📖 ~1 min read
Table of contents
Symptom & Impact
apt update fails signature checks, blocking package installation and patching.
Environment & Reproduction
Typical when third-party repository signing keys expire or rotate.
Root Cause Analysis
Keyring contains invalid or outdated signing key for active repo metadata.
Quick Triage
Identify failing repository and inspect trusted keyring entries.
Step-by-Step Diagnosis
Validate key fingerprint, expiry date, and repo signed-by directives.
Solution – Primary Fix
Import current trusted key and update repository configuration securely.
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Solution – Alternative Approaches
Disable affected repository temporarily while validating vendor trust chain.
Verification & Acceptance Criteria
apt update succeeds with no NO_PUBKEY or EXPKEYSIG errors.
Rollback Plan
Restore previous keyring and repository files if new key proves invalid.
Prevention & Hardening
Track key expiration dates and automate proactive rotation checks.
Related Errors & Cross-Refs
Related to mirror mismatch and repository metadata integrity failures.
Related tutorial: View the step-by-step tutorial for Debian 10.
View all Debian 10 tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Debian repository signing and apt-secure documentation.
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