x509: remove direct dependency from BIGNUM_C

Signed-off-by: Valerio Setti <vsetti@baylibre.com>
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index 661f8aa..f552345 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -197,7 +197,7 @@
 #define MBEDTLS_X509_CRT_VERSION_2              1
 #define MBEDTLS_X509_CRT_VERSION_3              2
 
-#define MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN 32
+#define MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN 20
 #define MBEDTLS_X509_RFC5280_UTC_TIME_LEN   15
 
 #if !defined(MBEDTLS_X509_MAX_FILE_PATH_LEN)
@@ -277,7 +277,8 @@
  */
 typedef struct mbedtls_x509write_cert {
     int MBEDTLS_PRIVATE(version);
-    mbedtls_mpi MBEDTLS_PRIVATE(serial);
+    unsigned char MBEDTLS_PRIVATE(serial)[MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN];
+    size_t MBEDTLS_PRIVATE(serial_len);
     mbedtls_pk_context *MBEDTLS_PRIVATE(subject_key);
     mbedtls_pk_context *MBEDTLS_PRIVATE(issuer_key);
     mbedtls_asn1_named_data *MBEDTLS_PRIVATE(subject);
@@ -986,15 +987,42 @@
  */
 void mbedtls_x509write_crt_set_version(mbedtls_x509write_cert *ctx, int version);
 
+#if defined(MBEDTLS_BIGNUM_C)
 /**
  * \brief           Set the serial number for a Certificate.
  *
+ * \deprecated      This function is deprecated and will be removed in a
+ *                  future version of the library. Please use
+ *                  mbedtls_x509write_crt_set_serial_new() instead.
+ *
+ * \note            Even though the MBEDTLS_BIGNUM_C guard looks redundant since
+ *                  X509 depends on PK and PK depends on BIGNUM, this emphasizes
+ *                  a direct dependency between X509 and BIGNUM which is going
+ *                  to be deprecated in the future.
+ *
  * \param ctx       CRT context to use
  * \param serial    serial number to set
  *
  * \return          0 if successful
  */
 int mbedtls_x509write_crt_set_serial(mbedtls_x509write_cert *ctx, const mbedtls_mpi *serial);
+#endif
+
+/**
+ * \brief           Set the serial number for a Certificate.
+ *
+ * \param ctx               CRT context to use
+ * \param serial_buff       Input buffer containing the serial number in big
+ *                          endian format
+ * \param serial_buff_len   Length of the previous input buffer buffer
+ *
+ * \return          0 if successful, or
+ *                  MBEDTLS_ERR_X509_BAD_INPUT_DATA if the provided input buffer:
+ *                  - is too big (longer than MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN)
+ *                  - contains invalid chars
+ */
+int mbedtls_x509write_crt_set_serial_new(mbedtls_x509write_cert *ctx,
+                                         char *serial_buff, size_t serial_buff_len);
 
 /**
  * \brief           Set the validity period for a Certificate
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index febd0e6..53c2ac4 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -52,14 +52,11 @@
 {
     memset(ctx, 0, sizeof(mbedtls_x509write_cert));
 
-    mbedtls_mpi_init(&ctx->serial);
     ctx->version = MBEDTLS_X509_CRT_VERSION_3;
 }
 
 void mbedtls_x509write_crt_free(mbedtls_x509write_cert *ctx)
 {
-    mbedtls_mpi_free(&ctx->serial);
-
     mbedtls_asn1_free_named_data_list(&ctx->subject);
     mbedtls_asn1_free_named_data_list(&ctx->issuer);
     mbedtls_asn1_free_named_data_list(&ctx->extensions);
@@ -103,15 +100,65 @@
     return mbedtls_x509_string_to_names(&ctx->issuer, issuer_name);
 }
 
+#if defined(MBEDTLS_BIGNUM_C)
 int mbedtls_x509write_crt_set_serial(mbedtls_x509write_cert *ctx,
                                      const mbedtls_mpi *serial)
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    int ret;
+    unsigned char tmp[MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN];
+    size_t tmp_len;
 
-    if ((ret = mbedtls_mpi_copy(&ctx->serial, serial)) != 0) {
+    /* Ensure that the MPI value fits into the buffer */
+    tmp_len = mbedtls_mpi_size(serial);
+    if (tmp_len > MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN) {
+        return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+    }
+
+    ctx->serial_len = tmp_len;
+
+    ret = mbedtls_mpi_write_binary(serial, tmp,
+                                   MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN);
+    if (ret < 0) {
         return ret;
     }
 
+    /* Reverse the string since "tmp" is in big endian format */
+    for (int i = 0; i < MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN; i++) {
+        ctx->serial[i] = tmp[MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN - 1 - i];
+    }
+
+    return 0;
+}
+#endif
+
+int mbedtls_x509write_crt_set_serial_new(mbedtls_x509write_cert *ctx,
+                                         char *serial_buff, size_t serial_buff_len)
+{
+    int i, j;
+    char c;
+    unsigned char val;
+
+    if (serial_buff_len > MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN) {
+        return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+    }
+
+    /* Store data in little endian format */
+    for (i = 0, j = serial_buff_len - 1; j == 0; i++, j--) {
+        c = serial_buff[j];
+        if (c >= 0x30 && c <= 0x39) {
+            val = c - 0x30;
+        } else if (c >= 0x41 && c <= 0x46) {
+            val = c - 0x37;
+        } else if (c >= 0x61 && c <= 0x66) {
+            val = c - 0x57;
+        } else {
+            return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+        }
+
+        ctx->serial[i] = val;
+    }
+    ctx->serial_len = i;
+
     return 0;
 }
 
@@ -510,9 +557,18 @@
 
     /*
      *  Serial   ::=  INTEGER
+     *
+     * Written data is:
+     * - [ctx->serial_len] bytes for the raw serial buffer
+     * - 1 byte for the length
+     * - 1 byte for the TAG
      */
-    MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_mpi(&c, buf,
-                                                     &ctx->serial));
+    MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_raw_buffer(&c, buf,
+                                                            ctx->serial, ctx->serial_len));
+    MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_len(&c, buf,
+                                                     ctx->serial_len));
+    MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_tag(&c, buf,
+                                                     MBEDTLS_ASN1_INTEGER));
 
     /*
      *  Version  ::=  INTEGER  {  v1(0), v2(1), v3(2)  }
diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index 3e134dd..363ed8d 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -514,12 +514,14 @@
     mbedtls_printf("  . Reading serial number...");
     fflush(stdout);
 
+#if defined(MBEDTLS_BIGNUM_C)
     if ((ret = mbedtls_mpi_read_string(&serial, 10, opt.serial)) != 0) {
         mbedtls_strerror(ret, buf, sizeof(buf));
         mbedtls_printf(" failed\n  !  mbedtls_mpi_read_string "
                        "returned -0x%04x - %s\n\n", (unsigned int) -ret, buf);
         goto exit;
     }
+#endif
 
     mbedtls_printf(" ok\n");
 
@@ -661,6 +663,7 @@
     mbedtls_x509write_crt_set_version(&crt, opt.version);
     mbedtls_x509write_crt_set_md_alg(&crt, opt.md);
 
+#if defined(MBEDTLS_BIGNUM_C)
     ret = mbedtls_x509write_crt_set_serial(&crt, &serial);
     if (ret != 0) {
         mbedtls_strerror(ret, buf, sizeof(buf));
@@ -668,6 +671,16 @@
                        "returned -0x%04x - %s\n\n", (unsigned int) -ret, buf);
         goto exit;
     }
+#else
+    ret = mbedtls_x509write_crt_set_serial_new(&crt, opt.serial,
+                                               strlen(opt.serial));
+    if (ret != 0) {
+        mbedtls_strerror(ret, buf, sizeof(buf));
+        mbedtls_printf(" failed\n  !  mbedtls_x509write_crt_set_serial_new "
+                       "returned -0x%04x - %s\n\n", (unsigned int) -ret, buf);
+        goto exit;
+    }
+#endif
 
     ret = mbedtls_x509write_crt_set_validity(&crt, opt.not_before, opt.not_after);
     if (ret != 0) {
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index a21ad47..2bf8024 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -384,13 +384,20 @@
         TEST_ASSERT(mbedtls_pk_get_type(&issuer_key) == MBEDTLS_PK_OPAQUE);
     }
 
+#if !defined(MBEDTLS_BIGNUM_C)
     TEST_ASSERT(mbedtls_test_read_mpi(&serial, serial_str) == 0);
+#endif
 
     if (ver != -1) {
         mbedtls_x509write_crt_set_version(&crt, ver);
     }
 
+#if !defined(MBEDTLS_BIGNUM_C)
     TEST_ASSERT(mbedtls_x509write_crt_set_serial(&crt, &serial) == 0);
+#else
+    TEST_ASSERT(mbedtls_x509write_crt_set_serial_new(&crt, serial_str,
+                                                     strlen(serial_str)) == 0);
+#endif
     TEST_ASSERT(mbedtls_x509write_crt_set_validity(&crt, not_before,
                                                    not_after) == 0);
     mbedtls_x509write_crt_set_md_alg(&crt, md_type);