Use ASN1 UTC tags for dates before 2000

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index ce475e6..6a9cfeb 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -208,8 +208,10 @@
     mbedtls_x509write_cert crt;
     unsigned char buf[4096];
     unsigned char check_buf[5000];
+    unsigned char *p, *end;
+    unsigned char tag, sz;
     mbedtls_mpi serial;
-    int ret;
+    int ret, before_tag, after_tag;
     size_t olen = 0, pem_len = 0, buf_index = 0;
     int der_len = -1;
     FILE *f;
@@ -287,14 +289,16 @@
         TEST_ASSERT( buf[buf_index] == 0 );
     }
 
-    f = fopen( cert_check_file, "r" );
-    TEST_ASSERT( f != NULL );
-    olen = fread( check_buf, 1, sizeof( check_buf ), f );
-    fclose( f );
-    TEST_ASSERT( olen < sizeof( check_buf ) );
-
-    TEST_ASSERT( olen >= pem_len - 1 );
-    TEST_ASSERT( memcmp( buf, check_buf, pem_len - 1 ) == 0 );
+    if( *cert_check_file != '\0' )
+    {
+        f = fopen( cert_check_file, "r" );
+        TEST_ASSERT( f != NULL );
+        olen = fread( check_buf, 1, sizeof( check_buf ), f );
+        fclose( f );
+        TEST_ASSERT( olen < sizeof( check_buf ) );
+        TEST_ASSERT( olen >= pem_len - 1 );
+        TEST_ASSERT( memcmp( buf, check_buf, pem_len - 1 ) == 0 );
+    }
 
     der_len = mbedtls_x509write_crt_der( &crt, buf, sizeof( buf ),
                                          mbedtls_test_rnd_pseudo_rand,
@@ -304,6 +308,54 @@
     if( der_len == 0 )
         goto exit;
 
+    // Not testing against file, check date format
+    if( *cert_check_file == '\0' )
+    {
+        // UTC tag if before 2050, 2 digits less for year
+        if( not_before[0] == '2' && ( not_before[1] > '0' || not_before[2] > '4' ) )
+        {
+            before_tag = MBEDTLS_ASN1_GENERALIZED_TIME;
+        }
+        else
+        {
+            before_tag = MBEDTLS_ASN1_UTC_TIME;
+            not_before += 2;
+        }
+        if( not_after[0] == '2' && ( not_after[1] > '0' || not_after[2] > '4' ) )
+        {
+            after_tag = MBEDTLS_ASN1_GENERALIZED_TIME;
+        }
+        else
+        {
+            after_tag = MBEDTLS_ASN1_UTC_TIME;
+            not_after += 2;
+        }
+        end = buf + sizeof( buf );
+        for( p = end - der_len ; p < end ; )
+        {
+            tag = *p++;
+            sz = *p++;
+            if( tag == MBEDTLS_ASN1_UTC_TIME || tag == MBEDTLS_ASN1_GENERALIZED_TIME )
+            {
+                // Check correct tag and time written
+                TEST_ASSERT( before_tag == tag );
+                TEST_ASSERT( memcmp( p, not_before, sz - 1 ) == 0 );
+                p += sz;
+                tag = *p++;
+                sz = *p++;
+                TEST_ASSERT( after_tag == tag );
+                TEST_ASSERT( memcmp( p, not_after, sz - 1 ) == 0 );
+                break;
+            }
+            // Increment if long form ASN1 length
+            if( sz & 0x80 )
+                p += sz & 0x0F;
+            if( tag != ( MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) )
+                p += sz;
+        }
+        TEST_ASSERT( p < end );
+    }
+
     ret = mbedtls_x509write_crt_der( &crt, buf, (size_t)( der_len - 1 ),
                                      mbedtls_test_rnd_pseudo_rand, &rnd_info );
     TEST_ASSERT( ret == MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );