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RSA

Algorithm​

RSA uses modular exponentiation. NoxTLS exposes key material as RsaPrivateKey / RsaPublicKey and provides high-level helpers for OAEP encryption and PSS signatures with SHA-256.

Purpose​

RSA key generation, OAEP encryption, PKCS#1 v1.5 (legacy), and RSASSA-PSS / PKCS#1 v1.5 signatures in noxtls-crypto.

Rust API​

  • Crate: noxtls-crypto
  • Module path (conceptual): noxtls_crypto::pkc
  • Primary symbols:
    • RsaPrivateKey
    • RsaPublicKey
    • RsaKeySizePolicy
    • noxtls_rsa_generate_keypair_secure_auto
    • noxtls_rsaes_oaep_sha256_encrypt_auto
    • noxtls_rsaes_oaep_sha256_decrypt
    • noxtls_rsassa_pss_sha256_sign_auto
    • noxtls_rsassa_pss_sha256_verify

Functions and types:

  • noxtls_rsa_generate_keypair_secure_auto(bits, policy, drbg) - Parameters: bits is target modulus length, policy enforces minimum key-size rules, and drbg provides random generation input. Behavior: generates RSA private/public key pair under secure policy checks. Returns: generated keypair in Result.
  • rsaes_oaep_sha256_* - Parameters: OAEP encrypt/decrypt calls take RSA key material, plaintext/ciphertext, optional label, and RNG where required. Behavior: performs RSAES-OAEP with SHA-256/MGF1 padding. Returns: encrypted or decrypted byte buffers in Result.
  • noxtls_rsassa_pss_sha256_sign_auto / noxtls_rsassa_pss_sha256_verify - Parameters: signer uses private key, message, DRBG, and salt_len; verifier uses public key, message, signature, and same salt_len. Behavior: produces and verifies RSASSA-PSS signatures. Returns: signature bytes for sign, and success/error for verify.

Feature flags and policy​

Some legacy RSA key-generation helpers require hazardous-legacy-crypto (see rustdoc).

Examples​

use noxtls_crypto::{
    noxtls_rsa_generate_keypair_secure_auto, noxtls_rsassa_pss_sha256_sign_auto,
    noxtls_rsassa_pss_sha256_verify, HmacDrbgSha256, RsaKeySizePolicy,
};

let mut drbg = HmacDrbgSha256::new(b"0123456789abcdef", b"nonce", b"")?;
let (private_key, public_key) =
    noxtls_rsa_generate_keypair_secure_auto(2048, RsaKeySizePolicy::Minimum2048, &mut drbg)?;
let message = b"firmware manifest bytes";
let salt_len = 32;
let signature =
    noxtls_rsassa_pss_sha256_sign_auto(&private_key, message, &mut drbg, salt_len)?;
noxtls_rsassa_pss_sha256_verify(&public_key, message, &signature, salt_len)?;
# Ok::<(), noxtls_core::Error>(())

Key generation can be CPU-heavy; run once at provisioning or in a background task on MCUs.

Security and compatibility​

Enforce minimum modulus via RsaKeySizePolicy; prefer RSASSA-PSS and RSAES-OAEP for new designs.