diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index ec4d325..05d7677 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -235,6 +235,42 @@
     return 0;
 }
 
+/*
+ * Convert the input "in_buff" string to a raw byte array "out_buff". The amount
+ * of converted data is returned on "written_data".
+ */
+static int parse_serial(const char *in_buff, size_t in_buff_len,
+                        unsigned char *out_buff, size_t out_buff_len,
+                        size_t *written_data)
+{
+    char c;
+    unsigned char val;
+    int i;
+
+    if (out_buff_len < in_buff_len) {
+        return -1;
+    }
+
+    *written_data = 0;
+
+    for (i = 0; i < (int) in_buff_len; i++, (*written_data)++) {
+        c = in_buff[i];
+        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 -1;
+        }
+
+        out_buff[i] = val;
+    }
+
+    return 0;
+}
+
 int main(int argc, char *argv[])
 {
     int ret = 1;
@@ -252,7 +288,8 @@
     mbedtls_x509_csr csr;
 #endif
     mbedtls_x509write_cert crt;
-    mbedtls_mpi serial;
+    unsigned char serial[MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN];
+    size_t serial_len;
     mbedtls_asn1_sequence *ext_key_usage;
     mbedtls_entropy_context entropy;
     mbedtls_ctr_drbg_context ctr_drbg;
@@ -264,7 +301,6 @@
     mbedtls_x509write_crt_init(&crt);
     mbedtls_pk_init(&loaded_issuer_key);
     mbedtls_pk_init(&loaded_subject_key);
-    mbedtls_mpi_init(&serial);
     mbedtls_ctr_drbg_init(&ctr_drbg);
     mbedtls_entropy_init(&entropy);
 #if defined(MBEDTLS_X509_CSR_PARSE_C)
@@ -514,15 +550,6 @@
     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");
 
     // Parse issuer certificate if present
@@ -663,24 +690,18 @@
     mbedtls_x509write_crt_set_version(&crt, opt.version);
     mbedtls_x509write_crt_set_md_alg(&crt, opt.md);
 
-#if defined(MBEDTLS_BIGNUM_C) && !defined(MBEDTLS_DEPRECATED_REMOVED)
-    ret = mbedtls_x509write_crt_set_serial(&crt, &serial);
-    if (ret != 0) {
-        mbedtls_strerror(ret, buf, sizeof(buf));
-        mbedtls_printf(" failed\n  !  mbedtls_x509write_crt_set_serial "
-                       "returned -0x%04x - %s\n\n", (unsigned int) -ret, buf);
+    if (parse_serial(opt.serial, strlen(opt.serial),
+                     serial, sizeof(serial), &serial_len) < 0) {
+        mbedtls_printf(" failed\n  !  Unable to parse serial\n\n");
         goto exit;
     }
-#else
-    ret = mbedtls_x509write_crt_set_serial_new(&crt, opt.serial,
-                                               strlen(opt.serial));
+    ret = mbedtls_x509write_crt_set_serial_new(&crt, serial, serial_len);
     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) {
@@ -820,7 +841,6 @@
     mbedtls_x509write_crt_free(&crt);
     mbedtls_pk_free(&loaded_subject_key);
     mbedtls_pk_free(&loaded_issuer_key);
-    mbedtls_mpi_free(&serial);
     mbedtls_ctr_drbg_free(&ctr_drbg);
     mbedtls_entropy_free(&entropy);