Cryptographic Primitives

Cryptographic primitives are the building blocks of secure systems.
They include encryption algorithms, signature schemes, key exchange methods, and key derivation functions.
SARE combines classical elliptic-curve cryptography with post-quantum algorithms to provide long-term security.

Symmetric Cryptography

Symmetric algorithms are used for encrypting file contents and wrapping keys.
They are efficient and secure, even in the post-quantum era.

• XChaCha20-Poly1305 — AEAD algorithm for streaming encryption with confidentiality and integrity.
• AES-256-GCM — AEAD alternative based on AES.
• AES-256 Key Wrap (AES-KW) — Used to wrap (encrypt) symmetric keys securely.

Each encrypted file includes a unique nonce in its metadata to prevent key/nonce reuse attacks.

Asymmetric Cryptography

Asymmetric primitives are used for exchanging keys, verifying identities, and creating digital signatures.

• Classical algorithms:

  • Ed25519 (signatures)
  • X25519 (key exchange)

• Post-quantum algorithms:

  • Dilithium3 (signatures)
  • Kyber (key encapsulation mechanism)

Hybrid KEM

SARE’s Hybrid KEM combines:

• A classical Diffie-Hellman exchange (X25519)
• A post-quantum KEM (Kyber)

This produces two shared secrets.
SARE then uses HKDF to derive a final symmetric key by hashing the concatenation of both secrets.
This ensures security against both classical and quantum adversaries.
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Hybrid Signatures

When signing data, SARE generates two signatures:

• One classical (Ed25519)
• One post-quantum (Dilithium3)

Both signatures must verify successfully.
This ensures trust even if one algorithm is broken in the future.
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Key Derivation Functions (KDFs)

Key derivation is central to SARE:

• HKDF: Derives symmetric keys from shared secrets in Hybrid KEM.
• PKDF: Derives keys from user passphrases when encrypting with a password.

Each derived key is tied to a salt and algorithm identifier, preventing reuse across contexts.
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Randomness & Nonces

Every encryption uses secure randomness:

• Nonces ensure ciphertext uniqueness.
• Salts in KDFs prevent rainbow-table attacks.

The randomness source is the system’s cryptographically secure RNG.

Summary

These primitives form the secure foundation for SARE’s encryption, signatures, and certificate model.