=
t is generally unsafe to use with SegWit since P2PKH addresses may be deriv=
ed from uncompressed public-keys.

Am I missing something here?





Referencing BIP141 and BIP143, specifically these sections:

https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#New_script_s=
emantics

"Only compressed public keys are accepted in P2WPKH and P2WSH"

https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki#Restrictions=
_on_public_key_type

"As a default policy, only compressed public keys are accepted in P2WPKH an=
d P2WSH. Each public key passed to a sigop inside version 0 witness program=
 must be a compressed key: the first byte MUST be either 0x02 or 0x03, and =
the size MUST be 33 bytes. Transactions that break this rule will not be re=
layed or mined by default.

Since this policy is preparation for a future softfork proposal, to avoid p=
otential future funds loss, users MUST NOT use uncompressed keys in version=
 0 witness programs."

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<p class=3D"MsoNormal">Let&#8217;s say Alice has a P2PKH address derived fr=
om an uncompressed public key, 1MsHWS1BnwMc3tLE8G35UXsS58fKipzB7a (from
<a href=3D"https://bitcoin.stackexchange.com/questions/3059/what-is-a-compr=
essed-bitcoin-key">
https://bitcoin.stackexchange.com/questions/3059/what-is-a-compressed-bitco=
in-key</a>).<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">If Alice gives Bob 1MsHWS1BnwMc3tLE8G35UXsS58fKipzB7=
a, is there any way Bob can safely issue Native P2WPKH outputs to Alice?<o:=
p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">BIPs 141 and 143 make it very clear that P2WPKH scri=
pts may only derive from compressed public-keys.&nbsp; Given this restricti=
on, assuming all you have is a P2PKH address &#8211; is there any way for B=
ob to safely issue spendable Native P2WPKH outputs
 to Alice?<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">The problem is Bob as no idea whether Alice&#8217;s =
P2PKH address represents a compressed or uncompressed public-key, so Bob ca=
nnot safely issue a Native P2WPKH output.<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">AFAICT all code is supposed to assume P2WPHK outputs=
 are compressed public-key derived.&nbsp; The conclusion would be that the =
existing P2PKH address format is generally unsafe to use with SegWit since =
P2PKH addresses may be derived from uncompressed
 public-keys.<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">Am I missing something here?<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">Referencing BIP141 and BIP143, specifically these se=
ctions:<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal"><a href=3D"https://github.com/bitcoin/bips/blob/mast=
er/bip-0141.mediawiki#New_script_semantics">https://github.com/bitcoin/bips=
/blob/master/bip-0141.mediawiki#New_script_semantics</a><o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">&#8220;Only compressed public keys are accepted in P=
2WPKH and P2WSH&#8221;<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal"><a href=3D"https://github.com/bitcoin/bips/blob/mast=
er/bip-0143.mediawiki#Restrictions_on_public_key_type">https://github.com/b=
itcoin/bips/blob/master/bip-0143.mediawiki#Restrictions_on_public_key_type<=
/a><o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">&#8220;As a default policy, only compressed public k=
eys are accepted in P2WPKH and P2WSH. Each public key passed to a sigop ins=
ide version 0 witness program must be a compressed key: the first byte MUST=
 be either 0x02 or 0x03, and the size MUST
 be 33 bytes. Transactions that break this rule will not be relayed or mine=
d by default.<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">Since this policy is preparation for a future softfo=
rk proposal, to avoid potential future funds loss, users MUST NOT use uncom=
pressed keys in version 0 witness programs.&#8221;<o:p></o:p></p>
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