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Let's finish by learning about conditional clauses and flow control, and taking a look at some examples of complex scripts.
Transaction identifiers
One of the greatest benefits of Segregated Witness is that it eliminates third-party transaction malleability.
Before segwit, transactions could have their signatures subtly modified by third parties, changing their transaction ID (hash) without changing any fundamental properties (inputs, outputs, amounts). This created opportunities for denial-of-service attacks as well as attacks against poorly written wallet software that assumed unconfirmed transaction hashes were immutable.
With the introduction of Segregated Witness, transactions have two identifiers, txid and wtxid. The traditional transaction ID txid is the double-SHA256 hash of the serialized transaction, without the witness data. A transaction wtxid is the double-SHA256 hash of the new serialization format of the transaction with witness data.
The traditional txid is calculated in exactly the same way as with a nonsegwit transaction. However, since a pure segwit transaction (a transaction that only contains segwit inputs) has empty scriptSigs in every input, there is no part of the transaction that can be modified by a third party. Therefore, in a pure segwit transaction, the txid is immutable by a third party, even when the transaction is unconfirmed.
The wtxid is like an "extended" ID, in that the hash also incorporates the witness data. If a transaction is transmitted without witness data, then the wtxid and txid are identical. Note that since the wtxid includes witness data (signatures) and since witness data may be malleable, the wtxid should be considered malleable until the transaction is confirmed. Only the txid of a pure segwit transaction can be considered immutable by third parties.
Tip: Segregated Witness transactions have two IDs: txid and wtxid. The txid is the hash of the transaction without the witness data and the wtxid is the hash inclusive of witness data. Only pure segwit transactions (transactions that only contain segwit inputs) have a txid that is not susceptible to third-party transaction malleability.