utils: Add constant-time comparison primitives for uint32_t

src/libstrongswan/Makefile.am

@@ -138,7 +138,8 @@ utils/parser_helper.h utils/test.h utils/integrity_checker.h utils/process.h \
 utils/utils/strerror.h utils/compat/windows.h utils/compat/apple.h utils/compat/android.h \
 utils/utils/atomics.h utils/utils/types.h utils/utils/byteorder.h \
 utils/utils/string.h utils/utils/memory.h utils/utils/tty.h utils/utils/path.h \
-utils/utils/status.h utils/utils/object.h utils/utils/time.h utils/utils/align.h
+utils/utils/status.h utils/utils/object.h utils/utils/time.h utils/utils/align.h \
+utils/utils/constant_time.h
 if USE_VM_SOCKETS
   nobase_strongswan_include_HEADERS += \
     networking/streams/stream_vsock.h \

src/libstrongswan/utils/utils.h

@@ -53,6 +53,7 @@
 #include "utils/atomics.h"
 #include "utils/align.h"
 #include "utils/byteorder.h"
+#include "utils/constant_time.h"
 #include "utils/string.h"
 #include "utils/memory.h"
 #include "utils/strerror.h"

src/libstrongswan/utils/utils/constant_time.h

@@ -0,0 +1,103 @@
+/*
+ * Copyright (C) 2026 Tobias Brunner
+ *
+ * Copyright (C) secunet Security Networks AG
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.  See <http://www.fsf.org/copyleft/gpl.txt>.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * for more details.
+ */
+
+/**
+ * @defgroup constant_time_i constant_time
+ * @{ @ingroup constant_time_i
+ */
+
+#ifndef CONSTANT_TIME_H_
+#define CONSTANT_TIME_H_
+
+#include <stdint.h>
+
+/**
+ * Check if the given values are not equal in constant time.
+ *
+ * @param x		first value to check
+ * @param y		second value to check
+ * @return		1 if values are not equal, 0 otherwise
+ */
+static inline u_int constant_time_neq(uint32_t x, uint32_t y)
+{
+	return ((x-y) | (y-x)) >> 31;
+}
+
+/**
+ * Check if the given values are equal in constant time.
+ *
+ * @param x		first value to check
+ * @param y		second value to check
+ * @return		1 if values are equal, 0 otherwise
+ */
+static inline u_int constant_time_eq(uint32_t x, uint32_t y)
+{
+	return 1 ^ constant_time_neq(x, y);
+}
+
+/**
+ * Compare the two values and return 1 if the first argument is lower than
+ * the second in constant time.
+ *
+ * @param x		first value to check
+ * @param y		second value to check
+ * @return		1 if first value is lower than second
+ */
+static inline u_int constant_time_lt(uint32_t x, uint32_t y)
+{
+	return (x ^ ((x^y) | ((x-y) ^ y))) >> 31;
+}
+
+/**
+ * Compare the two values and return 1 if the first argument greater or equal to
+ * the second in constant time.
+ *
+ * @param x		first value to check
+ * @param y		second value to check
+ * @return		1 if first value is greater or equal to the second
+ */
+static inline u_int constant_time_ge(uint32_t x, uint32_t y)
+{
+	return 1 ^ constant_time_lt(x, y);
+}
+
+/**
+ * Return a 32-bit all bit-set mask if the given value is not 0.
+ *
+ * @param x		value to check
+ * @return		0xffffffff if value is != 0, 0 otherwise
+ */
+static inline uint32_t constant_time_mask(uint32_t x)
+{
+	return -(uint32_t)constant_time_neq(x, 0);
+}
+
+/**
+ * Select one of two values depending on whether the condition is != 0 or not.
+ * Basically equivalent to 'c ? x : y'.
+ *
+ * @param x		first value to select
+ * @param y		second value to select
+ * @param c		condition
+ * @return		x if c is != 0, y otherwise
+ */
+static inline uint32_t constant_time_select(uint32_t x, uint32_t y, uint32_t c)
+{
+	uint32_t m = constant_time_mask(c);
+	return (x & m) | (y & ~m);
+}
+
+#endif /** CONSTANT_TIME_H_ @} */