Bitcoin Q&A: What is Segregated Witness?

What is Segregated Witness (SegWit)? How do you know you’re using SegWit? SegWit fixes transaction malleability and enables further upgrades towards Bitcoin smart contracts. Can transaction malleability lead to denial-of-service (DoS) attacks? Does SegWit put witness data in a different block? Did SegWit change transaction validation? versus? Are there any security risks of invalid blocks?