security: migrate AES-CBC to AES-GCM authenticated encryption

[3em0]

Summary

• Replaced AES-CBC (no message authentication) with AES-GCM in application/security/encryption.py, providing both confidentiality and integrity for stored credentials
• Backward-compatible decryption: existing CBC-encrypted data is auto-detected via a version byte prefix and decrypted through the legacy path
• Added tests for GCM round-trip, legacy CBC compatibility, and tamper detection (flipped ciphertext byte → returns {})

Motivation

AES-CBC without HMAC lacks ciphertext integrity protection. While the practical Padding Oracle attack surface is limited (no direct decrypt endpoint), the absence of authenticated encryption is a cryptographic deficiency. AES-GCM resolves this in a single primitive.

Changes

File	Change
application/security/encryption.py	encrypt_credentials → AES-256-GCM; decrypt_credentials → version-byte dispatch (GCM vs legacy CBC)
tests/security/test_encryption.py	Updated round-trip test; added test_decrypt_legacy_cbc_format and test_tampered_gcm_ciphertext_returns_empty

Test plan

• pytest tests/security/test_encryption.py -v — 8/8 passed
• GCM encrypt/decrypt round-trip verified
• Legacy CBC data still decrypts correctly
• Tampered GCM ciphertext correctly rejected (returns {})

🤖 Generated with Claude Code

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[3em0]

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