/* * Copyright 1995-2021 The OpenSSL Project Authors. All Rights Reserved. * * 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 #include #include #include /* Just for the SSL_MAX_MASTER_KEY_LENGTH value */ #include #include #include #include #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)) { ERR_raise(ERR_LIB_RSA, 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) { ERR_raise(ERR_LIB_RSA, RSA_R_INVALID_PADDING); return -1; } flen--; } if ((num != (flen + 1)) || (*(p++) != 0x01)) { ERR_raise(ERR_LIB_RSA, 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 { ERR_raise(ERR_LIB_RSA, RSA_R_BAD_FIXED_HEADER_DECRYPT); return -1; } } p++; } if (i == j) { ERR_raise(ERR_LIB_RSA, RSA_R_NULL_BEFORE_BLOCK_MISSING); return -1; } if (i < 8) { ERR_raise(ERR_LIB_RSA, RSA_R_BAD_PAD_BYTE_COUNT); return -1; } i++; /* Skip over the '\0' */ j -= i; if (j > tlen) { ERR_raise(ERR_LIB_RSA, RSA_R_DATA_TOO_LARGE); return -1; } memcpy(to, p, (unsigned int)j); return j; } int ossl_rsa_padding_add_PKCS1_type_2_ex(OSSL_LIB_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)) { ERR_raise(ERR_LIB_RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE); return 0; } else if (flen < 0) { ERR_raise(ERR_LIB_RSA, RSA_R_INVALID_LENGTH); 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) <= 0) return 0; for (i = 0; i < j; i++) { if (*p == '\0') do { if (RAND_bytes_ex(libctx, p, 1, 0) <= 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) { ERR_raise(ERR_LIB_RSA, RSA_R_PKCS_DECODING_ERROR); return -1; } em = OPENSSL_malloc(num); if (em == NULL) 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. */ ERR_raise(ERR_LIB_RSA, RSA_R_PKCS_DECODING_ERROR); err_clear_last_constant_time(1 & good); #endif return constant_time_select_int(good, mlen, -1); } static int ossl_rsa_prf(OSSL_LIB_CTX *ctx, unsigned char *to, int tlen, const char *label, int llen, const unsigned char *kdk, uint16_t bitlen) { int pos; int ret = -1; uint16_t iter = 0; unsigned char be_iter[sizeof(iter)]; unsigned char be_bitlen[sizeof(bitlen)]; HMAC_CTX *hmac = NULL; EVP_MD *md = NULL; unsigned char hmac_out[SHA256_DIGEST_LENGTH]; unsigned int md_len; if (tlen * 8 != bitlen) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); return ret; } be_bitlen[0] = (bitlen >> 8) & 0xff; be_bitlen[1] = bitlen & 0xff; hmac = HMAC_CTX_new(); if (hmac == NULL) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); goto err; } /* * we use hardcoded hash so that migrating between versions that use * different hash doesn't provide a Bleichenbacher oracle: * if the attacker can see that different versions return different * messages for the same ciphertext, they'll know that the message is * syntethically generated, which means that the padding check failed */ md = EVP_MD_fetch(ctx, "sha256", NULL); if (md == NULL) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); goto err; } if (HMAC_Init_ex(hmac, kdk, SHA256_DIGEST_LENGTH, md, NULL) <= 0) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); goto err; } for (pos = 0; pos < tlen; pos += SHA256_DIGEST_LENGTH, iter++) { if (HMAC_Init_ex(hmac, NULL, 0, NULL, NULL) <= 0) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); goto err; } be_iter[0] = (iter >> 8) & 0xff; be_iter[1] = iter & 0xff; if (HMAC_Update(hmac, be_iter, sizeof(be_iter)) <= 0) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); goto err; } if (HMAC_Update(hmac, (unsigned char *)label, llen) <= 0) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); goto err; } if (HMAC_Update(hmac, be_bitlen, sizeof(be_bitlen)) <= 0) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); goto err; } /* * HMAC_Final requires the output buffer to fit the whole MAC * value, so we need to use the intermediate buffer for the last * unaligned block */ md_len = SHA256_DIGEST_LENGTH; if (pos + SHA256_DIGEST_LENGTH > tlen) { if (HMAC_Final(hmac, hmac_out, &md_len) <= 0) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); goto err; } memcpy(to + pos, hmac_out, tlen - pos); } else { if (HMAC_Final(hmac, to + pos, &md_len) <= 0) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); goto err; } } } ret = 0; err: HMAC_CTX_free(hmac); EVP_MD_free(md); return ret; } /* * ossl_rsa_padding_check_PKCS1_type_2() checks and removes the PKCS#1 type 2 * padding from a decrypted RSA message. Unlike the * RSA_padding_check_PKCS1_type_2() it will not return an error in case it * detects a padding error, rather it will return a deterministically generated * random message. In other words it will perform an implicit rejection * of an invalid padding. This means that the returned value does not indicate * if the padding of the encrypted message was correct or not, making * side channel attacks like the ones described by Bleichenbacher impossible * without access to the full decrypted value and a brute-force search of * remaining padding bytes */ int ossl_rsa_padding_check_PKCS1_type_2(OSSL_LIB_CTX *ctx, unsigned char *to, int tlen, const unsigned char *from, int flen, int num, unsigned char *kdk) { /* * We need to generate a random length for the synthethic message, to avoid * bias towards zero and avoid non-constant timeness of DIV, we prepare * 128 values to check if they are not too large for the used key size, * and use 0 in case none of them are small enough, as 2^-128 is a good enough * safety margin */ #define MAX_LEN_GEN_TRIES 128 unsigned char *synthetic = NULL; int synthethic_length; uint16_t len_candidate; unsigned char candidate_lengths[MAX_LEN_GEN_TRIES * sizeof(len_candidate)]; uint16_t len_mask; uint16_t max_sep_offset; int synth_msg_index = 0; int ret = -1; int i, j; unsigned int good, found_zero_byte; int zero_index = 0, msg_index; /* * If these checks fail then either the message in publicly invalid, or * we've been called incorrectly. We can fail immediately. * Since this code is called only internally by openssl, those are just * sanity checks */ if (num != flen || tlen <= 0 || flen <= 0) { ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); return -1; } /* Generate a random message to return in case the padding checks fail */ synthetic = OPENSSL_malloc(flen); if (synthetic == NULL) { ERR_raise(ERR_LIB_RSA, ERR_R_MALLOC_FAILURE); return -1; } if (ossl_rsa_prf(ctx, synthetic, flen, "message", 7, kdk, flen * 8) < 0) goto err; /* decide how long the random message should be */ if (ossl_rsa_prf(ctx, candidate_lengths, sizeof(candidate_lengths), "length", 6, kdk, MAX_LEN_GEN_TRIES * sizeof(len_candidate) * 8) < 0) goto err; /* * max message size is the size of the modulus size less 2 bytes for * version and padding type and a minimum of 8 bytes padding */ len_mask = max_sep_offset = flen - 2 - 8; /* * we want a mask so lets propagate the high bit to all positions less * significant than it */ len_mask |= len_mask >> 1; len_mask |= len_mask >> 2; len_mask |= len_mask >> 4; len_mask |= len_mask >> 8; synthethic_length = 0; for (i = 0; i < MAX_LEN_GEN_TRIES * (int)sizeof(len_candidate); i += sizeof(len_candidate)) { len_candidate = (candidate_lengths[i] << 8) | candidate_lengths[i + 1]; len_candidate &= len_mask; synthethic_length = constant_time_select_int( constant_time_lt(len_candidate, max_sep_offset), len_candidate, synthethic_length); } synth_msg_index = flen - synthethic_length; /* we have alternative message ready, check the real one */ good = constant_time_is_zero(from[0]); good &= constant_time_eq(from[1], 2); /* then look for the padding|message separator (the first zero byte) */ found_zero_byte = 0; for (i = 2; i < flen; i++) { unsigned int equals0 = constant_time_is_zero(from[i]); zero_index = constant_time_select_int(~found_zero_byte & equals0, i, zero_index); found_zero_byte |= equals0; } /* * padding 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); /* * 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; /* * old code returned an error in case the decrypted message wouldn't fit * into the |to|, since that would leak information, return the synthethic * message instead */ good &= constant_time_ge(tlen, num - msg_index); msg_index = constant_time_select_int(good, msg_index, synth_msg_index); /* * since at this point the |msg_index| does not provide the signal * indicating if the padding check failed or not, we don't have to worry * about leaking the length of returned message, we still need to ensure * that we read contents of both buffers so that cache accesses don't leak * the value of |good| */ for (i = msg_index, j = 0; i < flen && j < tlen; i++, j++) to[j] = constant_time_select_8(good, from[i], synthetic[i]); ret = j; err: /* * the only time ret < 0 is when the ciphertext is publicly invalid * or we were called with invalid parameters, so we don't have to perform * a side-channel secure raising of the error */ if (ret < 0) ERR_raise(ERR_LIB_RSA, ERR_R_INTERNAL_ERROR); OPENSSL_free(synthetic); return ret; } /* * 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(OSSL_LIB_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) <= 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; }