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On Dec 18, 2015, at 10:30 AM, Pieter Wuille via bitcoin-dev <<a
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wrote:
<blockquote cite="mid:E76D5BF9-41BF-4AF5-BBAC-06F4EF574EBE@toom.im"
type="cite">
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<span style="color: rgb(34, 34, 34); font-family: 'Helvetica
Neue', Arial, sans-serif; font-size: 15px; font-style: normal;
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old full node wallet accepting a transaction whose</span><br
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<span style="color: rgb(34, 34, 34); font-family: 'Helvetica
Neue', Arial, sans-serif; font-size: 15px; font-style: normal;
font-variant: normal; font-weight: normal; letter-spacing:
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a script that depends on the softforked rules.</span></div>
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</blockquote>
There's that, but there's also a case where an attacker creates a
majority chain that follows the old rules but not the new ones.
Non-upgraded nodes would accept a transaction on what they believe
to be the consensus chain only to find that when they try to spend
those coins no one accepts them because they were part of an invalid
chain. <br>
<br>
This has the effect of dropping non upgraded nodes to a form of spv
security without their consent. <br>
<br>
This is in contrast to a hard fork where a full node operator could
explicitly set their node to accept higher version blocks that it
can't validate. They get the soft fork functionality back but they
have at least consented to it rather than have it forced on them.
Doing forks that way would also have the benefit of notifying the
user they are accepting unvalidated coins, whereas they wont know
that in a soft fork. <br>
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