Sync RSA_padding_check_PKCS1_OAEP_mgf1().

Update RSA_padding_check_PKCS1_OAEP_mgf1() with code from OpenSSL 1.1.1d
(with some improvements/corrections to comments).

This brings in code to make the padding check constant time.

ok inoguchi@ tb@

1 files changed, 111 insertions, 64 deletions

diff --git a/src/lib/libcrypto/rsa/rsa_oaep.c b/src/lib/libcrypto/rsa/rsa_oaep.c
index 6b1760da60..e54600b094 100644
--- a/src/lib/libcrypto/rsa/rsa_oaep.c
+++ b/src/lib/libcrypto/rsa/rsa_oaep.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: rsa_oaep.c,v 1.32 2019/10/09 16:17:59 jsing Exp $ */
+/* $OpenBSD: rsa_oaep.c,v 1.33 2019/10/17 14:31:56 jsing Exp $ */
 /*
  * Copyright 1999-2018 The OpenSSL Project Authors. All Rights Reserved.
  *
@@ -79,6 +79,7 @@
 #include <openssl/rsa.h>
 #include <openssl/sha.h>
 
+#include "constant_time_locl.h"
 #include "rsa_locl.h"
 
 int
@@ -169,13 +170,11 @@ RSA_padding_check_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
     const unsigned char *from, int flen, int num, const unsigned char *param,
     int plen, const EVP_MD *md, const EVP_MD *mgf1md)
 {
-        int i, dblen, mlen = -1;
-        const unsigned char *maskeddb;
-        int lzero;
-        unsigned char *db = NULL;
+        int i, dblen = 0, mlen = -1, one_index = 0, msg_index;
+        unsigned int good = 0, found_one_byte, mask;
+        const unsigned char *maskedseed, *maskeddb;
         unsigned char seed[EVP_MAX_MD_SIZE], phash[EVP_MAX_MD_SIZE];
-        unsigned char *padded_from;
-        int bad = 0;
+        unsigned char *db = NULL, *em = NULL;
         int mdlen;
 
         if (md == NULL)
@@ -184,88 +183,136 @@ RSA_padding_check_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
                 mgf1md = md;
 
         if ((mdlen = EVP_MD_size(md)) <= 0)
-                goto err;
+                return -1;
 
-        if (--num < 2 * mdlen + 1)
-                /*
-                 * 'num' is the length of the modulus, i.e. does not depend
-                 * on the particular ciphertext.
-                 */
-                goto decoding_err;
+        if (tlen <= 0 || flen <= 0)
+                return -1;
 
-        lzero = num - flen;
-        if (lzero < 0) {
-                /*
-                 * signalling this error immediately after detection might allow
-                 * for side-channel attacks (e.g. timing if 'plen' is huge
-                 * -- cf. James H. Manger, "A Chosen Ciphertext Attack on RSA
-                 * Optimal Asymmetric Encryption Padding (OAEP) [...]",
-                 * CRYPTO 2001), so we use a 'bad' flag
-                 */
-                bad = 1;
-                lzero = 0;
-                flen = num; /* don't overflow the memcpy to padded_from */
+        /*
+         * |num| is the length of the modulus; |flen| is the length of the
+         * encoded message. Therefore, for any |from| that was obtained by
+         * decrypting a ciphertext, we must have |flen| <= |num|. Similarly,
+         * |num| >= 2 * |mdlen| + 2 must hold for the modulus irrespective
+         * of the ciphertext, see PKCS #1 v2.2, section 7.1.2.
+         * This does not leak any side-channel information.
+         */
+        if (num < flen || num < 2 * mdlen + 2) {
+                RSAerror(RSA_R_OAEP_DECODING_ERROR);
+                return -1;
         }
 
-        dblen = num - mdlen;
-        if ((db = malloc(dblen + num)) == NULL) {
+        dblen = num - mdlen - 1;
+        if ((db = malloc(dblen)) == NULL) {
                 RSAerror(ERR_R_MALLOC_FAILURE);
-                return -1;
+                goto cleanup;
+        }
+        if ((em = malloc(num)) == NULL) {
+                RSAerror(ERR_R_MALLOC_FAILURE);
+                goto cleanup;
+        }
+
+        /*
+         * 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;
 
         /*
-         * Always do this zero-padding copy (even when lzero == 0)
-         * to avoid leaking timing info about the value of lzero.
+         * The first byte must be zero, however we must not leak if this is
+         * true. See James H. Manger, "A Chosen Ciphertext Attack on RSA
+         * Optimal Asymmetric Encryption Padding (OAEP) [...]", CRYPTO 2001).
          */
-        padded_from = db + dblen;
-        memset(padded_from, 0, lzero);
-        memcpy(padded_from + lzero, from, flen);
+        good = constant_time_is_zero(from[0]);
 
-        maskeddb = padded_from + mdlen;
+        maskedseed = from + 1;
+        maskeddb = from + 1 + mdlen;
 
         if (PKCS1_MGF1(seed, mdlen, maskeddb, dblen, mgf1md))
-                goto err;
+                goto cleanup;
         for (i = 0; i < mdlen; i++)
-                seed[i] ^= padded_from[i];
+                seed[i] ^= maskedseed[i];
+
         if (PKCS1_MGF1(db, dblen, seed, mdlen, mgf1md))
-                goto err;
+                goto cleanup;
         for (i = 0; i < dblen; i++)
                 db[i] ^= maskeddb[i];
 
         if (!EVP_Digest((void *)param, plen, phash, NULL, md, NULL))
-                goto err;
+                goto cleanup;
 
-        if (timingsafe_memcmp(db, phash, mdlen) != 0 || bad)
-                goto decoding_err;
-        else {
-                for (i = mdlen; i < dblen; i++)
-                        if (db[i] != 0x00)
-                                break;
-                if (i == dblen || db[i] != 0x01)
-                        goto decoding_err;
-                else {
-                        /* everything looks OK */
-
-                        mlen = dblen - ++i;
-                        if (tlen < mlen) {
-                                RSAerror(RSA_R_DATA_TOO_LARGE);
-                                mlen = -1;
-                        } else
-                                memcpy(to, db + i, mlen);
-                }
+        good &= constant_time_is_zero(timingsafe_memcmp(db, phash, mdlen));
+
+        found_one_byte = 0;
+        for (i = mdlen; i < dblen; i++) {
+                /*
+                 * Padding consists of a number of 0-bytes, followed by a 1.
+                 */
+                unsigned int equals1 = constant_time_eq(db[i], 1);
+                unsigned int equals0 = constant_time_is_zero(db[i]);
+
+                one_index = constant_time_select_int(~found_one_byte & equals1,
+                    i, one_index);
+                found_one_byte |= equals1;
+                good &= (found_one_byte | equals0);
+        }
+
+        good &= found_one_byte;
+
+        /*
+         * At this point |good| is zero unless the plaintext was valid,
+         * so plaintext-awareness ensures timing side-channels are no longer a
+         * concern.
+         */
+        msg_index = one_index + 1;
+        mlen = dblen - msg_index;
+
+        /*
+         * For good measure, do this check in constant time as well.
+         */
+        good &= constant_time_ge(tlen, mlen);
+
+        /*
+         * Even though we can't fake result's length, we can pretend copying
+         * |tlen| bytes where |mlen| bytes would be real. The last |tlen| of
+         * |dblen| bytes are viewed as a circular buffer starting at |tlen|-|mlen'|,
+         * where |mlen'| is the "saturated" |mlen| value. Deducing information
+         * about failure or |mlen| would require an attacker to observe
+         * memory access patterns 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(dblen, tlen), dblen, tlen);
+        msg_index = constant_time_select_int(good, msg_index, dblen - tlen);
+        mlen = dblen - msg_index;
+        for (from = db + msg_index, mask = good, i = 0; i < tlen; i++) {
+                unsigned int equals = constant_time_eq(i, mlen);
+
+                from -= dblen & equals; /* if (i == mlen) rewind   */
+                mask &= mask ^ equals;  /* if (i == mlen) mask = 0 */
+                to[i] = constant_time_select_8(mask, from[i], to[i]);
         }
-        free(db);
-        return mlen;
 
- decoding_err:
         /*
          * To avoid chosen ciphertext attacks, the error message should not
-         * reveal which kind of decoding error happened
+         * reveal which kind of decoding error happened.
          */
         RSAerror(RSA_R_OAEP_DECODING_ERROR);
- err:
-        free(db);
-        return -1;
+        err_clear_last_constant_time(1 & good);
+
+ cleanup:
+        explicit_bzero(seed, sizeof(seed));
+        freezero(db, dblen);
+        freezero(em, num);
+
+        return constant_time_select_int(good, mlen, -1);
 }
 
 int