Sign.ino

Minimal Cryptnox example: sign a 32-byte hash on the secp256k1 curve.

Minimal Cryptnox example: sign a 32-byte hash on the secp256k1 curve.Wiring & prerequisites:

• PN532 NFC reader on SPI, with SS on pin PN532_SS_PIN.
• A Cryptnox card initialised with a known PIN AND a loaded seed (use the Cryptnox CLI: cryptnox init then cryptnox seed generate).
• DEMO_PIN must match the PIN set on the card.

What the sketch does in each loop iteration:

1. Connect to the card and establish the secure channel.
2. Sign a 32-byte hash on the secp256k1 curve (key type CW_SIGN_CURR_K1, signature type CW_SIGN_SIG_ECDSA_LOW_S, PIN included in the sign payload CW_SIGN_WITH_PIN).
3. Print the result, securely wipe local secrets, disconnect.

Warning

On CW_SIGN_PIN_INCORRECT (0x82) the sketch enters an infinite halt: every wrong PIN attempt decrements the card's retry counter and reaching 0 permanently blocks the PIN. Recovery then requires the PUK via cryptnox unblock_pin. Verify DEMO_PIN before retrying.

Note

The PIN "000000000" used by the project examples is a demo placeholder. In real deployments set a strong PIN, never commit source files containing a real PIN, and keep this value out of any version-controlled config.

The hash filled with 0x01 below is a test pattern. In real use replace it with the SHA-256 (or Keccak-256 for Ethereum) digest of the transaction you want the card to sign.

/*
 * SPDX-License-Identifier: LGPL-3.0-or-later
 * Copyright (c) 2026 Cryptnox SA
 */

#include <CryptnoxWallet.h>
#include <SPI.h>

#define PN532_SS_PIN  (10U)

#define DEMO_PIN      "000000000"

ArduinoLoggerAdapter   serialAdapter;

PN532Adapter           nfc(serialAdapter, PN532_SS_PIN, &SPI);

ArduinoCryptoProvider  cryptoProvider;

ArduinoPlatform        platform;

CryptnoxWallet         wallet(nfc, serialAdapter, cryptoProvider, platform);

void setup() {
    serialAdapter.begin(115200);
    delay(1000);                              /* Arduino R4: wait for Serial */
    SPI.begin();
 
    if (!wallet.begin()) {
        serialAdapter.println(F("PN532 init failed"));
        while (1);
    }
}

void loop() {
    CW_SecureSession session;
    if (!wallet.connect(session)) {
        serialAdapter.println(F("Card not detected"));
        wallet.disconnect(session);
        delay(1000);
        return;
    }
 
    uint8_t hash[CW_HASH_SIZE];
    memset(hash, 0x01, sizeof(hash));   /* replace with SHA-256 of your tx */
 
    CW_SignRequest req(session,
                       CW_SIGN_CURR_K1,
                       CW_SIGN_SIG_ECDSA_LOW_S,
                       CW_SIGN_WITH_PIN);
    req.hash       = hash;
    req.hashLength = sizeof(hash);
    CW_Utils::safe_memcpy(req.pin, sizeof(req.pin),
                          reinterpret_cast<const uint8_t*>(DEMO_PIN), strlen(DEMO_PIN));
 
    CW_SignResult sig = wallet.sign(req);
    if (sig.errorCode == CW_OK) {
        serialAdapter.println(F("Signature OK"));
 
        serialAdapter.print(F("  r = "));
        for (uint8_t i = 0U; i < 32U; i++) {
            uint8_t b = sig.signature[CW_SIG_R_OFFSET + i];
            if (b < 0x10U) { serialAdapter.print(F("0")); }
            serialAdapter.print(b, HEX);
        }
        serialAdapter.println();
 
        serialAdapter.print(F("  s = "));
        for (uint8_t i = 0U; i < 32U; i++) {
            uint8_t b = sig.signature[CW_SIG_S_OFFSET + i];
            if (b < 0x10U) { serialAdapter.print(F("0")); }
            serialAdapter.print(b, HEX);
        }
        serialAdapter.println();
 
        serialAdapter.print(F("  errorCode = 0x"));
        serialAdapter.println(sig.errorCode, HEX);
    } else if (sig.errorCode == CW_SIGN_PIN_INCORRECT) {
        /* CRITICAL: do NOT loop — each wrong PIN attempt burns a retry. */
        serialAdapter.println(F("Wrong PIN — halting to protect retry counter"));
        CW_Utils::secure_wipe(hash, sizeof(hash));
        CW_Utils::secure_wipe(sig.signature, sizeof(sig.signature));
        wallet.disconnect(session);
        while (1);
    } else {
        serialAdapter.print(F("Sign failed: 0x"));
        serialAdapter.println(sig.errorCode, HEX);
    }
 
    CW_Utils::secure_wipe(hash, sizeof(hash));
    CW_Utils::secure_wipe(sig.signature, sizeof(sig.signature));
    wallet.disconnect(session);
    delay(1000);
}