diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index a439268..f453169 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -159,6 +159,27 @@
                                             size_t par_len);
 
 /**
+ * \brief           Write an AlgorithmIdentifier sequence in ASN.1 format.
+ *
+ * \note            This function works backwards in data buffer.
+ *
+ * \param p         The reference to the current position pointer.
+ * \param start     The start of the buffer, for bounds-checking.
+ * \param oid       The OID of the algorithm to write.
+ * \param oid_len   The length of the algorithm's OID.
+ * \param par_len   The length of the parameters, which must be already written.
+ * \param has_par   If there are any parameters. If 0, par_len must be 0. If 1
+ *                  and \p par_len is 0, NULL parameters are added.
+ *
+ * \return          The number of bytes written to \p p on success.
+ * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
+ */
+int mbedtls_asn1_write_algorithm_identifier_ext(unsigned char **p,
+                                                unsigned char *start,
+                                                const char *oid, size_t oid_len,
+                                                size_t par_len, int has_par);
+
+/**
  * \brief           Write a boolean tag (#MBEDTLS_ASN1_BOOLEAN) and value
  *                  in ASN.1 format.
  *
diff --git a/library/asn1write.c b/library/asn1write.c
index 0b487dc..f9c3830 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -195,24 +195,33 @@
                                             const char *oid, size_t oid_len,
                                             size_t par_len)
 {
+    return mbedtls_asn1_write_algorithm_identifier_ext(p, start, oid, oid_len, par_len, 1);
+}
+
+int mbedtls_asn1_write_algorithm_identifier_ext(unsigned char **p, unsigned char *start,
+                                                const char *oid, size_t oid_len,
+                                                size_t par_len, int has_par)
+{
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;
 
-    if (par_len == 0) {
-        MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_null(p, start));
-    } else {
-        len += par_len;
+    if (has_par) {
+        if (par_len == 0) {
+            MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_null(p, start));
+        } else {
+            len += par_len;
+        }
     }
 
     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_oid(p, start, oid, oid_len));
 
     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_len(p, start, len));
     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_tag(p, start,
-                                                     MBEDTLS_ASN1_CONSTRUCTED |
-                                                     MBEDTLS_ASN1_SEQUENCE));
+                                                    MBEDTLS_ASN1_CONSTRUCTED |
+                                                    MBEDTLS_ASN1_SEQUENCE));
 
     return (int) len;
-}
+ }
 
 int mbedtls_asn1_write_bool(unsigned char **p, unsigned char *start, int boolean)
 {