diff --git a/ChangeLog.d/00README.md b/ChangeLog.d/00README.md
index d2ea73d..2fbc989 100644
--- a/ChangeLog.d/00README.md
+++ b/ChangeLog.d/00README.md
@@ -21,6 +21,9 @@
 * Performance improvements, unless they are particularly significant.
 * Changes to parts of the code base that users don't interact with directly,
   such as test code and test data.
+* Fixes for compiler warnings. Releases typically contain a number of fixes
+  of this kind, so we will only mention them in the Changelog if they are
+  particularly significant.
 
 Until Mbed TLS 2.24.0, we required changelog entries in more cases.
 Looking at older changelog entries is good practice for how to write a
diff --git a/ChangeLog.d/x509-ec-algorithm-identifier-fix.txt b/ChangeLog.d/x509-ec-algorithm-identifier-fix.txt
new file mode 100644
index 0000000..c1de491
--- /dev/null
+++ b/ChangeLog.d/x509-ec-algorithm-identifier-fix.txt
@@ -0,0 +1,4 @@
+Bugfix
+   * Fix x509 certificate generation to conform to RFC 5480 / RFC 5758 when
+     using ECC key. The certificate was rejected by some crypto frameworks.
+     Fixes #2924.
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/include/mbedtls/x509.h b/include/mbedtls/x509.h
index 8fd321a..f00f3a6 100644
--- a/include/mbedtls/x509.h
+++ b/include/mbedtls/x509.h
@@ -361,7 +361,8 @@
                              mbedtls_asn1_named_data *first);
 int mbedtls_x509_write_sig(unsigned char **p, unsigned char *start,
                            const char *oid, size_t oid_len,
-                           unsigned char *sig, size_t size);
+                           unsigned char *sig, size_t size,
+                           mbedtls_pk_type_t pk_alg);
 
 #define MBEDTLS_X509_SAFE_SNPRINTF                          \
     do {                                                    \
diff --git a/library/asn1write.c b/library/asn1write.c
index 0b487dc..a499bea 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -195,13 +195,22 @@
                                             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));
diff --git a/library/x509_create.c b/library/x509_create.c
index cdfc82a..bd772d3 100644
--- a/library/x509_create.c
+++ b/library/x509_create.c
@@ -285,9 +285,11 @@
 
 int mbedtls_x509_write_sig(unsigned char **p, unsigned char *start,
                            const char *oid, size_t oid_len,
-                           unsigned char *sig, size_t size)
+                           unsigned char *sig, size_t size,
+                           mbedtls_pk_type_t pk_alg)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    int write_null_par;
     size_t len = 0;
 
     if (*p < start || (size_t) (*p - start) < size) {
@@ -310,8 +312,19 @@
 
     // Write OID
     //
-    MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_algorithm_identifier(p, start, oid,
-                                                                      oid_len, 0));
+    if (pk_alg == MBEDTLS_PK_ECDSA) {
+        /*
+         * The AlgorithmIdentifier's parameters field must be absent for DSA/ECDSA signature
+         * algorithms, see https://www.rfc-editor.org/rfc/rfc5480#page-17 and
+         * https://www.rfc-editor.org/rfc/rfc5758#section-3.
+         */
+        write_null_par = 0;
+    } else {
+        write_null_par = 1;
+    }
+    MBEDTLS_ASN1_CHK_ADD(len,
+                         mbedtls_asn1_write_algorithm_identifier_ext(p, start, oid, oid_len,
+                                                                     0, write_null_par));
 
     return (int) len;
 }
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index a8f4c28..e994411 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -342,6 +342,7 @@
     size_t sub_len = 0, pub_len = 0, sig_and_oid_len = 0, sig_len;
     size_t len = 0;
     mbedtls_pk_type_t pk_alg;
+    int write_sig_null_par;
 
     /*
      * Prepare data to be signed at the end of the target buffer
@@ -433,9 +434,20 @@
     /*
      *  Signature   ::=  AlgorithmIdentifier
      */
+    if (pk_alg == MBEDTLS_PK_ECDSA) {
+        /*
+         * The AlgorithmIdentifier's parameters field must be absent for DSA/ECDSA signature
+         * algorithms, see https://www.rfc-editor.org/rfc/rfc5480#page-17 and
+         * https://www.rfc-editor.org/rfc/rfc5758#section-3.
+         */
+        write_sig_null_par = 0;
+    } else {
+        write_sig_null_par = 1;
+    }
     MBEDTLS_ASN1_CHK_ADD(len,
-                         mbedtls_asn1_write_algorithm_identifier(&c, buf,
-                                                                 sig_oid, strlen(sig_oid), 0));
+                         mbedtls_asn1_write_algorithm_identifier_ext(&c, buf,
+                                                                     sig_oid, strlen(sig_oid),
+                                                                     0, write_sig_null_par));
 
     /*
      *  Serial   ::=  INTEGER
@@ -492,8 +504,8 @@
      * into the CRT buffer. */
     c2 = buf + size;
     MBEDTLS_ASN1_CHK_ADD(sig_and_oid_len, mbedtls_x509_write_sig(&c2, c,
-                                                                 sig_oid, sig_oid_len, sig,
-                                                                 sig_len));
+                                                                 sig_oid, sig_oid_len,
+                                                                 sig, sig_len, pk_alg));
 
     /*
      * Memory layout after this step:
diff --git a/library/x509write_csr.c b/library/x509write_csr.c
index a421fde..178b166 100644
--- a/library/x509write_csr.c
+++ b/library/x509write_csr.c
@@ -265,7 +265,7 @@
     c2 = buf + size;
     MBEDTLS_ASN1_CHK_ADD(sig_and_oid_len,
                          mbedtls_x509_write_sig(&c2, buf + len, sig_oid, sig_oid_len,
-                                                sig, sig_len));
+                                                sig, sig_len, pk_alg));
 
     /*
      * Compact the space between the CSR data and signature by moving the
diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index 17e0f90..222988c 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -1210,7 +1210,7 @@
 
 # The use of 'Server 1' in the DN is intentional here, as the DN is hardcoded in the x509_write test suite.'
 server5.req.ku.sha1: server5.key
-	$(MBEDTLS_CERT_REQ) output_file=$@ filename=$< key_usage=digital_signature,non_repudiation subject_name="C=NL,O=PolarSSL,CN=PolarSSL Server 1" md=SHA1
+	$(OPENSSL) req -key $< -out $@ -new -nodes -subj "/C=NL/O=PolarSSL/CN=PolarSSL Server 1" -sha1 -addext keyUsage=digitalSignature,nonRepudiation
 all_final += server5.req.ku.sha1
 
 # server6*
diff --git a/tests/data_files/Readme-x509.txt b/tests/data_files/Readme-x509.txt
index 84c775f..82f93d2 100644
--- a/tests/data_files/Readme-x509.txt
+++ b/tests/data_files/Readme-x509.txt
@@ -76,6 +76,10 @@
     -badsign.crt: S5 with corrupted signature
     -expired.crt: S5 with "not after" date in the past
     -future.crt: S5 with "not before" date in the future
+    -non-compliant.crt: S5, RFC non-compliant
+      (with forbidden EC algorithm identifier NULL parameter)
+      generated by (before fix):
+        cert_write subject_key=server5.key subject_name="CN=Test EC RFC non-compliant" issuer_crt=test-ca2.crt issuer_key=test-ca2.key
     -selfsigned.crt: Self-signed cert with S5 key
     -ss-expired.crt: Self-signed cert with S5 key, expired
     -ss-forgeca.crt: Copy of test-int-ca3 self-signed with S5 key
diff --git a/tests/data_files/parse_input/server5-non-compliant.crt b/tests/data_files/parse_input/server5-non-compliant.crt
new file mode 100644
index 0000000..abea17d
--- /dev/null
+++ b/tests/data_files/parse_input/server5-non-compliant.crt
@@ -0,0 +1,12 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
diff --git a/tests/data_files/server5.req.ku.sha1 b/tests/data_files/server5.req.ku.sha1
index 3281c94..c73a0e2 100644
--- a/tests/data_files/server5.req.ku.sha1
+++ b/tests/data_files/server5.req.ku.sha1
@@ -1,8 +1,8 @@
 -----BEGIN CERTIFICATE REQUEST-----
-MIIBFjCBvAIBADA8MQswCQYDVQQGEwJOTDERMA8GA1UECgwIUG9sYXJTU0wxGjAY
+MIIBFDCBvAIBADA8MQswCQYDVQQGEwJOTDERMA8GA1UECgwIUG9sYXJTU0wxGjAY
 BgNVBAMMEVBvbGFyU1NMIFNlcnZlciAxMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcD
 QgAEN8xW2XYJHlpyPsdZLf8gbu58+QaRdNCtFLX3aCJZYpJO5QDYIxH/6i/SNF1d
 Fr2KiMJrdw1VzYoqDvoByLTt/6AeMBwGCSqGSIb3DQEJDjEPMA0wCwYDVR0PBAQD
-AgbAMAsGByqGSM49BAEFAANIADBFAiEAnIKF+xKk0iEuN4MHd4FZWNvrznLQgkeg
-2n8ejjreTzcCIAH34z2TycuMpWQRhpV+YT988pBWR67LAg7REyZnjSAB
+AgbAMAkGByqGSM49BAEDSAAwRQIhAJyChfsSpNIhLjeDB3eBWVjb685y0IJHoNp/
+Ho463k83AiAB9+M9k8nLjKVkEYaVfmE/fPKQVkeuywIO0RMmZ40gAQ==
 -----END CERTIFICATE REQUEST-----
diff --git a/tests/suites/test_suite_x509parse.data b/tests/suites/test_suite_x509parse.data
index c796cdd..406a063 100644
--- a/tests/suites/test_suite_x509parse.data
+++ b/tests/suites/test_suite_x509parse.data
@@ -2890,6 +2890,14 @@
 depends_on:MBEDTLS_SHA256_C:MBEDTLS_RSA_C
 mbedtls_x509_crt_parse_file:"data_files/parse_input/cli-rsa-sha256-badalg.crt.der":MBEDTLS_ERR_X509_SIG_MISMATCH:0
 
+X509 File parse (does not conform to RFC 5480 / RFC 5758 - AlgorithmIdentifier's parameters field is present, mbedTLS generated before bugfix, OK)
+depends_on:MBEDTLS_ECDSA_C:MBEDTLS_ECP_DP_SECP256R1_ENABLED:MBEDTLS_SHA256_C
+x509parse_crt_file:"data_files/parse_input/server5-non-compliant.crt":0
+
+X509 File parse (conforms to RFC 5480 / RFC 5758 - AlgorithmIdentifier's parameters field must be absent for ECDSA)
+depends_on:MBEDTLS_ECDSA_C:MBEDTLS_ECP_DP_SECP256R1_ENABLED:MBEDTLS_SHA256_C
+x509parse_crt_file:"data_files/parse_input/server5.crt":0
+
 X509 Get time (UTC no issues)
 depends_on:MBEDTLS_X509_USE_C
 x509_get_time:MBEDTLS_ASN1_UTC_TIME:"500101000000Z":0:1950:1:1:0:0:0