Fix OPENSSL_BN_ASM_MONT5 for corner cases; add a test.

Submitted by: Emilia Kasper

diff --git a/crypto/bn/asm/x86_64-mont5.pl b/crypto/bn/asm/x86_64-mont5.pl
index fe335dfd44eb7314d326f35736ffece568bbb483..d6e4c63df1bf2918550a9a3ab44029734e27a925 100755 (executable)
--- a/crypto/bn/asm/x86_64-mont5.pl
+++ b/crypto/bn/asm/x86_64-mont5.pl
@@ -836,6 +836,8 @@ $code.=<<___;
 .type  bn_scatter5,\@abi-omnipotent
 .align 16
 bn_scatter5:
+       cmp     \$0, $num
+       jz      .Lscatter_epilogue
        lea     ($tbl,$idx,8),$tbl
 .Lscatter:
        mov     ($inp),%rax
@@ -844,6 +846,7 @@ bn_scatter5:
        lea     32*8($tbl),$tbl
        sub     \$1,$num
        jnz     .Lscatter
+.Lscatter_epilogue:
        ret
 .size  bn_scatter5,.-bn_scatter5
 ___

diff --git a/crypto/bn/bn_exp.c b/crypto/bn/bn_exp.c
index aae00491db21202ef1e484fa98837f8a1b7914fd..c69cd2c2f00c319d6c92c7779f682f5ab7b03275 100644 (file)
--- a/crypto/bn/bn_exp.c
+++ b/crypto/bn/bn_exp.c
@@ -693,9 +693,6 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
 
        if (!BN_copy(&computeTemp, am)) goto err;
 
-       if (bn_wexpand(am,top)==NULL || bn_wexpand(r,top)==NULL)
-               goto err;
-
 #if defined(OPENSSL_BN_ASM_MONT5)
     /* This optimization uses ideas from http://eprint.iacr.org/2011/239,
      * specifically optimization of cache-timing attack countermeasures
@@ -717,6 +714,24 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
        bn_scatter5(am->d,am->top,powerbuf,1);
 
        acc = computeTemp.d;
+       /* bn_mul_mont() and bn_mul_mont_gather5() assume fixed length inputs.
+        * Pad the inputs with zeroes.
+        */
+       if (bn_wexpand(am,top)==NULL || bn_wexpand(r,top)==NULL ||
+           bn_wexpand(&computeTemp,top)==NULL)
+               goto err;
+       for (i = am->top; i < top; ++i)
+               {
+               am->d[i] = 0;
+               }
+       for (i = computeTemp.top; i < top; ++i)
+               {
+               computeTemp.d[i] = 0;
+               }
+       for (i = r->top; i < top; ++i)
+               {
+               r->d[i] = 0;
+               }
 #if 0
        for (i=2; i<32; i++)
                {
@@ -1102,4 +1117,3 @@ err:
        bn_check_top(r);
        return(ret);
        }
-

diff --git a/crypto/bn/bntest.c b/crypto/bn/bntest.c
index 06f5954acc39083ea56981b6ccb8730d64ff5026..8bc6d462e733ef389434ac4779f21a08a62f5f0c 100644 (file)
--- a/crypto/bn/bntest.c
+++ b/crypto/bn/bntest.c
@@ -107,6 +107,7 @@ int test_mod(BIO *bp,BN_CTX *ctx);
 int test_mod_mul(BIO *bp,BN_CTX *ctx);
 int test_mod_exp(BIO *bp,BN_CTX *ctx);
 int test_mod_exp_mont_consttime(BIO *bp,BN_CTX *ctx);
+int test_mod_exp_mont5(BIO *bp, BN_CTX *ctx);
 int test_exp(BIO *bp,BN_CTX *ctx);
 int test_gf2m_add(BIO *bp);
 int test_gf2m_mod(BIO *bp);
@@ -249,6 +250,7 @@ int main(int argc, char *argv[])
 
        message(out,"BN_mod_exp_mont_consttime");
        if (!test_mod_exp_mont_consttime(out,ctx)) goto err;
+       if (!test_mod_exp_mont5(out,ctx)) goto err;
        (void)BIO_flush(out);
 
        message(out,"BN_exp");
@@ -1012,6 +1014,81 @@ int test_mod_exp_mont_consttime(BIO *bp, BN_CTX *ctx)
        return(1);
        }
 
+/* Test constant-time modular exponentiation with 1024-bit inputs,
+ * which on x86_64 cause a different code branch to be taken.
+ */
+int test_mod_exp_mont5(BIO *bp, BN_CTX *ctx)
+       {
+       BIGNUM *a,*p,*m,*d,*e;
+       int i;
+
+       BN_MONT_CTX *mont;
+
+       a=BN_new();
+       p=BN_new();
+       m=BN_new();
+       d=BN_new();
+       e=BN_new();
+
+       mont = BN_MONT_CTX_new();
+
+       BN_bntest_rand(m,1024,0,1); /* must be odd for montgomery */
+       /* Zero exponent */
+       BN_bntest_rand(a,1024,0,0);
+       BN_zero(p);
+       if(!BN_mod_exp_mont_consttime(d,a,p,m,ctx,NULL))
+               return 0;
+       if(!BN_is_one(d))
+               {
+               fprintf(stderr, "Modular exponentiation test failed!\n");
+               return 0;
+               }
+       /* Zero input */
+       BN_bntest_rand(p,1024,0,0);
+       BN_zero(a);
+       if(!BN_mod_exp_mont_consttime(d,a,p,m,ctx,NULL))
+               return 0;
+       if(!BN_is_zero(d))
+               {
+               fprintf(stderr, "Modular exponentiation test failed!\n");
+               return 0;
+               }
+       /* Craft an input whose Montgomery representation is 1,
+        * i.e., shorter than the modulus m, in order to test
+        * the const time precomputation scattering/gathering.
+        */
+       BN_one(a);
+       BN_MONT_CTX_set(mont,m,ctx);
+       if(!BN_from_montgomery(e,a,mont,ctx))
+               return 0;
+       if(!BN_mod_exp_mont_consttime(d,e,p,m,ctx,NULL))
+               return 0;
+       if(!BN_mod_exp_simple(a,e,p,m,ctx))
+               return 0;
+       if(BN_cmp(a,d) != 0)
+               {
+               fprintf(stderr,"Modular exponentiation test failed!\n");
+               return 0;
+               }
+       /* Finally, some regular test vectors. */
+       BN_bntest_rand(e,1024,0,0);
+       if(!BN_mod_exp_mont_consttime(d,e,p,m,ctx,NULL))
+               return 0;
+       if(!BN_mod_exp_simple(a,e,p,m,ctx))
+               return 0;
+       if(BN_cmp(a,d) != 0)
+               {
+               fprintf(stderr,"Modular exponentiation test failed!\n");
+               return 0;
+               }
+       BN_free(a);
+       BN_free(p);
+       BN_free(m);
+       BN_free(d);
+       BN_free(e);
+       return(1);
+       }
+
 int test_exp(BIO *bp, BN_CTX *ctx)
        {
        BIGNUM *a,*b,*d,*e,*one;