File 0002-rsa-rsa_eay.c-implement-variant-of-Smooth-CRT-RSA.patch of Package compat-openssl098.12531
From f9381fd323303316282331a8cced6e030e809794 Mon Sep 17 00:00:00 2001
From: Andy Polyakov <appro@openssl.org>
Date: Mon, 13 Aug 2018 20:20:28 +0200
Subject: [PATCH 2/4] rsa/rsa_eay.c: implement variant of "Smooth CRT-RSA."
In [most common] case of p and q being of same width, it's possible to
replace CRT modulo operations with Montgomery reductions. And those are
even fixed-length Montgomery reductions...
(cherry picked from commit 41bfd5e7c8ac3a0874a94e4d15c006ad5eb48e59)
Resolved conflicts:
crypto/rsa/rsa_eay.c
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/6942)
---
crypto/rsa/rsa_eay.c | 101 +++++++++++++++++++++++++++++++++++--------
1 file changed, 83 insertions(+), 18 deletions(-)
Index: openssl-1.0.1i/crypto/rsa/rsa_eay.c
===================================================================
--- openssl-1.0.1i.orig/crypto/rsa/rsa_eay.c
+++ openssl-1.0.1i/crypto/rsa/rsa_eay.c
@@ -805,7 +805,7 @@ static int RSA_eay_mod_exp(BIGNUM *r0, c
BIGNUM *r1,*m1,*vrfy;
BIGNUM local_dmp1,local_dmq1,local_c,local_r1;
BIGNUM *dmp1,*dmq1,*c,*pr1;
- int ret=0;
+ int ret = 0, smooth = 0;
BN_CTX_start(ctx);
r1 = BN_CTX_get(ctx);
@@ -841,6 +841,9 @@ static int RSA_eay_mod_exp(BIGNUM *r0, c
goto err;
if (!BN_MONT_CTX_set_locked(&rsa->_method_mod_q, CRYPTO_LOCK_RSA, q, ctx))
goto err;
+
+ smooth = (rsa->meth->bn_mod_exp == BN_mod_exp_mont)
+ && (BN_num_bits(q) == BN_num_bits(p));
}
}
@@ -848,6 +851,46 @@ static int RSA_eay_mod_exp(BIGNUM *r0, c
if (!BN_MONT_CTX_set_locked(&rsa->_method_mod_n, CRYPTO_LOCK_RSA, rsa->n, ctx))
goto err;
+ if (smooth) {
+ /*
+ * Conversion from Montgomery domain, a.k.a. Montgomery reduction,
+ * accepts values in [0-m*2^w) range. w is m's bit width rounded up
+ * to limb width. So that at the very least if |I| is fully reduced,
+ * i.e. less than p*q, we can count on from-to round to perform
+ * below modulo operations on |I|. Unlike BN_mod it's constant time.
+ */
+ if (/* m1 = I moq q */
+ !bn_from_mont_fixed_top(m1, I, rsa->_method_mod_q, ctx)
+ || !bn_to_mont_fixed_top(m1, m1, rsa->_method_mod_q, ctx)
+ /* m1 = m1^dmq1 mod q */
+ || !BN_mod_exp_mont_consttime(m1, m1, rsa->dmq1, rsa->q, ctx,
+ rsa->_method_mod_q)
+ /* r1 = I mod p */
+ || !bn_from_mont_fixed_top(r1, I, rsa->_method_mod_p, ctx)
+ || !bn_to_mont_fixed_top(r1, r1, rsa->_method_mod_p, ctx)
+ /* r1 = r1^dmp1 mod p */
+ || !BN_mod_exp_mont_consttime(r1, r1, rsa->dmp1, rsa->p, ctx,
+ rsa->_method_mod_p)
+ /* r1 = (r1 - m1) mod p */
+ /*
+ * bn_mod_sub_fixed_top is not regular modular subtraction,
+ * it can tolerate subtrahend to be larger than modulus, but
+ * not bit-wise wider. This makes up for uncommon q>p case,
+ * when |m1| can be larger than |rsa->p|.
+ */
+ || !bn_mod_sub_fixed_top(r1, r1, m1, rsa->p)
+
+ /* r0 = r0 * iqmp mod p */
+ || !bn_to_mont_fixed_top(r1, r1, rsa->_method_mod_p, ctx)
+ || !bn_mul_mont_fixed_top(r1, r1, rsa->iqmp, rsa->_method_mod_p,
+ ctx)
+ || !bn_mul_fixed_top(r0, r1, rsa->q, ctx)
+ || !bn_mod_add_fixed_top(r0, r0, m1, rsa->n))
+ goto err;
+
+ goto tail;
+ }
+
/* compute I mod q */
if (!(rsa->flags & RSA_FLAG_NO_CONSTTIME))
{
@@ -924,14 +967,29 @@ static int RSA_eay_mod_exp(BIGNUM *r0, c
if (!BN_mul(r1,r0,rsa->q,ctx)) goto err;
if (!BN_add(r0,r1,m1)) goto err;
- if (rsa->e && rsa->n)
- {
- if (!rsa->meth->bn_mod_exp(vrfy,r0,rsa->e,rsa->n,ctx,rsa->_method_mod_n)) goto err;
+
+tail:
+ if (rsa->e && rsa->n) {
+ if (rsa->meth->bn_mod_exp == BN_mod_exp_mont) {
+ if (!BN_mod_exp_mont(vrfy, r0, rsa->e, rsa->n, ctx,
+ rsa->_method_mod_n))
+ goto err;
+ } else {
+ bn_correct_top(r0);
+ if (!rsa->meth->bn_mod_exp(vrfy, r0, rsa->e, rsa->n, ctx,
+ rsa->_method_mod_n))
+ goto err;
+ }
/* If 'I' was greater than (or equal to) rsa->n, the operation
* will be equivalent to using 'I mod n'. However, the result of
* the verify will *always* be less than 'n' so we don't check
* for absolute equality, just congruency. */
if (!BN_sub(vrfy, vrfy, I)) goto err;
+ if (BN_is_zero(vrfy)) {
+ bn_correct_top(r0);
+ ret = 1;
+ goto err; /* not actually error */
+ }
if (!BN_mod(vrfy, vrfy, rsa->n, ctx)) goto err;
if (BN_is_negative(vrfy))
if (!BN_add(vrfy, vrfy, rsa->n)) goto err;
@@ -955,6 +1013,15 @@ static int RSA_eay_mod_exp(BIGNUM *r0, c
rsa->_method_mod_n)) goto err;
}
}
+ /*
+ * It's unfortunate that we have to bn_correct_top(r0). What hopefully
+ * saves the day is that correction is highly unlike, and private key
+ * operations are customarily performed on blinded message. Which means
+ * that attacker won't observe correlation with chosen plaintext.
+ * Secondly, remaining code would still handle it in same computational
+ * time and even conceal memory access pattern around corrected top.
+ */
+ bn_correct_top(r0);
ret=1;
err:
BN_CTX_end(ctx);
