Add constant time memcmp and use it for comparing keys.

There is perhaps no problem now, but it is a good practise to use
constant time for key comaprison to avoid possible side channel
issues.

lib/crypto_backend/crypto_backend.h

@@ -149,4 +149,7 @@ static inline void crypt_backend_memzero(void *s, size_t n)
 #endif
 }
 
+/* Memcmp helper (memcmp in constant time) */
+int crypt_backend_memeq(const void *m1, const void *m2, size_t n);
+
 #endif /* _CRYPTO_BACKEND_H */

lib/crypto_backend/crypto_backend_internal.h

@@ -58,4 +58,18 @@ int crypt_bitlk_decrypt_key_kernel(const void *key, size_t key_length,
 				   const char *iv, size_t iv_length,
 				   const char *tag, size_t tag_length);
 
+/* Internal implementation for constant time memory comparison */
+static inline int crypt_internal_memeq(const void *m1, const void *m2, size_t n)
+{
+	const unsigned char *_m1 = (const unsigned char *) m1;
+	const unsigned char *_m2 = (const unsigned char *) m2;
+	unsigned char result = 0;
+	size_t i;
+
+	for (i = 0; i < n; i++)
+		result |= _m1[i] ^ _m2[i];
+
+	return result;
+}
+
 #endif /* _CRYPTO_BACKEND_INTERNAL_H */

lib/crypto_backend/crypto_gcrypt.c

@@ -550,3 +550,8 @@ out:
 	return -ENOTSUP;
 #endif
 }
+
+int crypt_backend_memeq(const void *m1, const void *m2, size_t n)
+{
+	return crypt_internal_memeq(m1, m2, n);
+}

lib/crypto_backend/crypto_kernel.c

@@ -416,3 +416,8 @@ int crypt_bitlk_decrypt_key(const void *key, size_t key_length,
 	return crypt_bitlk_decrypt_key_kernel(key, key_length, in, out, length,
 					      iv, iv_length, tag, tag_length);
 }
+
+int crypt_backend_memeq(const void *m1, const void *m2, size_t n)
+{
+	return crypt_internal_memeq(m1, m2, n);
+}

lib/crypto_backend/crypto_nettle.c

@@ -26,6 +26,7 @@
 #include <nettle/sha3.h>
 #include <nettle/hmac.h>
 #include <nettle/pbkdf2.h>
+#include <nettle/memops.h>
 #include "crypto_backend_internal.h"
 
 #if HAVE_NETTLE_VERSION_H
@@ -446,3 +447,9 @@ int crypt_bitlk_decrypt_key(const void *key, size_t key_length,
 	return crypt_bitlk_decrypt_key_kernel(key, key_length, in, out, length,
 					      iv, iv_length, tag, tag_length);
 }
+
+int crypt_backend_memeq(const void *m1, const void *m2, size_t n)
+{
+	/* The logic is inverse to memcmp... */
+	return !memeql_sec(m1, m2, n);
+}

lib/crypto_backend/crypto_nss.c

@@ -395,3 +395,8 @@ int crypt_bitlk_decrypt_key(const void *key, size_t key_length,
 	return crypt_bitlk_decrypt_key_kernel(key, key_length, in, out, length,
 					      iv, iv_length, tag, tag_length);
 }
+
+int crypt_backend_memeq(const void *m1, const void *m2, size_t n)
+{
+	return NSS_SecureMemcmp(m1, m2, n);
+}

lib/crypto_backend/crypto_openssl.c

@@ -30,6 +30,7 @@
 
 #include <string.h>
 #include <errno.h>
+#include <openssl/crypto.h>
 #include <openssl/evp.h>
 #include <openssl/hmac.h>
 #include <openssl/rand.h>
@@ -806,3 +807,8 @@ out:
 	return -ENOTSUP;
 #endif
 }
+
+int crypt_backend_memeq(const void *m1, const void *m2, size_t n)
+{
+	return CRYPTO_memcmp(m1, m2, n);
+}