Your theoretical approach using password-based encryption (PBE) for authentication introduces interesting trade-offs compared to traditional password hashing.
Below is a breakdown of the security implications, pros/cons, and recommendations:
Key Comparisons: Password Hashing vs. PBE
- Password Hashing (Traditional Approach)
Mechanism:
A password is hashed with a salt using a secure key derivation function (KDF) like Argon2.
Validation involves rehashing the input password with the stored salt and comparing the result.
Advantages:
Simple, widely understood, and battle-tested.
No encryption/decryption overhead; minimal implementation complexity.
Explicitly designed for password storage (e.g., tools like bcrypt and argon2 handle salting and work factors).
Disadvantages:
Does not encrypt user data at rest (if that’s a requirement).
- Password-Based Encryption (Your Proposed Approach)
Mechanism:
Derive a key from the password using a KDF (e.g., Argon2).
Encrypt arbitrary data (e.g., a fixed string or JSON blob) with this key and store the ciphertext.
Validate passwords by attempting decryption: Success implies the correct key (and password).
Advantages:
Encrypts user data at rest (if the data column contains sensitive information).
Obscures the authentication mechanism (security through obscurity, though not a robust defense).
Disadvantages:
Complexity: Requires secure encryption parameters (nonce/IV, authentication tags).
False Positives: Without authenticated encryption, garbage decryption might accidentally match expected plaintext.
Key Management: Changing passwords requires re-encrypting all data with a new key.
Performance: Encryption/decryption adds minor overhead, but KDFs like Argon2 dominate the cost.
Critical Security Considerations
Authentication and Integrity:
Stream ciphers (e.g., ChaCha20) require authenticated encryption (e.g., ChaCha20-Poly1305). Without an authentication tag, attackers could tamper with ciphertexts or exploit false positives during decryption.
Known Plaintext Attacks:
If the encrypted data is predictable (e.g., "valid"), attackers could target it similarly to cracking hashes. Use randomized plaintext (e.g., a UUID) to mitigate this.
Security Through Obscurity:
If the database is breached, attackers likely also have server code (via other exploits), revealing KDF/encryption details. Do not rely on hidden algorithms for security.
Salt and Nonce Management:
Salts must be unique per user. Nonces/IVs must never repeat for the same key. Store these with the ciphertext.
Password Changes:
Updating passwords requires re-encrypting all data linked to the old key, adding complexity.
When to Use PBE Over Hashing?
Use PBE if:
You need to encrypt user data at rest (e.g., sensitive user attributes).
You want to combine authentication and data encryption into one workflow.
Stick to Hashing if:
You only need to validate passwords (no data encryption requirement).
Simplicity and maintainability are priorities.
Recommendation
For most applications, traditional password hashing is preferable due to:
Lower complexity and fewer failure points.
Explicit design for password storage (e.g., built-in handling of salts and work factors).
No need to manage encryption keys or ciphertexts.
If encryption at rest is required, combine both approaches:
Hash the password (for authentication).
Use a separate key (derived from the password) to encrypt data.
Example Secure Workflow (Hybrid Approach)
Registration:
Generate a random salt.
Hash the password with Argon2 (or similar) and store the hash.
Derive an encryption key from the password and salt.
Encrypt user data with this key (using AEAD like AES-GCM) and store the ciphertext.
Login:
Verify the password against the stored hash.
If valid, derive the key and decrypt user data.
This separates concerns (authentication vs. encryption) while leveraging the strengths of both methods.
Final Notes
Never use raw encryption without authentication (e.g., AES-CBC, ChaCha20 alone). Always use AEAD modes (e.g., AES-GCM, ChaCha20-Poly1305).
Avoid inventing custom schemes. Use established libraries (e.g., Libsodium, OpenSSL) for KDFs and encryption.
Prioritize code audits for cryptographic implementations, as subtle flaws can compromise security.
While your PBE approach is theoretically viable, it introduces risks that often outweigh its benefits unless encryption at rest is explicitly required. Stick to hashing for most use cases.
