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Re: [ADVISORY] Timing Attack on OpenSSL
From: Christopher Fowler <cfowler () outpostsentinel com>
Date: 17 Mar 2003 09:17:28 -0500

Is this a new advisory.  I've patched for a previous timing attack 2
weeks ago.
On Mon, 2003-03-17 at 03:47, Ben Laurie wrote:
I expect a release to follow shortly.

-- 
http://www.apache-ssl.org/ben.html       http://www.thebunker.net/

"There is no limit to what a man can do or how far he can go if he
doesn't mind who gets the credit." - Robert Woodruff
----


OpenSSL v0.9.7a and 0.9.6i vulnerability
----------------------------------------

Researchers have discovered a timing attack on RSA keys, to which
OpenSSL is generally vulnerable, unless RSA blinding has been turned
on.

Typically, it will not have been, because it is not easily possible to
do so when using OpenSSL to provide SSL or TLS.

The enclosed patch switches blinding on by default. Applications that
wish to can remove the blinding with RSA_blinding_off(), but this is
not generally advised. It is also possible to disable it completely by
defining OPENSSL_NO_FORCE_RSA_BLINDING at compile-time.

The performance impact of blinding appears to be small (a few
percent).

This problem affects many applications using OpenSSL, in particular,
almost all SSL-enabled Apaches. You should rebuild and reinstall
OpenSSL, and all affected applications.

The Common Vulnerabilities and Exposures project (cve.mitre.org) has
assigned the name CAN-2003-0147 to this issue.

We strongly advise upgrading OpenSSL in all cases, as a precaution.
----


Index: crypto/rsa/rsa_eay.c
===================================================================
RCS file: /e/openssl/cvs/openssl/crypto/rsa/rsa_eay.c,v
retrieving revision 1.28.2.3
diff -u -r1.28.2.3 rsa_eay.c
--- crypto/rsa/rsa_eay.c      30 Jan 2003 17:37:46 -0000      1.28.2.3
+++ crypto/rsa/rsa_eay.c      16 Mar 2003 10:34:13 -0000
@@ -195,6 +195,25 @@
      return(r);
      }
 
+static int rsa_eay_blinding(RSA *rsa, BN_CTX *ctx)
+     {
+     int ret = 1;
+     CRYPTO_w_lock(CRYPTO_LOCK_RSA);
+     /* Check again inside the lock - the macro's check is racey */
+     if(rsa->blinding == NULL)
+             ret = RSA_blinding_on(rsa, ctx);
+     CRYPTO_w_unlock(CRYPTO_LOCK_RSA);
+     return ret;
+     }
+
+#define BLINDING_HELPER(rsa, ctx, err_instr) \
+     do { \
+             if(((rsa)->flags & RSA_FLAG_BLINDING) && \
+                             ((rsa)->blinding == NULL) && \
+                             !rsa_eay_blinding(rsa, ctx)) \
+                     err_instr \
+     } while(0)
+
 /* signing */
 static int RSA_eay_private_encrypt(int flen, const unsigned char *from,
           unsigned char *to, RSA *rsa, int padding)
@@ -239,8 +258,8 @@
              goto err;
              }
 
-     if ((rsa->flags & RSA_FLAG_BLINDING) && (rsa->blinding == NULL))
-             RSA_blinding_on(rsa,ctx);
+     BLINDING_HELPER(rsa, ctx, goto err;);
+
      if (rsa->flags & RSA_FLAG_BLINDING)
              if (!BN_BLINDING_convert(&f,rsa->blinding,ctx)) goto err;
 
@@ -318,8 +337,8 @@
              goto err;
              }
 
-     if ((rsa->flags & RSA_FLAG_BLINDING) && (rsa->blinding == NULL))
-             RSA_blinding_on(rsa,ctx);
+     BLINDING_HELPER(rsa, ctx, goto err;);
+
      if (rsa->flags & RSA_FLAG_BLINDING)
              if (!BN_BLINDING_convert(&f,rsa->blinding,ctx)) goto err;
 
Index: crypto/rsa/rsa_lib.c
===================================================================
RCS file: /e/openssl/cvs/openssl/crypto/rsa/rsa_lib.c,v
retrieving revision 1.30.2.2
diff -u -r1.30.2.2 rsa_lib.c
--- crypto/rsa/rsa_lib.c      30 Jan 2003 17:37:46 -0000      1.30.2.2
+++ crypto/rsa/rsa_lib.c      16 Mar 2003 10:34:13 -0000
@@ -72,7 +72,13 @@
 
 RSA *RSA_new(void)
      {
-     return(RSA_new_method(NULL));
+     RSA *r=RSA_new_method(NULL);
+
+#ifndef OPENSSL_NO_FORCE_RSA_BLINDING
+     r->flags|=RSA_FLAG_BLINDING;
+#endif
+
+     return r;
      }
 
 void RSA_set_default_method(const RSA_METHOD *meth)


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