Greg was one of the key architects of the two-way peg which makes sidechains possible. He has been a Bitcoin core developer since 2011, and is one of the most active reviewers of cryptographic protocol proposals in the Bitcoin industrial ecosystem. He has contributed to many widely-used techniques in the Bitcoin space, such as the homomorphic key derivation used in BIP32 and trustless privacy-preserving techniques such as CoinJoin and blinded proof of solvency. Greg is a long-time free software developer and comes to Blockstream from Mozilla where he contributed to the Daala video compression project and coauthored the Opus audio codec (RFC 6716). He also has over 15 years of experience developing, implementing and operating embedded systems and protocols for large-scale networking. For many in the Bitcoin community, Greg is likely the person telling you that your protocol is broken and why, but he usually feels pretty bad about it.