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)  }