Cryptography and key custody: Ed25519 transaction signing, Merkle-committed state, threshold and shard-based key handling, AWS KMS and on-premise HSM integration, and trusted execution environments (Intel SGX via Fortanix, AWS Nitro Enclaves). Hands-on experience with blockchain or distributed ledger internals: a consensus protocol (BFT, Raft, Paxos, or similar), transaction validation, state commitment such as Merkle trees, block or state storage, or node-to-node networking.