Add an internal constant-time comparison function

k5_bcmp acts similarly to the deprecated Unix bcmp() function,
returning zero if two memory regions are equal and nonzero if they are
not.  It is implemented such that it should take the same amount of
time regardless of how many bytes are equal within the memory regions.

src/include/k5-platform.h

@@ -40,6 +40,7 @@
  * + [v]asprintf
  * + mkstemp
  * + zap (support function; macro is in k5-int.h)
+ * + constant time memory comparison
  * + path manipulation
  * + _, N_, dgettext, bindtextdomain (for localization)
  */
@@ -1048,6 +1049,13 @@ extern int krb5int_gettimeofday(struct timeval *tp, void *ignore);
 
 extern void krb5int_zap(void *ptr, size_t len);
 
+/*
+ * Return 0 if the n-byte memory regions p1 and p2 are equal, and nonzero if
+ * they are not.  The function is intended to take the same amount of time
+ * regardless of how many bytes of p1 and p2 are equal.
+ */
+int k5_bcmp(const void *p1, const void *p2, size_t n);
+
 /*
  * Split a path into parent directory and basename.  Either output parameter
  * may be NULL if the caller doesn't need it.  parent_out will be empty if path

src/util/support/Makefile.in

@@ -71,6 +71,7 @@ STLIBOBJS= \
 	path.o \
 	base64.o \
 	json.o \
+	bcmp.o \
 	$(GETTIMEOFDAY_ST_OBJ) \
 	$(IPC_ST_OBJ) \
 	$(STRLCPY_ST_OBJ) \
@@ -92,6 +93,7 @@ LIBOBJS= \
 	$(OUTPRE)path.$(OBJEXT) \
 	$(OUTPRE)base64.$(OBJEXT) \
 	$(OUTPRE)json.$(OBJEXT) \
+	$(OUTPRE)bcmp.$(OBJEXT) \
 	$(GETTIMEOFDAY_OBJ) \
 	$(IPC_OBJ) \
 	$(STRLCPY_OBJ) \
@@ -121,7 +123,8 @@ SRCS=\
 	$(srcdir)/zap.c \
 	$(srcdir)/path.c \
 	$(srcdir)/base64.c \
-	$(srcdir)/json.c
+	$(srcdir)/json.c \
+	$(srcdir)/bcmp.c
 
 SHLIB_EXPDEPS =
 # Add -lm if dumping thread stats, for sqrt.

src/util/support/bcmp.c

@@ -0,0 +1,44 @@
+/* -*- mode: c; c-basic-offset: 4; indent-tabs-mode: nil -*- */
+/* util/support/bitcmp.c - Constant-time byte comparison function */
+/*
+ * Copyright (C) 2013 by the Massachusetts Institute of Technology.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in
+ *   the documentation and/or other materials provided with the
+ *   distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "k5-platform.h"
+
+int
+k5_bcmp(const void *p1, const void *p2, size_t n)
+{
+    const unsigned char *c1 = p1, *c2 = p2;
+    unsigned char x = 0;
+
+    for (; n > 0; c1++, c2++, n--)
+        x |= *c1 ^ *c2;
+    return x;
+}

src/util/support/libkrb5support-fixed.exports

@@ -1,5 +1,6 @@
 k5_base64_decode
 k5_base64_encode
+k5_bcmp
 k5_buf_init_fixed
 k5_buf_init_dynamic
 k5_buf_add