Does this mean, that basically any functional change in the block verification code can be used to leverage the hashing power of all users of one version into a 51% attack?
The attack would be essentially to create the current situation artificially by crafting a block that is only valid by one version of bitcoind, therefore splitting the block chain. Then one needs to include two different transactions into the two blockchains. ( In the theoretical scenario, by mining another block for one of the chains. ) For this I do not need to have 51% of the hashing power, but the branch of the blockchain needs to be supported by 50% of the hashing power.
EDIT: Thinking a bit more about this, it seems that the probability to suceed with such an attack seems to rely on the attackers abbility to craft a 'splitting block', which should be proportional to the share of the attacker controlled hashing power and the ability of the 'incompatible' clients to outgrow the standard block chain. ( So that a merchant with an 'incompatible' client accepts a payment based on the fork of the blockchain.) This means that the attack is probably impossible, if the one CPU one vote approach is not only used for valid transactions, but also for valid clients.
The attack would be essentially to create the current situation artificially by crafting a block that is only valid by one version of bitcoind, therefore splitting the block chain. Then one needs to include two different transactions into the two blockchains. ( In the theoretical scenario, by mining another block for one of the chains. ) For this I do not need to have 51% of the hashing power, but the branch of the blockchain needs to be supported by 50% of the hashing power.
EDIT: Thinking a bit more about this, it seems that the probability to suceed with such an attack seems to rely on the attackers abbility to craft a 'splitting block', which should be proportional to the share of the attacker controlled hashing power and the ability of the 'incompatible' clients to outgrow the standard block chain. ( So that a merchant with an 'incompatible' client accepts a payment based on the fork of the blockchain.) This means that the attack is probably impossible, if the one CPU one vote approach is not only used for valid transactions, but also for valid clients.