by the miner nodes which make sure that a transaction is valid.

by every single node in the blockchain network executing the same transaction.

by a cryptographic secure signature algorithm called ECDSA which makes sure that cheating is impossible.

it's good to use the block timestamp, as this is always different.

it's good to use the block hash as this is clearly always very different.

it's good to use the RANDAO smart contract.

it's not possible to have a random number in a deterministic environment such as the Ethereum blockchain.

Address.call() is used for calling other contracts using the scope of the called contract in terms of storage variables. Address.delegatecall() is used for libraries, which uses the storage variables of the contract who called. Libraries are a great way to re-use already existing code and delegatecall can make sure that no storage is used from the library, instead it looks like the code is directly copied into the calling contract.

Address.delegatecall() is used for calling other contracts using the scope of the called contract in terms of storage variables. Address.call() is used for libraries, which uses the storage variables of the contract who called. Libraries are a great way to re-use already existing code and call() can make sure that no storage is used from the library, instead it looks like the code is directly copied into the calling contract.

a 20 bytes value

a 32 bytes value

a string

a 20 characters long hex number

to bytecode that can't be understood by humans.

to bytecodes which are essentially opcodes running instruction by instruction.

string1 == string2

the internal function "str_compare(str1,str2)"

you can't directly compare two strings, but one method would be to hash both strings and compare the hashes.

bytes32(string1) == bytes32(string2)

will cascade exceptions and address.transfer() will return a false on error.

will return false on error while address.transfer() will cascade transactions.

can only be accessed during calls.

can be accessed during transactions and calls.

working with either // or ///

working with /* comment */ or /** @.. natspec style */

not possible, all comments must be multi-line.

all gas is consumed.

all remaining gas is returned.

to move all existing logic to the blockchain, so everything runs on the same system. This way it might be more complex, but easier to maintain.

to move only those parts to the blockchain that really need the blockchain. This way smart contracts can be easier to read, easier to test and are not so complex.

to move those parts to the blockchain that deal with Ether transfers. All other parts can remain in traditional database systems. This way only the value-transfer is on the blockchain.

is used to communicate between blockchain nodes.

is used to ensure safe communication between miners.

is a mean of dumping the blockchain data in a so-called consensus export.

is used to communicate between the blockchain node and externally running applications.