Hi friends,
This is both an alert and a request for help at the same time.
First: internally when libcurl wants a random value for formdata/ntlm/digest
etc, it uses the function that the underlying TLS backend provides in order to
get as strong random as possible. Of course, building libcurl completely
without TLS takes that away and then users are left with weaker random and as
a consequence weaker security properties. But then I suppose that's not a
surprise if you opt to do it without TLS.
Now, mbedTLS and its precursor PolarSSL are two backends that don't have any
function setup for the vtls API to provide random data to libcurl. This makes
libcurl use an as weak random for mbedTLS as it does when built completely
without TLS. It would be good to get this fixed, as I believe most people who
opt to use mbedTLS still would like to get the best possible security level.
My attempt to implement such a function for the mbedtls backend can be found
in the attached patch. IT DOES NOT WORK. I'll of course appreciate if someone
with greater insights and understanding of mbedTLS would take a look at this
problem and send us a working fix instead.
--
/ daniel.haxx.se
From a2f920185b727c23fee544783255bf15e6971742 Mon Sep 17 00:00:00 2001
From: Daniel Stenberg <[email protected]>
Date: Sat, 12 Nov 2016 16:35:08 +0100
Subject: [PATCH] mbedtls: provide random to libcurl via the vtls API
---
lib/vtls/mbedtls.c | 17 +++++++++++++++++
lib/vtls/mbedtls.h | 8 +++-----
2 files changed, 20 insertions(+), 5 deletions(-)
diff --git a/lib/vtls/mbedtls.c b/lib/vtls/mbedtls.c
index 7797c3e..28f4207 100644
--- a/lib/vtls/mbedtls.c
+++ b/lib/vtls/mbedtls.c
@@ -881,6 +881,23 @@ int Curl_mbedtls_data_pending(const struct connectdata *conn, int sockindex)
mbedtls_ssl_context *ssl =
(mbedtls_ssl_context *)&conn->ssl[sockindex].ssl;
return ssl->in_msglen != 0;
}
+int Curl_mbedtls_random(struct Curl_easy *data,
+ unsigned char *entropy,
+ size_t length)
+{
+ static mbedtls_ctr_drbg_context ctr_drbg;
+ static bool inited = FALSE;
+ (void)data;
+
+ if(!inited) {
+ mbedtls_ctr_drbg_init(&ctr_drbg);
+ inited = TRUE;
+ }
+
+ /* non-zero return means error */
+ return mbedtls_ctr_drbg_random(&ctr_drbg, entropy, length);
+}
+
#endif /* USE_MBEDTLS */
diff --git a/lib/vtls/mbedtls.h b/lib/vtls/mbedtls.h
index 1021d54..0e5c1a6 100644
--- a/lib/vtls/mbedtls.h
+++ b/lib/vtls/mbedtls.h
@@ -47,10 +47,12 @@ void Curl_mbedtls_close_all(struct Curl_easy *data);
void Curl_mbedtls_close(struct connectdata *conn, int sockindex);
void Curl_mbedtls_session_free(void *ptr);
size_t Curl_mbedtls_version(char *buffer, size_t size);
int Curl_mbedtls_shutdown(struct connectdata *conn, int sockindex);
+int Curl_mbedtls_random(struct Curl_easy *data, unsigned char *entropy,
+ size_t length);
/* this backends supports CURLOPT_PINNEDPUBLICKEY */
#define have_curlssl_pinnedpubkey 1
/* API setup for mbedTLS */
@@ -68,13 +70,9 @@ int Curl_mbedtls_shutdown(struct connectdata *conn, int sockindex);
#define curlssl_version Curl_mbedtls_version
#define curlssl_check_cxn(x) (x=x, -1)
#define curlssl_data_pending(x,y) Curl_mbedtls_data_pending(x, y)
#define CURL_SSL_BACKEND CURLSSLBACKEND_MBEDTLS
#define curlssl_sha256sum(a,b,c,d) mbedtls_sha256(a,b,c,0)
-
-/* This might cause libcurl to use a weeker random!
- TODO: implement proper use of Polarssl's CTR-DRBG or HMAC-DRBG and use that
-*/
-#define curlssl_random(x,y,z) (x=x, y=y, z=z, CURLE_NOT_BUILT_IN)
+#define curlssl_random(x,y,z) Curl_mbedtls_random(x,y,z)
#endif /* USE_MBEDTLS */
#endif /* HEADER_CURL_MBEDTLS_H */
--
2.10.2
-------------------------------------------------------------------
List admin: https://cool.haxx.se/list/listinfo/curl-library
Etiquette: https://curl.haxx.se/mail/etiquette.html
