diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 70d7e93..bcc9cb0 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -31,6 +31,7 @@
 #include "mbedtls/asn1write.h"
 #include "mbedtls/error.h"
 #include "mbedtls/oid.h"
+#include "mbedtls/platform.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/md.h"
 
@@ -152,6 +153,128 @@
     return 0;
 }
 
+
+int mbedtls_x509write_crt_set_subject_alternative_name(mbedtls_x509write_cert *ctx,
+                                                       const mbedtls_x509_san_list *san_list)
+{
+    int ret = 0;
+    const mbedtls_x509_san_list *cur;
+    unsigned char *buf;
+    unsigned char *p;
+    size_t len;
+    size_t buflen = 0;
+
+    /* Determine the maximum size of the SubjectAltName list */
+    for (cur = san_list; cur != NULL; cur = cur->next) {
+        /* Calculate size of the required buffer */
+        switch (cur->node.type) {
+            case MBEDTLS_X509_SAN_DNS_NAME:
+            case MBEDTLS_X509_SAN_UNIFORM_RESOURCE_IDENTIFIER:
+            case MBEDTLS_X509_SAN_IP_ADDRESS:
+                /* length of value for each name entry,
+                 * maximum 4 bytes for the length field,
+                 * 1 byte for the tag/type.
+                 */
+                buflen += cur->node.san.unstructured_name.len + 4 + 1;
+                break;
+            case MBEDTLS_X509_SAN_DIRECTORY_NAME:
+                const mbedtls_asn1_named_data *chunk = &cur->node.san.directory_name;
+                while (chunk != NULL) {
+                    // 5 bytes for OID, max 4 bytes for length, +1 for tag,
+                    // additional 4 max for length, +1 for tag.
+                    // See x509_write_name for more information.
+                    buflen += chunk->val.len + 5 + 4 + 1 + 4 + 1;
+                    chunk = chunk->next;
+                }
+                buflen += cur->node.san.unstructured_name.len + 4 + 1;
+                break;
+            default:
+                /* Not supported - skip. */
+                break;
+        }
+    }
+
+    /* Add the extra length field and tag */
+    buflen += 4 + 1;
+
+    /* Allocate buffer */
+    buf = mbedtls_calloc(1, buflen);
+    if (buf == NULL) {
+        return MBEDTLS_ERR_ASN1_ALLOC_FAILED;
+    }
+
+    mbedtls_platform_zeroize(buf, buflen);
+    p = buf + buflen;
+
+    /* Write ASN.1-based structure */
+    cur = san_list;
+    len = 0;
+    while (cur != NULL) {
+        size_t single_san_len = 0;
+        switch (cur->node.type) {
+            case MBEDTLS_X509_SAN_DNS_NAME:
+            case MBEDTLS_X509_SAN_RFC822_NAME:
+            case MBEDTLS_X509_SAN_UNIFORM_RESOURCE_IDENTIFIER:
+            case MBEDTLS_X509_SAN_IP_ADDRESS:
+            {
+                const unsigned char *unstructured_name =
+                    (const unsigned char *) cur->node.san.unstructured_name.p;
+                size_t unstructured_name_len = cur->node.san.unstructured_name.len;
+
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_asn1_write_raw_buffer(
+                                                 &p, buf,
+                                                 unstructured_name, unstructured_name_len));
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len, mbedtls_asn1_write_len(
+                                                 &p, buf, unstructured_name_len));
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_asn1_write_tag(
+                                                 &p, buf,
+                                                 MBEDTLS_ASN1_CONTEXT_SPECIFIC | cur->node.type));
+            }
+            break;
+            case MBEDTLS_X509_SAN_DIRECTORY_NAME:
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_x509_write_names(&p, buf,
+                                                                      (mbedtls_asn1_named_data *) &
+                                                                      cur->node
+                                                                      .san.directory_name));
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_asn1_write_len(&p, buf, single_san_len));
+                MBEDTLS_ASN1_CHK_CLEANUP_ADD(single_san_len,
+                                             mbedtls_asn1_write_tag(&p, buf,
+                                                                    MBEDTLS_ASN1_CONTEXT_SPECIFIC |
+                                                                    MBEDTLS_ASN1_CONSTRUCTED |
+                                                                    MBEDTLS_X509_SAN_DIRECTORY_NAME));
+                break;
+            default:
+                /* Skip unsupported names. */
+                break;
+        }
+        cur = cur->next;
+        len += single_san_len;
+    }
+
+    MBEDTLS_ASN1_CHK_CLEANUP_ADD(len, mbedtls_asn1_write_len(&p, buf, len));
+    MBEDTLS_ASN1_CHK_CLEANUP_ADD(len,
+                                 mbedtls_asn1_write_tag(&p, buf,
+                                                        MBEDTLS_ASN1_CONSTRUCTED |
+                                                        MBEDTLS_ASN1_SEQUENCE));
+
+    ret = mbedtls_x509write_crt_set_extension(
+        ctx,
+        MBEDTLS_OID_SUBJECT_ALT_NAME,
+        MBEDTLS_OID_SIZE(MBEDTLS_OID_SUBJECT_ALT_NAME),
+        0,
+        buf + buflen - len,
+        len);
+
+cleanup:
+    mbedtls_free(buf);
+    return ret;
+}
+
+
 int mbedtls_x509write_crt_set_extension(mbedtls_x509write_cert *ctx,
                                         const char *oid, size_t oid_len,
                                         int critical,