Modular scripting, cryptographic signing, and a tamper-evident ledger for Linux integrity management
When it comes to integrity, trust isn't a setting — it's a habit.
This framework transforms AIDE from a passive checker into an active guardian, verifying its own history before trusting the present.
Introduction
Security in IT is a funny thing.
Everyone agrees it's important — but how much is enough?
The answer depends on context:
- What does the server do?
- Does it store personal or sensitive data?
- Who are the stakeholders — developers, security teams, compliance auditors?
Your SOC team might see things very differently from your developers.
For some, this framework may feel like overkill; for others, it may not go far enough.
That's okay — the point here is to present a clear, modular integrity framework that can be scaled to match your environment.
🧭 Table of Contents
- Introduction
- Recap: Where We've Been
- Where We're Going: Tamper-Evident Logging with a Ledger
- Why Use a Ledger?
- How the Ledger Works
- 1️⃣ The First Entry — The Genesis Block
- 2️⃣ The Second Entry — Linking the Chain
- ♻️ The Nth Entry — Immutable History
Recap: Where We've Been
In the previous parts of this series, we:
- Installed and configured AIDE (Advanced Intrusion Detection Environment)
- Ran
aide --initto create a trusted baseline - Signed and encrypted that baseline to prevent tampering
- Created a
systemdservice to runaide --checkdaily - Captured, encrypted, and hashed each AIDE log for post-run verification
🔗 Where We're Going: Tamper-Evident Logging with a Ledger
The next step is to introduce a ledger — a chained, tamper-evident record of every AIDE run.
Each log entry is:
- Signed and hashed
- Added to a cryptographic ledger
- Linked to the previous entry (like a lightweight blockchain)
This prevents attackers from silently erasing or altering past logs. Even with root access, they'd have to reconstruct the entire chain, which is both difficult and detectable.
🧠 Why Use a Ledger?
Hashing and encryption protect individual logs, but not the sequence of events over time.
Without a ledger, a compromised system could regenerate a clean log, hash it, encrypt it — and make it look like nothing happened.
The ledger prevents this by enforcing historical integrity. If even one previous log or entry is modified, all subsequent hashes break.
🧱 How the Ledger Works
Each AIDE run logs a single line in the ledger:
<log_hash> <log_path> <chain_hash>
Let's walk through it:
1️⃣ The First Entry — The Genesis Block
When AIDE runs for the first time:
log_hash = SHA512(first_log)
chain_hash = SHA512(log_hash)
There's no prior chain — just the hash of the log itself. This is the anchor of the chain.
2️⃣ The Second Entry — Linking the Chain
On the second run:
log_hash = SHA512(second_log)
chain_hash = SHA512(log_hash + previous_chain_hash)
Here, the + means byte-concatenation, not addition. Now, the new hash depends on both the current log and the entire prior chain.
♻️ The Nth Entry — Immutable History
From here on:
chain_hash_n = SHA512(log_hash_n + chain_hash_(n-1))
This cumulative design makes the ledger tamper-evident. One altered entry corrupts everything that follows.
📘 Example Ledger Chain
| Run | Source Log | What chain_hash Protects |
|---|---|---|
| 1 | aide-check-01.log |
Only the first report |
| 2 | aide-check-02.log + c₁ |
Reports from Run 1 → Run 2 |
| 3 | aide-check-03.log + c₂ |
Reports from Run 1 → Run 3 |
| … | … | Full integrity history |
By Run 10, the ledger proves that none of the previous nine logs were altered — not even a byte.
🗂️ Hiding the Evidence: Relocating AIDE Logs
By default, AIDE logs to /var/log/aide/. That's fine — until someone with access goes looking.
Obvious logs are obvious targets. We hide ours in plain sight.
We relocate AIDE's operational files into a less visible structure under /var/lib/system_metrics/, using dot-prefixed folders:
/var/lib/system_metrics/
├── .l/ → AIDE logs
├── .h/ → SHA512 hashes
├── .s/ → GPG signatures
├── .c → Ledger
└── .db/ → AIDE baseline database + signature
This doesn't replace encryption — it simply adds operational stealth.
Think of it like moving your surveillance footage from the coffee table into a locked cabinet.
🔒 Why Secure and Hide Them?
| Reason | Benefit |
|---|---|
| Reduce visibility | Dot-folders (.l, .h, etc.) don't show in casual ls commands |
| Isolate from syslogs | Keeps AIDE separate from noisy application logs |
| Tight access controls |
/var/lib/system_metrics can be root-owned, mode 700
|
| Support stealth checks | You can validate system integrity without broadcasting it |
🧠 Final Thoughts — Is This Overkill?
That depends.
- In enterprise environments, you likely already have commercial-grade IDS/IPS systems. This may be redundant.
- But in small offices, personal servers, or edge deployments, AIDE plus ledgering offers high-integrity security without requiring external tools.
Ultimately, how much security you need depends on your risks, responsibilities, and resources.
But one thing is certain:
It's always safer to be a little over-secured than to be one clever script away from compromise.
🧩 Related Articles in the Series
- AIDE - File Integrity Monitoring for System Security
- AIDE in Motion Automating and Signing System Integrity Checks
- AIDE Automation Framework From Integrity Checks to Self-Verification
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