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- Fraud Proofs present evidence that a state transition was incorrect. They reflect an optimistic view of the world: the
*assumption*is that blocks represent only correct states of L2 data, until proven otherwise. In reality, a committed block could well include an incorrect state transition. - A method for proving that a state transition is invalid. Optimistic rollups are a type of blockchain that utilize fraud proofs to prove invalid state transitions. Since fraud proofs only assess if fraud took place, they are only required during situations where a state transition is disputed.
- A simple construction of a fraud proof requires nodes to re-execute the transaction(s) that were part of the disputed state transition. However, this can become prohibitively expensive if the blockchain they are being re-executed on has high gas fees. To avoid this, fraud proofs have been constructed that involve interactive verification games (IVGs) that narrow down the disputed state transition to just the key computation step in question, which is then re-executed to assess if it is fraudulent.

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- Validity Proofs present evidence that a state transition is correct. They reflect a more pessimistic view of the world. Blocks include values representing L2 state if, and only if, that state is correct.
- A type of cryptographic proof that can be used to attest to the validity of a state transition. For example, zk-Rollups utilize validity proofs to prove valid state transitions to a parent chain - commonly used with proof systems such as SNARKs and STARKs.

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