# Gavin Andresen
# 2011-03-07 15:59:53
# https://bitcointalk.org/index.php?topic=191.msg61586#msg61586

@s{quotedtext}
@s{quotedtext}
 @p{brk}
So lets say I can create SHA-256 collisions fairly easily, and I want to replace an old transaction somewhere in the block chain. @p{par}

I create an alternate version of the transaction with the same hash... and then?  Whenever clients happen to connect to my node to get old transactions I feed them the bogus version? @p{par}

How do I get a majority of the network to accept the bogus version as valid, when the majority of the network probably already has already downloaded the old, valid version? @p{par}

Same question if I'm creating duplicate (old) block hashes instead of duplicate transaction hashes. @p{par}

 @p{brk}
I suppose I could try to double-spend with two transactions that hash to the same value... and hope that the merchant's bitcoin accepts Transaction Version 1 while the majority of the rest of the network accepts Transaction Version 2 (where I pay myself).   But if SHA-256 ever gets close to being broken I'm sure bitcoin will be upgraded so new clients only accept upgraded hashes for new blocks/transactions. @p{brk}