--- /n/sources/plan9/sys/include/libsec.h	Sat Jun 14 00:00:00 2025
+++ /sys/include/libsec.h	Tue Jun 23 00:00:00 2026
@@ -466,3 +466,5 @@
 int	hkdfExpand(uchar*, DigestState*(*)(uchar*, ulong, uchar*, DigestState*), int, uchar*, int, uchar*, int, int);
 int	hkdfKey(uchar*, DigestState*(*x)(uchar*, ulong, uchar*, DigestState*), int xlen, uchar*, int, uchar*, int, uchar*, int, int);
 
+/* timing safe memcmp() */
+int	tsmemcmp(void*, void*, ulong);
--- /dev/null	Thu Jan  1 00:00:00 1970
+++ /sys/src/libsec/port/tsmemcmp.c	Tue Jun 23 00:00:00 2026
@@ -0,0 +1,26 @@
+#include <u.h>
+#include <libc.h>
+#include <libsec.h>
+
+/*
+ * timing safe memcmp()
+ */
+int
+tsmemcmp(void *a1, void *a2, ulong n)
+{
+	int lt, gt, c1, c2, r, m;
+	uchar *s1, *s2;
+
+	r = m = 0;
+	s1 = a1;
+	s2 = a2;
+	while(n--){
+		c1 = *s1++;
+		c2 = *s2++;
+		lt = (c1 - c2) >> 8;
+		gt = (c2 - c1) >> 8;
+		r |= (lt - gt) & ~m;
+		m |= lt | gt;
+	}
+	return r;
+}
--- /n/sources/plan9/sys/src/libsec/port/mkfile	Sat Jun 14 00:00:00 2025
+++ /sys/src/libsec/port/mkfile	Tue Jun 23 00:00:00 2026
@@ -20,6 +20,7 @@
 	tlshand.c thumb.c readcert.c \
 	pbkdf2.c\
 	hkdf.c\
+	tsmemcmp.c\
 
 ALLOFILES=${CFILES:%.c=%.$O}
 
--- /n/sources/plan9/sys/man/2/memory	Sat Jun 14 00:00:00 2025
+++ /sys/man/2/memory	Tue Jun 23 00:00:00 2026
@@ -1,6 +1,6 @@
 .TH MEMORY 2
 .SH NAME
-memccpy, memchr, memcmp, memcpy, memmove, memset \- memory operations
+memccpy, memchr, memcmp, memcpy, memmove, memset, tsmemcmp \- memory operations
 .SH SYNOPSIS
 .B #include <u.h>
 .br
@@ -17,6 +17,9 @@
 int	memcmp(void *s1, void *s2, ulong n)
 .PP
 .B
+int	tsmemcmp(void *s1, void *s2, ulong n)
+.PP
+.B
 void*	memcpy(void *s1, void *s2, ulong n)
 .PP
 .B
@@ -124,3 +127,12 @@
 and
 .I memmove
 are handed a negative count, they abort.
+.PP
+.I Tsmemcmp
+is a variant of
+.I memcmp
+that is safe against timing attacks: its runtime depends only on
+.I n
+and not on where the inputs first differ.
+.I Memcmp
+should not be used to compare sensitive data such as authenticators.