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rsa/rsa_ssl.c: make RSA_padding_check_SSLv23 constant-time.

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Copy of RSA_padding_check_PKCS1_type_2 with a twist that rejects padding
if nul delimiter is preceded by 8 consecutive 0x03 bytes.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
This commit is contained in:
Andy Polyakov 2018-09-14 17:24:13 +02:00 committed by Matt Caswell
parent 75f5e944be
commit 603221407d
1 changed files with 96 additions and 37 deletions
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133
crypto/rsa/rsa_ssl.c
View file

@ -12,6 +12,7 @@
#include <openssl/bn.h>
#include <openssl/rsa.h>
#include <openssl/rand.h>
#include "internal/constant_time_locl.h"
int RSA_padding_add_SSLv23(unsigned char *to, int tlen,
const unsigned char *from, int flen)
@ -52,57 +53,115 @@ int RSA_padding_add_SSLv23(unsigned char *to, int tlen,
return 1;
}
/*
* Copy of RSA_padding_check_PKCS1_type_2 with a twist that rejects padding
* if nul delimiter is preceded by 8 consecutive 0x03 bytes. It also
* preserves error code reporting for backward compatibility.
*/
int RSA_padding_check_SSLv23(unsigned char *to, int tlen,
const unsigned char *from, int flen, int num)
{
int i, j, k;
const unsigned char *p;
int i;
/* |em| is the encoded message, zero-padded to exactly |num| bytes */
unsigned char *em = NULL;
unsigned int good, found_zero_byte, mask, threes_in_row;
int zero_index = 0, msg_index, mlen = -1, err;
p = from;
if (flen < 10) {
RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_DATA_TOO_SMALL);
return -1;
}
/* Accept even zero-padded input */
if (flen == num) {
if (*(p++) != 0) {
RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_BLOCK_TYPE_IS_NOT_02);
return -1;
}
flen--;
}
if ((num != (flen + 1)) || (*(p++) != 02)) {
RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_BLOCK_TYPE_IS_NOT_02);
em = OPENSSL_malloc(num);
if (em == NULL) {
RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, ERR_R_MALLOC_FAILURE);
return -1;
}
/*
* Caller is encouraged to pass zero-padded message created with
* BN_bn2binpad. Trouble is that since we can't read out of |from|'s
* bounds, it's impossible to have an invariant memory access pattern
* in case |from| was not zero-padded in advance.
*/
for (from += flen, em += num, i = 0; i < num; i++) {
mask = ~constant_time_is_zero(flen);
flen -= 1 & mask;
from -= 1 & mask;
*--em = *from & mask;
}
from = em;
good = constant_time_is_zero(from[0]);
good &= constant_time_eq(from[1], 2);
err = constant_time_select_int(good, 0, RSA_R_BLOCK_TYPE_IS_NOT_02);
mask = ~good;
/* scan over padding data */
j = flen - 1; /* one for type */
for (i = 0; i < j; i++)
if (*(p++) == 0)
break;
found_zero_byte = 0;
threes_in_row = 0;
for (i = 2; i < num; i++) {
unsigned int equals0 = constant_time_is_zero(from[i]);
if ((i == j) || (i < 8)) {
RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23,
RSA_R_NULL_BEFORE_BLOCK_MISSING);
return -1;
}
for (k = -9; k < -1; k++) {
if (p[k] != 0x03)
break;
}
if (k == -1) {
RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_SSLV3_ROLLBACK_ATTACK);
return -1;
zero_index = constant_time_select_int(~found_zero_byte & equals0,
i, zero_index);
found_zero_byte |= equals0;
threes_in_row += 1 & ~found_zero_byte;
threes_in_row &= found_zero_byte | constant_time_eq(from[i], 3);
}
i++; /* Skip over the '\0' */
j -= i;
if (j > tlen) {
RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_DATA_TOO_LARGE);
return -1;
}
memcpy(to, p, (unsigned int)j);
/*
* PS must be at least 8 bytes long, and it starts two bytes into |from|.
* If we never found a 0-byte, then |zero_index| is 0 and the check
* also fails.
*/
good &= constant_time_ge(zero_index, 2 + 8);
err = constant_time_select_int(mask | good, err,
RSA_R_NULL_BEFORE_BLOCK_MISSING);
mask = ~good;
return j;
good &= constant_time_lt(threes_in_row, 8);
err = constant_time_select_int(mask | good, err,
RSA_R_SSLV3_ROLLBACK_ATTACK);
mask = ~good;
/*
* Skip the zero byte. This is incorrect if we never found a zero-byte
* but in this case we also do not copy the message out.
*/
msg_index = zero_index + 1;
mlen = num - msg_index;
/*
* For good measure, do this check in constant time as well.
*/
good &= constant_time_ge(tlen, mlen);
err = constant_time_select_int(mask | good, err, RSA_R_DATA_TOO_LARGE);
/*
* Even though we can't fake result's length, we can pretend copying
* |tlen| bytes where |mlen| bytes would be real. Last |tlen| of |num|
* bytes are viewed as circular buffer with start at |tlen|-|mlen'|,
* where |mlen'| is "saturated" |mlen| value. Deducing information
* about failure or |mlen| would take attacker's ability to observe
* memory access pattern with byte granularity *as it occurs*. It
* should be noted that failure is indistinguishable from normal
* operation if |tlen| is fixed by protocol.
*/
tlen = constant_time_select_int(constant_time_lt(num, tlen), num, tlen);
msg_index = constant_time_select_int(good, msg_index, num - tlen);
mlen = num - msg_index;
for (from += msg_index, mask = good, i = 0; i < tlen; i++) {
unsigned int equals = constant_time_eq(i, mlen);
from -= tlen & equals; /* if (i == mlen) rewind */
mask &= mask ^ equals; /* if (i == mlen) mask = 0 */
to[i] = constant_time_select_8(mask, from[i], to[i]);
}
OPENSSL_clear_free(em, num);
RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, err);
err_clear_last_constant_time(1 & good);
return constant_time_select_int(good, mlen, -1);
}