Encryption

Encryption for Medical Devices

Purpose

Guide selection and use of cryptography for medical devices: algorithms, key derivation, data at rest/in transit, library choice, and FIPS considerations.

When to Apply

• Any handling of PHI/PII, credentials, or safety-control data.
• Transport security (BLE/WiFi/USB app-layer), storage encryption, OTA/update integrity.

Requirements (testable)

1. Algorithms: Use modern primitives (AES-256-GCM or ChaCha20-Poly1305; SHA-256/384; P-256 or X25519 for key exchange). Rationale: strong security.
2. TLS: Use TLS 1.3 (or strong 1.2 with AEAD); disable legacy ciphers; validate server certs; prefer mTLS for devices. Rationale: confidentiality/integrity.
3. Key Derivation: Use HKDF (or SP800-108/Argon2 for passwords); never reuse keys/IVs; unique IV per message. Rationale: cryptographic safety.
4. Data at Rest: Encrypt PHI/secrets at rest; bind to device (e.g., hardware unique key or secure element); protect keys separately. Rationale: protect on theft/loss.
5. Key Storage: Store long-term keys in secure hardware if available; otherwise protect with KDF + sealed storage; avoid plaintext keys in flash. Rationale: key protection.
6. Randomness: Use hardware TRNG or approved DRBG seeded correctly; no rand()/LFSR. Rationale: unpredictability.
7. Library Selection: Use vetted libraries (mbedTLS, OpenSSL/BoringSSL, wolfSSL) with security updates; avoid homegrown crypto. Rationale: reduce defects.
8. Integrity/Authentication: Use AEAD for confidentiality+integrity; for integrity-only, use HMAC-SHA-256. Rationale: tamper detection.

Recommended Practices

• Keep a crypto policy document listing allowed algorithms and parameters.
• Enforce certificate pinning where appropriate; rotate pins safely.
• Zeroize key material after use; avoid swapping sensitive buffers.
• Add versioning to encrypted blobs; include AEAD AAD with context.

Patterns

AEAD encryption (pseudocode):

// REQ-ENC-AEAD-01; TEST-ENC-04
int encrypt_payload(const uint8_t *pt, size_t pt_len,
                    const uint8_t *aad, size_t aad_len,
                    uint8_t *ct, uint8_t *tag) {
    uint8_t iv[12];
    get_random(iv, sizeof(iv)); // per-message IV
    return aes_gcm_encrypt(key, iv, pt, pt_len, aad, aad_len, ct, tag);
}

HKDF for session key:

// REQ-ENC-KDF-02; TEST-ENC-06
hkdf_sha256(master_secret, salt, info, session_key, 32);

Randomness:

// REQ-ENC-RAND-03; TEST-ENC-08
size_t n = hwrng_fill(buf, len);

Anti-Patterns (risks)

• Using ECB/RC4/MD5/SHA1 -> risk: weak encryption/integrity.
• Reusing IVs/nonces -> risk: catastrophic AEAD failure.
• Homegrown crypto or unreviewed parameter changes -> risk: design flaws.
• Storing keys unprotected in flash -> risk: extraction and compromise.
• Using rand() for keys/IVs -> risk: predictable values.

Verification Checklist

• Approved algorithms only (AES-GCM/ChaCha20-Poly1305, SHA-256+, P-256/X25519).
• TLS 1.3 (or hardened 1.2) with cert validation; weak ciphers disabled.
• HKDF/KDF used; IVs unique per message; no key/IV reuse.
• Data at rest encrypted; keys protected/bound to device; zeroization applied.
• Secure RNG/DRBG used; no rand()/LFSR.
• Vetted crypto library with updates; no homegrown primitives.
• AEAD or HMAC used for integrity/authentication; AAD/context applied.

Traceability

• Requirements REQ-ENC-### to tests TEST-ENC-###; link to security risk controls and SBOM for crypto libs.

References

• NIST SP 800-56, 800-57, 800-90 (DRBG), 800-38D (GCM).
• IETF RFCs: TLS 1.3 (8446), ChaCha20-Poly1305 (8439), HKDF (5869).
• FIPS 140-2/3 considerations when required.

Changelog

• 1.0.0 (2026-01-04): Initial encryption skill with AEAD/TLS, KDF, RNG, and key protection guidance.

Audit History

• 2026-01-04: Audit performed. Verified:
  • Algorithm recommendations (AES-256-GCM, ChaCha20-Poly1305, SHA-256/384) are current and secure
  • TLS 1.3 preference with hardened TLS 1.2 fallback is accurate
  • NIST SP 800-series references (800-56, 800-57, 800-90, 800-38D) are appropriate
  • HKDF (RFC 5869) reference accurate
  • No deprecated algorithms recommended