KSI (Keyless Signature Infrastructure) is a blockchain-based digital signature technology developed by Guardtime. It provides cryptographic authentication for electronic data, machines, and humans using hash-function cryptography exclusively, without reliance on asymmetric key cryptography or trusted authorities. Unlike traditional PKI, KSI does not use private/public key pairs or certificate authorities. Instead, it submits hash values of data to a distributed public ledger, returning a signature that proves the time of signing, data integrity, and origin attribution. Key architectural properties include: - Scalability at O(t) complexity (grows with time, not transaction volume), enabling exabyte-scale signature generation - Sub-second settlement time achieved via a limited-participant consensus protocol, eliminating Proof of Work - Quantum immunity through exclusive use of hash-function cryptography - Independent verification without reliance on any trusted third party or key infrastructure - Data privacy by design — only hash values leave customer premises; raw data never does KSI introduces several security concepts: - Active Integrity: Continuous verification of electronic data to detect, alert, and mitigate tampering in firmware, OS, logs, memory, and network components - Clean State Proof: Mathematical assurance that a network is free of compromise - Attributable Network: Full non-repudiation via tagging, tracking, and locating all digital assets and network components - Forensic Auditability: Legally admissible, independently verifiable digital evidence - Portability of Evidence: Signed data can be authenticated across geographic and organizational boundaries, independent of the original service provider