Remove attachment salt hex decode fallback

[careck]

Security Review Finding — MEDIUM Priority

Source: Krillnotes Security Review v1.0.1 (April 2026)
Location: krillnotes-core/src/core/workspace/attachments.rs:424

Description

In restore_attachment(), hex::decode(&meta.salt) falls back to meta.salt.as_bytes().to_vec() on decode failure. Malformed hex strings produce incorrect key material, which means:

1. The derived encryption key will be wrong
2. Decryption will silently fail or produce garbage
3. The error is masked — the user sees a corrupt file rather than a clear error message

Impact

A corrupted or tampered salt value in attachment metadata would not be caught — instead of failing fast with a clear error, it silently derives the wrong key.

Recommendation

Remove the unwrap_or_else fallback. The salt should always be valid hex — if it isn't, return an explicit error.

// Before (problematic)
let salt = hex::decode(&meta.salt).unwrap_or_else(|_| meta.salt.as_bytes().to_vec());

// After (correct)
let salt = hex::decode(&meta.salt)
    .map_err(|_| KrillnotesError::CryptoError("invalid attachment salt hex".into()))?;

Acceptance Criteria

• Fallback removed, replaced with explicit error
• Test added for malformed salt handling