diff --git a/src/config.c b/src/config.c
index b8394a5..e0b4b7c 100644
--- a/src/config.c
+++ b/src/config.c
@@ -5,7 +5,6 @@
 
 #include <arpa/inet.h>
 #include <limits.h>
-#include <ctype.h>
 #include <netdb.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -19,6 +18,7 @@
 #include "containers.h"
 #include "ipc.h"
 #include "encoding.h"
+#include "ctype.h"
 
 #define COMMENT_CHAR '#'
 
@@ -86,7 +86,7 @@ static inline bool parse_fwmark(uint32_t *fwmark, uint32_t *flags, const char *v
 		return true;
 	}
 
-	if (!isdigit(value[0]))
+	if (!char_is_digit(value[0]))
 		goto err;
 
 	if (strlen(value) > 2 && value[0] == '0' && value[1] == 'x')
@@ -141,7 +141,7 @@ static bool parse_keyfile(uint8_t key[static WG_KEY_LEN], const char *path)
 	dst[WG_KEY_LEN_BASE64 - 1] = '\0';
 
 	while ((c = getc(f)) != EOF) {
-		if (!isspace(c)) {
+		if (!char_is_space(c)) {
 			fprintf(stderr, "Found trailing character in key file: `%c'\n", c);
 			goto out;
 		}
@@ -290,7 +290,7 @@ static inline bool parse_persistent_keepalive(uint16_t *interval, uint32_t *flag
 		return true;
 	}
 
-	if (!isdigit(value[0]))
+	if (!char_is_digit(value[0]))
 		goto err;
 
 	ret = strtoul(value, &end, 10);
@@ -375,7 +375,7 @@ static inline bool parse_allowedips(struct wgpeer *peer, struct wgallowedip **la
 		}
 
 		if (mask) {
-			if (!isdigit(mask[0]))
+			if (!char_is_digit(mask[0]))
 				goto err;
 			cidr = strtoul(mask, &end, 10);
 			if (*end || (cidr > 32 && new_allowedip->family == AF_INET) || (cidr > 128 && new_allowedip->family == AF_INET6))
@@ -501,7 +501,7 @@ bool config_read_line(struct config_ctx *ctx, const char *input)
 	}
 
 	for (size_t i = 0; i < len; ++i) {
-		if (!isspace(input[i]))
+		if (!char_is_space(input[i]))
 			line[cleaned_len++] = input[i];
 	}
 	if (!cleaned_len)
@@ -555,7 +555,7 @@ static char *strip_spaces(const char *in)
 		return NULL;
 	}
 	for (i = 0, l = 0; i < t; ++i) {
-		if (!isspace(in[i]))
+		if (!char_is_space(in[i]))
 			out[l++] = in[i];
 	}
 	return out;
diff --git a/src/ctype.h b/src/ctype.h
new file mode 100644
index 0000000..98b2283
--- /dev/null
+++ b/src/ctype.h
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2015-2020 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ *
+ * Specialized constant-time ctype.h reimplementations that aren't locale-specific.
+ */
+
+#ifndef CTYPE_H
+#define CTYPE_H
+
+#include <stdbool.h>
+
+static inline bool char_is_space(int c)
+{
+	unsigned char d = c - 9;
+	return (0x80001FU >> (d & 31)) & (1U >> (d >> 5));
+}
+
+static inline bool char_is_digit(int c)
+{
+	return (unsigned int)(('0' - 1 - c) & (c - ('9' + 1))) >> (sizeof(c) * 8 - 1);
+}
+
+static inline bool char_is_alpha(int c)
+{
+	return (unsigned int)(('a' - 1 - (c | 32)) & ((c | 32) - ('z' + 1))) >> (sizeof(c) * 8 - 1);
+}
+
+#endif
diff --git a/src/ipc-uapi.h b/src/ipc-uapi.h
index 3b1fff3..f464be7 100644
--- a/src/ipc-uapi.h
+++ b/src/ipc-uapi.h
@@ -4,7 +4,6 @@
  */
 
 #include <arpa/inet.h>
-#include <ctype.h>
 #include <errno.h>
 #include <net/if.h>
 #include <netdb.h>
@@ -17,6 +16,7 @@
 #include "containers.h"
 #include "curve25519.h"
 #include "encoding.h"
+#include "ctype.h"
 
 #ifdef _WIN32
 #include "ipc-uapi-windows.h"
@@ -102,7 +102,7 @@ static int userspace_set_device(struct wgdevice *dev)
 #define NUM(max) ({ \
 	unsigned long long num; \
 	char *end; \
-	if (!isdigit(value[0])) \
+	if (!char_is_digit(value[0])) \
 		break; \
 	num = strtoull(value, &end, 10); \
 	if (*end || num > max) \
@@ -223,7 +223,7 @@ static int userspace_get_device(struct wgdevice **out, const char *iface)
 			struct wgallowedip *new_allowedip;
 			char *end, *mask = value, *ip = strsep(&mask, "/");
 
-			if (!mask || !isdigit(mask[0]))
+			if (!mask || !char_is_digit(mask[0]))
 				break;
 			new_allowedip = calloc(1, sizeof(*new_allowedip));
 			if (!new_allowedip) {
diff --git a/src/pubkey.c b/src/pubkey.c
index a78fe11..b4478dc 100644
--- a/src/pubkey.c
+++ b/src/pubkey.c
@@ -5,11 +5,11 @@
 
 #include <errno.h>
 #include <stdio.h>
-#include <ctype.h>
 
 #include "curve25519.h"
 #include "encoding.h"
 #include "subcommands.h"
+#include "ctype.h"
 
 int pubkey_main(int argc, char *argv[])
 {
@@ -31,7 +31,7 @@ int pubkey_main(int argc, char *argv[])
 
 	for (;;) {
 		trailing_char = getc(stdin);
-		if (!trailing_char || isspace(trailing_char))
+		if (!trailing_char || char_is_space(trailing_char))
 			continue;
 		if (trailing_char == EOF)
 			break;
diff --git a/src/terminal.c b/src/terminal.c
index bea27ba..7c293cd 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -3,7 +3,6 @@
  * Copyright (C) 2015-2020 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
  */
 
-#include <ctype.h>
 #include <stdarg.h>
 #include <stddef.h>
 #include <stdio.h>
@@ -11,6 +10,7 @@
 #include <string.h>
 #include <stdbool.h>
 #include <unistd.h>
+#include "ctype.h"
 
 static bool color_mode(void)
 {
@@ -46,7 +46,7 @@ static void filter_ansi(const char *fmt, va_list args)
 			if (str[i] == '\x1b' && str[i + 1] == '[') {
 				str[i] = str[i + 1] = '\0';
 				for (j = i + 2; j < len; ++j) {
-					if (isalpha(str[j]))
+					if (char_is_alpha(str[j]))
 						break;
 					str[j] = '\0';
 				}