X-Git-Url: https://git.openssl.org/gitweb/?p=openssl.git;a=blobdiff_plain;f=crypto%2Frsa%2Frsa_pk1.c;h=64f68647400a469f4bdce502fb93d55752509152;hp=2791291b948df9e36b2e053a0d663cb180b827a0;hb=23b2fc0b501d6635dd821014aec8ff16689cbb6a;hpb=7dfb0b774e6592dcbfe47015168a0ac8b44e2a17

diff --git a/crypto/rsa/rsa_pk1.c b/crypto/rsa/rsa_pk1.c
index 2791291b94..64f6864740 100644
--- a/crypto/rsa/rsa_pk1.c
+++ b/crypto/rsa/rsa_pk1.c
@@ -1,233 +1,400 @@
-/* crypto/rsa/rsa_pk1.c */
-/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
- * All rights reserved.
+/*
+ * Copyright 1995-2020 The OpenSSL Project Authors. All Rights Reserved.
  *
- * This package is an SSL implementation written
- * by Eric Young (eay@cryptsoft.com).
- * The implementation was written so as to conform with Netscapes SSL.
- * 
- * This library is free for commercial and non-commercial use as long as
- * the following conditions are aheared to.  The following conditions
- * apply to all code found in this distribution, be it the RC4, RSA,
- * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
- * included with this distribution is covered by the same copyright terms
- * except that the holder is Tim Hudson (tjh@cryptsoft.com).
- * 
- * Copyright remains Eric Young's, and as such any Copyright notices in
- * the code are not to be removed.
- * If this package is used in a product, Eric Young should be given attribution
- * as the author of the parts of the library used.
- * This can be in the form of a textual message at program startup or
- * in documentation (online or textual) provided with the package.
- * 
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- * 1. Redistributions of source code must retain the copyright
- *    notice, this list of conditions and the following disclaimer.
- * 2. Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in the
- *    documentation and/or other materials provided with the distribution.
- * 3. All advertising materials mentioning features or use of this software
- *    must display the following acknowledgement:
- *    "This product includes cryptographic software written by
- *     Eric Young (eay@cryptsoft.com)"
- *    The word 'cryptographic' can be left out if the rouines from the library
- *    being used are not cryptographic related :-).
- * 4. If you include any Windows specific code (or a derivative thereof) from 
- *    the apps directory (application code) you must include an acknowledgement:
- *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
- * 
- * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
- * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
- * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
- * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
- * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
- * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
- * SUCH DAMAGE.
- * 
- * The licence and distribution terms for any publically available version or
- * derivative of this code cannot be changed.  i.e. this code cannot simply be
- * copied and put under another distribution licence
- * [including the GNU Public Licence.]
+ * Licensed under the Apache License 2.0 (the "License").  You may not use
+ * this file except in compliance with the License.  You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
  */
 
+/*
+ * RSA low level APIs are deprecated for public use, but still ok for
+ * internal use.
+ */
+#include "internal/deprecated.h"
+
+#include "internal/constant_time.h"
+
 #include <stdio.h>
-#include "cryptlib.h"
-#include "bn.h"
-#include "rsa.h"
-#include "rand.h"
-
-#ifndef NOPROTO
-int RSA_padding_add_PKCS1_type_1();
-int RSA_padding_check_PKCS1_type_1();
-int RSA_padding_add_PKCS1_type_2();
-int RSA_padding_check_PKCS1_type_2();
-int RSA_padding_add_SSLv23();
-int RSA_padding_check_SSLv23();
-int RSA_padding_add_none();
-int RSA_padding_check_none();
+#include <openssl/bn.h>
+#include <openssl/rsa.h>
+#include <openssl/rand.h>
+/* Just for the SSL_MAX_MASTER_KEY_LENGTH value */
+#include <openssl/ssl.h>
+#include "internal/cryptlib.h"
+#include "crypto/rsa.h"
+#include "rsa_local.h"
+
+int RSA_padding_add_PKCS1_type_1(unsigned char *to, int tlen,
+                                 const unsigned char *from, int flen)
+{
+    int j;
+    unsigned char *p;
+
+    if (flen > (tlen - RSA_PKCS1_PADDING_SIZE)) {
+        RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_TYPE_1,
+               RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
+        return 0;
+    }
+
+    p = (unsigned char *)to;
+
+    *(p++) = 0;
+    *(p++) = 1;                 /* Private Key BT (Block Type) */
+
+    /* pad out with 0xff data */
+    j = tlen - 3 - flen;
+    memset(p, 0xff, j);
+    p += j;
+    *(p++) = '\0';
+    memcpy(p, from, (unsigned int)flen);
+    return 1;
+}
+
+int RSA_padding_check_PKCS1_type_1(unsigned char *to, int tlen,
+                                   const unsigned char *from, int flen,
+                                   int num)
+{
+    int i, j;
+    const unsigned char *p;
+
+    p = from;
+
+    /*
+     * The format is
+     * 00 || 01 || PS || 00 || D
+     * PS - padding string, at least 8 bytes of FF
+     * D  - data.
+     */
+
+    if (num < RSA_PKCS1_PADDING_SIZE)
+        return -1;
+
+    /* Accept inputs with and without the leading 0-byte. */
+    if (num == flen) {
+        if ((*p++) != 0x00) {
+            RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1,
+                   RSA_R_INVALID_PADDING);
+            return -1;
+        }
+        flen--;
+    }
+
+    if ((num != (flen + 1)) || (*(p++) != 0x01)) {
+        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1,
+               RSA_R_BLOCK_TYPE_IS_NOT_01);
+        return -1;
+    }
+
+    /* scan over padding data */
+    j = flen - 1;               /* one for type. */
+    for (i = 0; i < j; i++) {
+        if (*p != 0xff) {       /* should decrypt to 0xff */
+            if (*p == 0) {
+                p++;
+                break;
+            } else {
+                RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1,
+                       RSA_R_BAD_FIXED_HEADER_DECRYPT);
+                return -1;
+            }
+        }
+        p++;
+    }
+
+    if (i == j) {
+        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1,
+               RSA_R_NULL_BEFORE_BLOCK_MISSING);
+        return -1;
+    }
+
+    if (i < 8) {
+        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1,
+               RSA_R_BAD_PAD_BYTE_COUNT);
+        return -1;
+    }
+    i++;                        /* Skip over the '\0' */
+    j -= i;
+    if (j > tlen) {
+        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1, RSA_R_DATA_TOO_LARGE);
+        return -1;
+    }
+    memcpy(to, p, (unsigned int)j);
+
+    return j;
+}
+
+int ossl_rsa_padding_add_PKCS1_type_2_ex(OPENSSL_CTX *libctx, unsigned char *to,
+                                         int tlen, const unsigned char *from,
+                                         int flen)
+{
+    int i, j;
+    unsigned char *p;
+
+    if (flen > (tlen - RSA_PKCS1_PADDING_SIZE)) {
+        RSAerr(0, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
+        return 0;
+    }
+
+    p = (unsigned char *)to;
+
+    *(p++) = 0;
+    *(p++) = 2;                 /* Public Key BT (Block Type) */
+
+    /* pad out with non-zero random data */
+    j = tlen - 3 - flen;
+
+    if (RAND_bytes_ex(libctx, p, j) <= 0)
+        return 0;
+    for (i = 0; i < j; i++) {
+        if (*p == '\0')
+            do {
+                if (RAND_bytes_ex(libctx, p, 1) <= 0)
+                    return 0;
+            } while (*p == '\0');
+        p++;
+    }
+
+    *(p++) = '\0';
+
+    memcpy(p, from, (unsigned int)flen);
+    return 1;
+}
+
+int RSA_padding_add_PKCS1_type_2(unsigned char *to, int tlen,
+                                 const unsigned char *from, int flen)
+{
+    return ossl_rsa_padding_add_PKCS1_type_2_ex(NULL, to, tlen, from, flen);
+}
 
+int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen,
+                                   const unsigned char *from, int flen,
+                                   int num)
+{
+    int i;
+    /* |em| is the encoded message, zero-padded to exactly |num| bytes */
+    unsigned char *em = NULL;
+    unsigned int good, found_zero_byte, mask;
+    int zero_index = 0, msg_index, mlen = -1;
+
+    if (tlen <= 0 || flen <= 0)
+        return -1;
+
+    /*
+     * PKCS#1 v1.5 decryption. See "PKCS #1 v2.2: RSA Cryptography Standard",
+     * section 7.2.2.
+     */
+
+    if (flen > num || num < RSA_PKCS1_PADDING_SIZE) {
+        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2,
+               RSA_R_PKCS_DECODING_ERROR);
+        return -1;
+    }
+
+    em = OPENSSL_malloc(num);
+    if (em == NULL) {
+        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2, 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;
+    }
+
+    good = constant_time_is_zero(em[0]);
+    good &= constant_time_eq(em[1], 2);
+
+    /* scan over padding data */
+    found_zero_byte = 0;
+    for (i = 2; i < num; i++) {
+        unsigned int equals0 = constant_time_is_zero(em[i]);
+
+        zero_index = constant_time_select_int(~found_zero_byte & equals0,
+                                              i, zero_index);
+        found_zero_byte |= equals0;
+    }
+
+    /*
+     * PS must be at least 8 bytes long, and it starts two bytes into |em|.
+     * 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);
+
+    /*
+     * 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);
+
+    /*
+     * Move the result in-place by |num|-RSA_PKCS1_PADDING_SIZE-|mlen| bytes to the left.
+     * Then if |good| move |mlen| bytes from |em|+RSA_PKCS1_PADDING_SIZE to |to|.
+     * Otherwise leave |to| unchanged.
+     * Copy the memory back in a way that does not reveal the size of
+     * the data being copied via a timing side channel. This requires copying
+     * parts of the buffer multiple times based on the bits set in the real
+     * length. Clear bits do a non-copy with identical access pattern.
+     * The loop below has overall complexity of O(N*log(N)).
+     */
+    tlen = constant_time_select_int(constant_time_lt(num - RSA_PKCS1_PADDING_SIZE, tlen),
+                                    num - RSA_PKCS1_PADDING_SIZE, tlen);
+    for (msg_index = 1; msg_index < num - RSA_PKCS1_PADDING_SIZE; msg_index <<= 1) {
+        mask = ~constant_time_eq(msg_index & (num - RSA_PKCS1_PADDING_SIZE - mlen), 0);
+        for (i = RSA_PKCS1_PADDING_SIZE; i < num - msg_index; i++)
+            em[i] = constant_time_select_8(mask, em[i + msg_index], em[i]);
+    }
+    for (i = 0; i < tlen; i++) {
+        mask = good & constant_time_lt(i, mlen);
+        to[i] = constant_time_select_8(mask, em[i + RSA_PKCS1_PADDING_SIZE], to[i]);
+    }
+
+    OPENSSL_clear_free(em, num);
+#ifndef FIPS_MODULE
+    /*
+     * This trick doesn't work in the FIPS provider because libcrypto manages
+     * the error stack. Instead we opt not to put an error on the stack at all
+     * in case of padding failure in the FIPS provider.
+     */
+    RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2, RSA_R_PKCS_DECODING_ERROR);
+    err_clear_last_constant_time(1 & good);
 #endif
 
-int RSA_padding_add_PKCS1_type_1(to,tlen,from,flen)
-unsigned char *to;
-int tlen;
-unsigned char *from;
-int flen;
-	{
-	int j;
-	unsigned char *p;
-
-	if (flen > (tlen-11))
-		{
-		RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_TYPE_1,RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
-		return(0);
-		}
-	
-	p=(unsigned char *)to;
-
-	*(p++)=0;
-	*(p++)=1; /* Private Key BT (Block Type) */
-
-	/* padd out with 0xff data */
-	j=tlen-3-flen;
-	memset(p,0xff,j);
-	p+=j;
-	*(p++)='\0';
-	memcpy(p,from,(unsigned int)flen);
-	return(1);
-	}
-
-int RSA_padding_check_PKCS1_type_1(to,tlen,from,flen)
-unsigned char *to;
-int tlen;
-unsigned char *from;
-int flen;
-	{
-	int i,j;
-	unsigned char *p;
-
-	p=from;
-	if (*(p++) != 01)
-		{
-		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1,RSA_R_BLOCK_TYPE_IS_NOT_01);
-		return(-1);
-		}
-
-	/* scan over padding data */
-	j=flen-1; /* one for type. */
-	for (i=0; i<j; i++)
-		{
-		if (*p != 0xff) /* should decrypt to 0xff */
-			{
-			if (*p == 0)
-				{ p++; break; }
-			else	{
-				RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1,RSA_R_BAD_FIXED_HEADER_DECRYPT);
-				return(-1);
-				}
-			}
-		p++;
-		}
-
-	if (i == j)
-		{
-		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1,RSA_R_NULL_BEFORE_BLOCK_MISSING);
-		return(-1);
-		}
-
-	if (i < 8)
-		{
-		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_1,RSA_R_BAD_PAD_BYTE_COUNT);
-		return(-1);
-		}
-	i++; /* Skip over the '\0' */
-	j-=i;
-	memcpy(to,p,(unsigned int)j);
-
-	return(j);
-	}
-
-int RSA_padding_add_PKCS1_type_2(to,tlen,from,flen)
-unsigned char *to;
-int tlen;
-unsigned char *from;
-int flen;
-	{
-	int i,j;
-	unsigned char *p;
-	
-	if (flen > (tlen-11))
-		{
-		RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_TYPE_2,RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
-		return(0);
-		}
-	
-	p=(unsigned char *)to;
-
-	*(p++)=0;
-	*(p++)=2; /* Public Key BT (Block Type) */
-
-	/* pad out with non-zero random data */
-	j=tlen-3-flen;
-
-	RAND_bytes(p,j);
-	for (i=0; i<j; i++)
-		{
-		if (*p == '\0')
-			do	{
-				RAND_bytes(p,1);
-				} while (*p == '\0');
-		p++;
-		}
-
-	*(p++)='\0';
-
-	memcpy(p,from,(unsigned int)flen);
-	return(1);
-	}
-
-int RSA_padding_check_PKCS1_type_2(to,tlen,from,flen)
-unsigned char *to;
-int tlen;
-unsigned char *from;
-int flen;
-	{
-	int i,j;
-	unsigned char *p;
-
-	p=from;
-	if (*(p++) != 02)
-		{
-		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2,RSA_R_BLOCK_TYPE_IS_NOT_02);
-		return(-1);
-		}
-
-	/* scan over padding data */
-	j=flen-1; /* one for type. */
-	for (i=0; i<j; i++)
-		if (*(p++) == 0) break;
-
-	if (i == j)
-		{
-		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2,RSA_R_NULL_BEFORE_BLOCK_MISSING);
-		return(-1);
-		}
-
-	if (i < 8)
-		{
-		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2,RSA_R_BAD_PAD_BYTE_COUNT);
-		return(-1);
-		}
-	i++; /* Skip over the '\0' */
-	j-=i;
-	memcpy(to,p,(unsigned int)j);
-
-	return(j);
-	}
+    return constant_time_select_int(good, mlen, -1);
+}
+
+/*
+ * ossl_rsa_padding_check_PKCS1_type_2_TLS() checks and removes the PKCS1 type 2
+ * padding from a decrypted RSA message in a TLS signature. The result is stored
+ * in the buffer pointed to by |to| which should be |tlen| bytes long. |tlen|
+ * must be at least SSL_MAX_MASTER_KEY_LENGTH. The original decrypted message
+ * should be stored in |from| which must be |flen| bytes in length and padded
+ * such that |flen == RSA_size()|. The TLS protocol version that the client
+ * originally requested should be passed in |client_version|. Some buggy clients
+ * can exist which use the negotiated version instead of the originally
+ * requested protocol version. If it is necessary to work around this bug then
+ * the negotiated protocol version can be passed in |alt_version|, otherwise 0
+ * should be passed.
+ *
+ * If the passed message is publicly invalid or some other error that can be
+ * treated in non-constant time occurs then -1 is returned. On success the
+ * length of the decrypted data is returned. This will always be
+ * SSL_MAX_MASTER_KEY_LENGTH. If an error occurs that should be treated in
+ * constant time then this function will appear to return successfully, but the
+ * decrypted data will be randomly generated (as per
+ * https://tools.ietf.org/html/rfc5246#section-7.4.7.1).
+ */
+int ossl_rsa_padding_check_PKCS1_type_2_TLS(OPENSSL_CTX *libctx,
+                                            unsigned char *to, size_t tlen,
+                                            const unsigned char *from,
+                                            size_t flen, int client_version,
+                                            int alt_version)
+{
+    unsigned int i, good, version_good;
+    unsigned char rand_premaster_secret[SSL_MAX_MASTER_KEY_LENGTH];
+
+    /*
+     * If these checks fail then either the message in publicly invalid, or
+     * we've been called incorrectly. We can fail immediately.
+     */
+    if (flen < RSA_PKCS1_PADDING_SIZE + SSL_MAX_MASTER_KEY_LENGTH
+            || tlen < SSL_MAX_MASTER_KEY_LENGTH) {
+        ERR_raise(ERR_LIB_RSA, RSA_R_PKCS_DECODING_ERROR);
+        return -1;
+    }
+
+    /*
+     * Generate a random premaster secret to use in the event that we fail
+     * to decrypt.
+     */
+    if (RAND_priv_bytes_ex(libctx, rand_premaster_secret,
+                           sizeof(rand_premaster_secret)) <= 0) {
+        ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR);
+        return -1;
+    }
+
+    good = constant_time_is_zero(from[0]);
+    good &= constant_time_eq(from[1], 2);
+
+    /* Check we have the expected padding data */
+    for (i = 2; i < flen - SSL_MAX_MASTER_KEY_LENGTH - 1; i++)
+        good &= ~constant_time_is_zero_8(from[i]);
+    good &= constant_time_is_zero_8(from[flen - SSL_MAX_MASTER_KEY_LENGTH - 1]);
+
+
+    /*
+     * If the version in the decrypted pre-master secret is correct then
+     * version_good will be 0xff, otherwise it'll be zero. The
+     * Klima-Pokorny-Rosa extension of Bleichenbacher's attack
+     * (http://eprint.iacr.org/2003/052/) exploits the version number
+     * check as a "bad version oracle". Thus version checks are done in
+     * constant time and are treated like any other decryption error.
+     */
+    version_good =
+        constant_time_eq(from[flen - SSL_MAX_MASTER_KEY_LENGTH],
+                         (client_version >> 8) & 0xff);
+    version_good &=
+        constant_time_eq(from[flen - SSL_MAX_MASTER_KEY_LENGTH + 1],
+                         client_version & 0xff);
+
+    /*
+     * The premaster secret must contain the same version number as the
+     * ClientHello to detect version rollback attacks (strangely, the
+     * protocol does not offer such protection for DH ciphersuites).
+     * However, buggy clients exist that send the negotiated protocol
+     * version instead if the server does not support the requested
+     * protocol version. If SSL_OP_TLS_ROLLBACK_BUG is set then we tolerate
+     * such clients. In that case alt_version will be non-zero and set to
+     * the negotiated version.
+     */
+    if (alt_version > 0) {
+        unsigned int workaround_good;
+
+        workaround_good =
+            constant_time_eq(from[flen - SSL_MAX_MASTER_KEY_LENGTH],
+                             (alt_version >> 8) & 0xff);
+        workaround_good &=
+            constant_time_eq(from[flen - SSL_MAX_MASTER_KEY_LENGTH + 1],
+                             alt_version & 0xff);
+        version_good |= workaround_good;
+    }
+
+    good &= version_good;
+
+
+    /*
+     * Now copy the result over to the to buffer if good, or random data if
+     * not good.
+     */
+    for (i = 0; i < SSL_MAX_MASTER_KEY_LENGTH; i++) {
+        to[i] =
+            constant_time_select_8(good,
+                                   from[flen - SSL_MAX_MASTER_KEY_LENGTH + i],
+                                   rand_premaster_secret[i]);
+    }
+
+    /*
+     * We must not leak whether a decryption failure occurs because of
+     * Bleichenbacher's attack on PKCS #1 v1.5 RSA padding (see RFC 2246,
+     * section 7.4.7.1). The code follows that advice of the TLS RFC and
+     * generates a random premaster secret for the case that the decrypt
+     * fails. See https://tools.ietf.org/html/rfc5246#section-7.4.7.1
+     * So, whether we actually succeeded or not, return success.
+     */
 
+    return SSL_MAX_MASTER_KEY_LENGTH;
+}