feat: HMAC-SHA256 signing module with native tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/lib/hmac/hmac.cpp

@@ -0,0 +1,64 @@
+// firmware/src/hmac.cpp
+#include "hmac.h"
+#include "mbedtls/md.h"
+#include <stdio.h>
+#include <string.h>
+
+static HString bytes_to_hex(const uint8_t* bytes, size_t len) {
+    HString out;
+    char buf[3];
+    for (size_t i = 0; i < len; i++) {
+        snprintf(buf, sizeof(buf), "%02x", bytes[i]);
+        out += buf;
+    }
+    return out;
+}
+
+static void hex_to_bytes(const HString& hex, uint8_t* out, size_t out_len) {
+    for (size_t i = 0; i < out_len && (i * 2 + 1) < hex.size(); i++) {
+        char byte_str[3] = {hex[i*2], hex[i*2+1], 0};
+        out[i] = (uint8_t)strtol(byte_str, nullptr, 16);
+    }
+}
+
+static void sha256(const uint8_t* data, size_t len, uint8_t out[32]) {
+    mbedtls_md_context_t ctx;
+    const mbedtls_md_info_t* info = mbedtls_md_info_from_type(MBEDTLS_MD_SHA256);
+    mbedtls_md_init(&ctx);
+    mbedtls_md_setup(&ctx, info, 0);
+    mbedtls_md_starts(&ctx);
+    mbedtls_md_update(&ctx, data, len);
+    mbedtls_md_finish(&ctx, out);
+    mbedtls_md_free(&ctx);
+}
+
+HString hmac_sign(const HString& secret_hex, const HString& device_id,
+                  uint32_t timestamp, const HString& body) {
+    // 1. SHA256(body)
+    uint8_t body_hash[32];
+    sha256((const uint8_t*)body.c_str(), body.size(), body_hash);
+    HString body_hash_hex = bytes_to_hex(body_hash, 32);
+
+    // 2. Build message
+    char ts_buf[12];
+    snprintf(ts_buf, sizeof(ts_buf), "%u", (unsigned)timestamp);
+    HString message = device_id + ":" + ts_buf + ":" + body_hash_hex;
+
+    // 3. Decode secret from hex
+    size_t secret_len = secret_hex.size() / 2;
+    uint8_t secret[64] = {};
+    hex_to_bytes(secret_hex, secret, secret_len);
+
+    // 4. HMAC-SHA256(secret, message)
+    uint8_t hmac_result[32];
+    mbedtls_md_context_t ctx;
+    const mbedtls_md_info_t* info = mbedtls_md_info_from_type(MBEDTLS_MD_SHA256);
+    mbedtls_md_init(&ctx);
+    mbedtls_md_setup(&ctx, info, 1);
+    mbedtls_md_hmac_starts(&ctx, secret, secret_len);
+    mbedtls_md_hmac_update(&ctx, (const uint8_t*)message.c_str(), message.size());
+    mbedtls_md_hmac_finish(&ctx, hmac_result);
+    mbedtls_md_free(&ctx);
+
+    return bytes_to_hex(hmac_result, 32);
+}