Pemanfaatan Ekstensi PRF WebAuthn untuk Pembungkusan Kunci Privat Sisi Klien pada Penandatanganan Transaksi Offline Hyperledger Fabric

Purwarupa dan Evaluasi Kelayakan

Abstract

Private key management remains a practical barrier to adopting consortium blockchains such as Hyperledger Fabric, where transactions must be signed with the ECDSA private key associated with an X.509 certificate. Server custody simplifies implementation but creates a single point of compromise, whereas delegating key files to users reduces usability and raises key-loss risk. This study proposes and evaluates a non-custodial prototype that uses the WebAuthn Pseudo-Random Function (PRF) extension to derive a client-side key-wrapping key. Unlike existing Fabric key management (file-based wallets, server custody, or HSM/PKCS#11) and prior WebAuthn PRF uses that mainly encrypt user data, this work combines PRF-derived key wrapping, envelope encryption, and Fabric offline signing to protect rather than replace the ECDSA signing key. The ECDSA P-256 key is generated in the browser and protected by envelope encryption: it is encrypted with a random Data Encryption Key (DEK, AES-256-GCM), while the DEK is wrapped with a Key Encryption Key derived from PRF output via HKDF-SHA256. The server stores only ciphertexts and the user certificate. During submission, the key is briefly decrypted in browser memory for client-side offline signing before being sent to the Fabric network. Evaluation with the asset-transfer-basic chaincode shows that browser-signed transactions are accepted and validated without exposing the plaintext private key to the server. Average cryptographic overhead is 0.248 ms (SD 0.022 ms). It remains feasible for interactive use, although PRF support remains uneven across authenticators. This study is a proof of concept for technical feasibility, not a comprehensive security analysis.