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ESPCrypto

ESPCrypto wraps the ESP32 hardware crypto blocks (SHA, AES-GCM/CTR, RSA/ECC) with guardrails, automatic fallbacks, and high-level helpers (JWTs, salted hashes) that work in both ESP-IDF and Arduino builds.

CI / Release / License

Toolchain Compatibility

• GitHub Actions builds against the ESP32 Arduino core 3.3.3 via Espressif's board manager URL (IDF 5.x generation) and caches the toolchains to keep PlatformIO/Arduino builds in sync for esp32, esp32-s3, esp32-c3, and esp32-p4 boards.
• Runtime code gates the mbedTLS 2.x (ESP-IDF 4.x) and 3.x (ESP-IDF 5.x) API differences—including the ESP AES-GCM alt streaming signatures—so PlatformIO/Arduino builds succeed regardless of which ESP-IDF revision a board package ships.
• Device fingerprinting prefers esp_read_mac from esp_mac.h when present and falls back to esp_efuse_mac_get_default, so Arduino/PlatformIO board packages that dropped esp_efuse_mac.h still build.

Features

• SHA256/384/512 helpers that try the ESP parallel SHA engine first and fall back to mbedTLS when the accelerator (or platform) is unavailable.
• AES-GCM and AES-CTR utilities with a safe aesGcmEncryptAuto that generates a random 12-byte IV, optional nonce-reuse debug guard, and capability introspection via ESPCrypto::caps().
• RSA/ECC signing + verification helpers (PKCS#1 v1.5 + ECDSA) that power HS256/RS256/ES256 JWT flows or stand-alone signatures.
• CryptoKey + KeyHandle abstractions with MemoryKeyStore, NvsKeyStore, and LittleFsKeyStore for alias/versioned key rotation plus cached mbedTLS contexts to avoid repeated parsing.
• Device-bound HKDF helper deriveDeviceKey(...) that derives stable per-device keys from an optional persistent NVS secret plus the chip fingerprint, mainly to avoid hard-coded symmetric keys rather than to provide hardware-backed key protection.
• Buffer-friendly span overloads for SHA digests and AES-GCM encrypt/decrypt that write into caller-provided buffers to reduce heap churn on large payloads, plus streaming contexts (ShaCtx, HmacCtx, AesCtrStream, AesGcmCtx) for chunked workloads.
• Nonce strategies for AES-GCM auto IVs: random 96-bit (default), counter+random hybrid, or boot-counter based with optional NVS persistence to avoid reuse under long-lived keys.
• Modern lanes: ChaCha20-Poly1305 for CPUs without AES accel, X25519 ECDH helper, and ECDSA DER↔raw helpers to interop with JOSE stacks. (Ed25519/EdDSA stay capability-gated to platform support.)
• HMAC/HKDF/PBKDF2 (SHA-256/384/512) building blocks with policy enforcement for PBKDF2 iteration counts; password hashing uses these primitives and constant-time verification.
• Structured CryptoStatus + CryptoResult<T> with span-friendly overloads to reduce heap churn and keep error handling uniform; SecureBuffer/SecureString zeroize sensitive data on scope exit.
• Full JWT builder/validator that uses ArduinoJson v7 JsonDocuments, fills iat/exp/nbf fields, enforces issuer/audience, and exposes both friendly errors and structured status codes.
• Ready-to-flash example plus Unity tests under test/test_esp_crypto with NIST/RFC vectors for SHA, AES-GCM, HKDF, PBKDF2, JWT, and password hashing regressions.

Examples

• examples/basic_hash_and_aes – SHA plus AES-GCM with auto IV/tag handling and structured status.
• examples/jwt_and_password – HS256 JWT creation/verification and password hashing/verification.
• examples/advanced_primitives – Capability/policy introspection, SecureBuffer/String, HMAC/HKDF/PBKDF2, AES-CTR streaming, and RSA/ECDSA signing flows.
• examples/keys_and_streaming – Keystore usage, streaming SHA/AES-GCM, nonce strategies, and device-bound key derivation.
• examples/bench_crypto – Tiny on-device timing loops for SHA and AES-GCM to gauge perf per board.
• examples/jwks_rotation – JWKS verification with rotating kid values for HS256.

The basic AES example shows SHA and AES-GCM in one go:

#include<Arduino.h>
#include<ESPCrypto.h>
#include<vector>voidsetup() {
Serial.begin(115200);
std::vector<uint8_t> key(32, 0x01);
std::vector<uint8_t> plaintext = {'h', 'e', 'l', 'l', 'o'};
String digest = ESPCrypto::shaHex("esptoolkit");
auto gcm = ESPCrypto::aesGcmEncryptAuto(key, plaintext);
if (gcm.ok()) {
auto decrypted = ESPCrypto::aesGcmDecrypt(key, gcm.value.iv, gcm.value.ciphertext, gcm.value.tag);
(void)decrypted;
}
// Release ESPCrypto runtime caches/state before deep sleep or shutdown paths.ESPCrypto::deinit();
}
voidloop() {}

Run examples/basic_hash_and_aes via PlatformIO/Arduino to see the full output.

Lifecycle and Teardown

• ESPCrypto is static-style, so there is no instance destructor to release global runtime state.
• Call ESPCrypto::deinit() when your app no longer needs crypto helpers (for example before deep sleep, app shutdown, or full subsystem restart).
• deinit() is safe before any crypto call and safe to call repeatedly.
• Use ESPCrypto::isInitialized() to check whether runtime state (policy/caches/counters) is currently active.

Key management and device-bound helpers

Cache parsed keys, rotate aliases, and derive symmetric keys without shipping long-lived secrets in firmware:

#include<ESPCrypto.h>voidrotate_keys() {
MemoryKeyStore memory;
KeyHandle current{String("jwt_auth"), 2};
// Store a PEM private key and reload it as a cached CryptoKeyconstchar *pem = "-----BEGIN PRIVATE KEY-----...";
ESPCrypto::storeKey(memory, current, CryptoSpan<constuint8_t>(reinterpret_cast<constuint8_t *>(pem), strlen(pem)));
auto loaded = ESPCrypto::loadKey(memory, current, KeyFormat::Pem, KeyKind::Private);
if (loaded.ok()) {
auto sig = ESPCrypto::rsaSign(
loaded.value,
CryptoSpan<constuint8_t>(reinterpret_cast<constuint8_t *>("payload"), 7),
ShaVariant::SHA256
);
(void)sig;
}
// Derive a device-bound symmetric key using HKDF + NVS-backed seedauto derived = ESPCrypto::deriveDeviceKey("provisioning", CryptoSpan<constuint8_t>(), 32);
if (derived.ok()) {
// use derived.value as an AES or HMAC key without embedding long-term secrets
}
}

MemoryKeyStore keeps key material only in RAM for tests or ephemeral rotations. LittleFsKeyStore stores blobs on LittleFS when mounted, and NvsKeyStore persists blobs in NVS with whatever flash/NVS protection the device is configured for. None of these backends should be treated as a secure-element substitute.

API Highlights

• CryptoResult<std::vector<uint8_t>> shaResult(...) / shaHex(...) – SHA256/384/512 with optional hardware preference (default on) and structured status codes.
• CryptoResult<GcmMessage> aesGcmEncryptAuto(...) + aesGcmDecrypt(...) – 128/192/256-bit AES-GCM with random IVs, optional AAD, 16-byte tags, and policy-enforced IV length; aesCtrCrypt(...) covers stream-like CTR use cases.
• CryptoResult<std::vector<uint8_t>> rsaSign/eccSign and rsaVerify/eccVerify – Wrap mbedTLS PK contexts while enforcing minimum key sizes unless allowLegacy is enabled.
• CryptoKey helpers for RSA/ECC reuse parsed PK contexts; pair them with KeyHandle aliases in a KeyStore to rotate versions without reparsing PEM/DER.
• CryptoResult<void> sha(...) and aesGcmEncrypt/Decrypt(...) span overloads – write digests/ciphertext/tag into caller-owned buffers to avoid heap allocations on large payloads.
• Streaming helpers: ShaCtx/HmacCtx for incremental hashing/HMAC, AesCtrStream for chunked CTR flows, and AesGcmCtx for AAD + payload streaming with tag verification.
• GcmNonceOptions lets you pick random, counter+random, or boot-counter IV strategies (with optional NVS persistence) when using aesGcmEncryptAuto(...).
• JWT additions: JWK/JWKS verification helper, leeway support, multi-audience/typ enforcement, and DER↔raw ECDSA helpers to match JOSE encodings.
• ChaCha20-Poly1305 encrypt/decrypt helpers and X25519 shared-secret derivation for devices where AES accel varies. XChaCha20-Poly1305 and Ed25519/EdDSA APIs currently return Unsupported until the toolchain provides those primitives.
• CryptoResult<String> createJwtResult(...) / verifyJwtResult(...) – Build HS256/RS256/ES256 JWTs with auto iat/exp fields and get back structured status plus the friendly error string versions.
• CryptoResult<std::vector<uint8_t>> hmac/hkdf/pbkdf2 and hashString/verifyString – HMAC/HKDF/PBKDF2 building blocks; password hashes stay in the $esphash$v1$cost$salt$hash envelope and compare in constant time.
• CryptoCaps caps() and SecureBuffer/SecureString – Introspect hardware acceleration availability and zeroize sensitive buffers on scope exit.

JWT Helpers

JwtSignOptions lets you set issuer, subject, audience, expiresInSeconds, notBefore, issuedAt, and keyId. JwtVerifyOptions can enforce issuer/audience matches, require expiration, and accept externally supplied clocks (e.g., SNTP time). Header/payload data stays as ArduinoJson v7 JsonDocuments, so you can merge them with doc.set(...) or stream them over serial for debugging. Use createJwt/verifyJwt for friendly strings or createJwtResult/verifyJwtResult for structured status codes.

verifyJwtWithJwks consumes an in-memory JWKS (JsonDocument) and picks keys by kid, with support for leeway, multi-audience payloads, typ enforcement, and crit header allowlists. ECDSA raw/DER conversion helpers are available when interoping with JOSE stacks that send compact raw signatures.

Policy & Guardrails

• CryptoPolicy (default: RSA ≥ 2048 bits, PBKDF2 iterations ≥ 1024, GCM IV ≥ 12 bytes) is readable via ESPCrypto::policy() and adjustable with setPolicy(...); set allowLegacy = true to opt into weaker parameters.
• AES-GCM can enable debug nonce-reuse detection via ESPCRYPTO_ENABLE_NONCE_GUARD (tiny LRU cache keyed by IV + key fingerprint).
• constantTimeEq performs content-constant-time comparison only when both inputs already have the same length; a length mismatch returns false immediately. SecureBuffer and SecureString zeroize owned memory on cleanup.

Security Posture

• Constant-time coverage: constantTimeEq underpins password verification and HS256 JWT checks when compared buffers are the same length; it does not hide input length. Other primitives lean on ESP-IDF/mbedTLS implementations and should be treated as best-effort constant-time rather than hardened side-channel countermeasures.
• Hardware acceleration: SHA, AES-CTR, and AES-GCM try the ESP hardware blocks first and fall back to mbedTLS software paths; ESPCrypto::caps() reports what is active at runtime. Random bytes come from esp_fill_random on-device and from std::random_device only for host builds/tests.
• Best-effort hardening: password hashes stay in a structured envelope with policy-enforced PBKDF2 costs, AES-GCM enforces IV length and offers an optional nonce-reuse guard, and sensitive buffers zeroize on scope exit or failure paths.
• Threat model: aimed at network-connected ESP32-class devices where attackers can send arbitrary inputs. It does not attempt to defend against physical capture, power/EM/fault-injection side channels, or secure element/key storage requirements; review your board’s secure boot/flash encryption story separately.

Password Hashing

hashString emits $esphash$v1$<cost>$<salt>$<hash> so you can persist passwords without storing secrets. Costs map to 2^cost PBKDF2 iterations (default 10 ⇒ 1024) and will auto-bump to the policy minimum iteration count unless allowLegacy is enabled. verifyString accepts any string in that envelope, decodes the salt/hash, replays PBKDF2, and compares in constant time.

Tests

Hardware exercises run via PlatformIO Unity tests under test/test_esp_crypto, including KATs for SHA-2, AES-GCM (with tag checks), HKDF, PBKDF2, JWT HS256 round-trips, and password hashing. Host-side CMake just stubs out tests (ESP-IDF primitives are unavailable when cross-compiling for CI).

Formatting Baseline

This repository follows the firmware formatting baseline from esptoolkit-template:

• .clang-format is the source of truth for C/C++/INO layout.
• .editorconfig enforces tabs (tab_width = 4), LF endings, and final newline.
• Format all tracked firmware sources with bash scripts/format_cpp.sh.

License

MIT — see LICENSE.md.

ESPToolKit

• Discover other libraries: https://github.com/orgs/ESPToolKit/repositories
• Website: https://www.esptoolkit.hu/
• Support the project: https://ko-fi.com/esptoolkit
• Visit the website: https://www.esptoolkit.hu/