implementing-log-integrity-with-blockchain - SKILL.md Agent Skill

name: implementing-log-integrity-with-blockchain description: Build an append-only log integrity chain using SHA-256 hash chaining for tamper detection. Each log entry is hashed with the previous entry's hash to create a blockchain-like structure where modifying any entry invalidates all subsequent hashes. Implements log ingestion, chain verification, tamper detection with pinpoint identification, and periodic checkpoint anchoring to external timestamping services. domain: cybersecurity subdomain: security-operations tags:

implementing
log
integrity
with version: '1.0' author: mahipal license: Apache-2.0 nist_csf:
DE.CM-01
RS.MA-01
GV.OV-01
DE.AE-02

Implementing Log Integrity with Blockchain

When to Use

When deploying or configuring implementing log integrity with blockchain capabilities in your environment
When establishing security controls aligned to compliance requirements
When building or improving security architecture for this domain
When conducting security assessments that require this implementation

Common Misconfigurations & Verification

Genesis not anchored: if the first prev_hash is a hardcoded zero with no external witness, an attacker who rewrites the whole chain (including index 0) produces a valid-looking ledger. Anchor periodic checkpoint hashes to an external timestamping authority (RFC 3161 TSA / OpenTimestamps) so a full rewrite is detectable.
Hashing fields that omit ordering/index: if chain_hash covers only prev_hash + content but not the index and timestamp, entries can be reordered or replayed without breaking the chain. Include the index and a monotonic timestamp in the hashed material.
Ledger stored writable next to the logs: if log_chain.json has the same write perms on the same host as the source logs, tamper-then-rechain is trivial. Append-only storage (WORM / S3 Object Lock) or off-host replication is required for the integrity claim to hold.
Verification only checks the tail: recompute and compare EVERY hash from index 0, not just the latest — a break at entry 42 must flag 42..N. Confirm by mutating one byte of a middle entry and re-running --verify; the report must pinpoint the first broken index.
Confirm anchoring round-trips: after a checkpoint, independently re-verify the checkpoint hash against the external service's response, not just your local record of it.

Prerequisites

Familiarity with security operations concepts and tools
Access to a test or lab environment for safe execution
Python 3.8+ with required dependencies installed
Appropriate authorization for any testing activities

Instructions

Install dependencies: pip install requests
Ingest log entries from syslog, JSON, or plain text files.
For each entry, compute SHA-256 hash of: previous_hash + timestamp + log_content.
Store the chain as a JSON ledger with entry index, timestamp, content hash, previous hash, and chain hash.
Verify chain integrity by recomputing all hashes and detecting breaks.
Optionally anchor checkpoint hashes to an external timestamping service.

python scripts/agent.py --log-file /var/log/syslog --chain-file log_chain.json --verify --output integrity_report.json

Examples

Chain Entry Structure

{"index": 42, "timestamp": "2024-01-15T10:30:00Z", "content_hash": "a1b2c3...",
 "prev_hash": "d4e5f6...", "chain_hash": "SHA256(prev_hash + timestamp + content_hash)"}

Tamper Detection

If entry 42 is modified, chain_hash[42] will not match SHA256(chain_hash[41] + ...), and all entries from 42 onward will be flagged as invalid.