The ductility of a transaction is also called ductility, which means that the essence and quality of the same thing (such as a mass of metal) have not changed, but the shape has changed. However, this extensibility will lead to the inconsistency between the transaction ID and TXID, which will lead to the user not finding the sent transaction.

Now in the transaction data format of Bitcoin, the transaction signature part is also included in the whole transaction. Finally, the whole transaction is hashed. There are many ways to write the transaction signature, and the attacker tampered with them. Their functions are the same, but the bytes have changed, resulting in different signatures. Some time ago, even a mine dug up a file, which contained all the transactions that were attacked by expansion, which brought trouble to some applications.

The change of TXID may cause some applications to fail to find TXID, thus affecting the status of recharging or withdrawing some wallets, and bringing troubles to operators and users. Isolation witness is put forward to solve this problem, which separates the transaction data from the signature data, so that the TXID of a transaction must be unique.