From d1dec41f7656d7937f1cce64df1cc41121f3e44d Mon Sep 17 00:00:00 2001
From: Matt Johnston <matt@ucc.asn.au>
Date: Thu, 3 Oct 2013 22:25:30 +0800
Subject: [PATCH] Constant time memcmp for the hmac and password crypt
---
dbutil.c | 13 +++++++++++++
dbutil.h | 3 +++
packet.c | 2 +-
svr-authpasswd.c | 13 ++++++++++++-
4 files changed, 29 insertions(+), 2 deletions(-)
diff --git a/dbutil.c b/dbutil.c
index 8ee6050c..2589e1a7 100644
--- a/dbutil.c
+++ b/dbutil.c
@@ -884,3 +884,16 @@ int m_str_to_uint(const char* str, unsigned int *val) {
return DROPBEAR_SUCCESS;
}
}
+
+int constant_time_memcmp(const void* a, const void *b, size_t n)
+{
+ const char *xa = a, *xb = b;
+ uint8_t c = 0;
+ size_t i;
+ for (i = 0; i < n; i++)
+ {
+ c |= (xa[i] ^ xb[i]);
+ }
+ return c;
+}
+
diff --git a/dbutil.h b/dbutil.h
index 3af79f72..a34ddd75 100644
--- a/dbutil.h
+++ b/dbutil.h
@@ -94,4 +94,7 @@ int m_str_to_uint(const char* str, unsigned int *val);
/* Dropbear assertion */
#define dropbear_assert(X) do { if (!(X)) { fail_assert(#X, __FILE__, __LINE__); } } while (0)
+/* Returns 0 if a and b have the same contents */
+int constant_time_memcmp(const void* a, const void *b, size_t n);
+
#endif /* _DBUTIL_H_ */
diff --git a/packet.c b/packet.c
index d458ccf7..4ebd0d71 100644
--- a/packet.c
+++ b/packet.c
@@ -376,7 +376,7 @@ static int checkmac() {
/* compare the hash */
buf_setpos(ses.readbuf, contents_len);
- if (memcmp(mac_bytes, buf_getptr(ses.readbuf, mac_size), mac_size) != 0) {
+ if (constant_time_memcmp(mac_bytes, buf_getptr(ses.readbuf, mac_size), mac_size) != 0) {
return DROPBEAR_FAILURE;
} else {
return DROPBEAR_SUCCESS;
diff --git a/svr-authpasswd.c b/svr-authpasswd.c
index 17dd2a1b..7a5a1210 100644
--- a/svr-authpasswd.c
+++ b/svr-authpasswd.c
@@ -33,6 +33,17 @@
#ifdef ENABLE_SVR_PASSWORD_AUTH
+static int constant_time_strcmp(const char* a, const char* b) {
+ size_t la = strlen(a);
+ size_t lb = strlen(b);
+
+ if (la != lb) {
+ return 1;
+ }
+
+ return constant_time_memcmp(a, b, la);
+}
+
/* Process a password auth request, sending success or failure messages as
* appropriate */
void svr_auth_password() {
@@ -82,7 +93,7 @@ void svr_auth_password() {
return;
}
- if (strcmp(testcrypt, passwdcrypt) == 0) {
+ if (constant_time_strcmp(testcrypt, passwdcrypt) == 0) {
/* successful authentication */
dropbear_log(LOG_NOTICE,
"Password auth succeeded for '%s' from %s",
--
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