Developer Cryptography Best Practices

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1 Developer Cryptography ì Best Practices Secure Software Systems Fall 2018 2 Design Considerations ì How do I update my crypto? ì In an installed application ì With users in the field ì Gracefully? ì So that over time, all active accounts are transparently upgraded to the latest secure standards? ì Examples ì New KDF (function, or just parameters) after old KDF found to be weak ì New crypto library after security bugs fixed Secure Software Systems Fall 2018 3 ì Application Use Cases Secure Software Systems Fall 2018 4 Application Use Cases ì Password handling (Storing passwords in a database, using human- intelligible secret keys, …) ì Plaintext? NO NO NO ì Old cryptographic hash (MD5)? NO NO ì Old cryptographic hash (MD5) with salt? NO NO ì New cryptographic hash (SHA-2) with salt? NO ì CPU and memory intensive Key Derivation Function? YES! ì bcrypt ì scrypt Secure Software Systems https://blog.tjll.net/please-stop-hashing-passwords/ Fall 2018 5 Application Use Cases ì Encryption of Bulk Data (Hiding from users / hiding from network) ì NaCl / Libsodium default – YES! ì Chacha20 Cipher w/ Poly1305 MAC – YES! ì AES-GCM – YES! ì All options provide AEAD ì Authenticated Encryption with Associated Data Secure Software Systems Fall 2018 6 Application Use Cases ì Symmetric Signatures (Authenticating an API, authenticating cookies, but not encrypting) ì HMAC – YES! ì GMAC – YES! ì But are you sure you don’t also want to be encrypting the data via AEAD? Secure Software Systems Fall 2018 7 Application Use Cases ì Asymmetric Encryption, i.e. Public Keys (Sending messages to many people, who are strangers, who need to decrypt and process it offline) ì NaCl / Libsodium – YES! ì Prefer elliptic curves over RSA ì Assuming curve parameters are safe, fewer ways to go wrong in implementation ì Fewer downgrade attacks possible Secure Software Systems Fall 2018 8 Application Use Cases ì Website security ì TLS – YES! ì BoringSSL (Google) ì LibreSSL (OpenBSD) ì Client-Server Application Security ì TLS! – YES! ì Consider hard-coding TLS 1.2+, ECDHE, AES-GCM (to prevent downgrade attacks) ì Consider self-signing and shipping certificate vs relying on certificate authority Secure Software Systems Fall 2018.

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