🛡️ Sentinel: [HIGH/ENHANCEMENT] Fix timing attacks and improve charset consistency

[richkmeli]

🚨 Severity: HIGH (Timing Attacks) / MEDIUM (Charset Consistency)
💡 Vulnerability:

1. Sensitive comparisons for password hashes and tokens used String.equalsIgnoreCase(), which is susceptible to timing attacks.
2. Cryptographic operations relied on the platform's default charset, potentially leading to inconsistencies.
3. RandomStringGenerator created a new SecureRandom instance on every call.
4. Temporary test files were not properly ignored in .gitignore.

🎯 Impact:

1. Timing attacks could allow an attacker to guess valid tokens or password hashes by measuring response times.
2. Charset inconsistencies could break cryptographic verification across different environments.
3. Inefficient random number generation.
4. Sensitive session/encrypted data could be accidentally committed.

🔧 Fix:

1. Implemented constant-time comparison using java.security.MessageDigest.isEqual().
2. Explicitly specified StandardCharsets.UTF_8 for all byte conversions.
3. Refactored RandomStringGenerator to use a static SecureRandom instance.
4. Updated .gitignore to include all temporary test files.

✅ Verification:

1. Ran mvn test and all tests passed (BUILD SUCCESS).
2. Verified changes in PasswordManager.java, TokenManager.java, SHA256.java, RandomStringGenerator.java, and .gitignore.

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PR created automatically by Jules for task 6593343978773098799 started by @richkmeli

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