I have read sometimes in the web, that in the blockchain are checkpoints, so that an 51% could not the blockchain before that checkpoint even with over 51%. Is this true?
In this answer this is missing completely: Can an attacker with 51% of hash power change old blocks?
For this question i would assume that the 51% attacker would change the client he uses any way that would help to do this attack
Cheers!
Checkpoints are features of certain clients, not inherent to the blockchain itself. So, if an attacker did re-write the chain all the way back to a popular checkpoint, he could maybe get the network to disagree. Some would go with the 'chain with most work wins' approach, and some would go with the 'chain that meets my checkpoints with the most work' rule.
Checkpoints do not inherently help prevent a 51% attack though. In fact, their usefulness is predicated on the assumption that a 51% attack all the way back to a checkpoint will never happen. For example, some clients skip ECDSA verification for anything older than a certain checkpoint, so it's useful to have these checkpoints because initial-block-download syncing will be faster.
