Introduce mbedtls_ctr_increment_counter
Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/ctr.h b/library/ctr.h
new file mode 100644
index 0000000..a6b84cd
--- /dev/null
+++ b/library/ctr.h
@@ -0,0 +1,30 @@
+/**
+ * \file ctr.h
+ *
+ * \brief This file contains common functionality for counter algorithms.
+ *
+ * Copyright The Mbed TLS Contributors
+ * SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+
+#include "common.h"
+
+/**
+ * \brief Increment a big-endian 16-byte value.
+ * This is quite performance-sensitive for AES-CTR and CTR-DRBG.
+ *
+ * \param n A 16-byte value to be incremented.
+ */
+static inline void mbedtls_ctr_increment_counter(uint8_t n[16])
+{
+ // The 32-bit version seems to perform about the same as a 64-bit version
+ // on 64-bit architectures, so no need to define a 64-bit version.
+ for (int i = 3;; i--) {
+ uint32_t x = MBEDTLS_GET_UINT32_BE(n, i << 2);
+ x += 1;
+ MBEDTLS_PUT_UINT32_BE(x, n, i << 2);
+ if (x != 0 || i == 0) {
+ break;
+ }
+ }
+}