diff options
Diffstat (limited to 'crypto/ec/ec_key.c')
-rw-r--r-- | crypto/ec/ec_key.c | 76 |
1 files changed, 73 insertions, 3 deletions
diff --git a/crypto/ec/ec_key.c b/crypto/ec/ec_key.c index 08aaac5d8a..90698b9664 100644 --- a/crypto/ec/ec_key.c +++ b/crypto/ec/ec_key.c @@ -1,5 +1,5 @@ /* - * Copyright 2002-2018 The OpenSSL Project Authors. All Rights Reserved. + * Copyright 2002-2020 The OpenSSL Project Authors. All Rights Reserved. * Copyright (c) 2002, Oracle and/or its affiliates. All rights reserved * * Licensed under the OpenSSL license (the "License"). You may not use @@ -14,6 +14,7 @@ #include "internal/refcount.h" #include <openssl/err.h> #include <openssl/engine.h> +#include "crypto/bn.h" EC_KEY *EC_KEY_new(void) { @@ -416,17 +417,86 @@ const BIGNUM *EC_KEY_get0_private_key(const EC_KEY *key) int EC_KEY_set_private_key(EC_KEY *key, const BIGNUM *priv_key) { + int fixed_top; + const BIGNUM *order = NULL; + BIGNUM *tmp_key = NULL; + if (key->group == NULL || key->group->meth == NULL) return 0; + + /* + * Not only should key->group be set, but it should also be in a valid + * fully initialized state. + * + * Specifically, to operate in constant time, we need that the group order + * is set, as we use its length as the fixed public size of any scalar used + * as an EC private key. + */ + order = EC_GROUP_get0_order(key->group); + if (order == NULL || BN_is_zero(order)) + return 0; /* This should never happen */ + if (key->group->meth->set_private != NULL && key->group->meth->set_private(key, priv_key) == 0) return 0; if (key->meth->set_private != NULL && key->meth->set_private(key, priv_key) == 0) return 0; + + /* + * We should never leak the bit length of the secret scalar in the key, + * so we always set the `BN_FLG_CONSTTIME` flag on the internal `BIGNUM` + * holding the secret scalar. + * + * This is important also because `BN_dup()` (and `BN_copy()`) do not + * propagate the `BN_FLG_CONSTTIME` flag from the source `BIGNUM`, and + * this brings an extra risk of inadvertently losing the flag, even when + * the called specifically set it. + * + * The propagation has been turned on and off a few times in the past + * years because in some conditions has shown unintended consequences in + * some code paths, so at the moment we can't fix this in the BN layer. + * + * In `EC_KEY_set_private_key()` we can work around the propagation by + * manually setting the flag after `BN_dup()` as we know for sure that + * inside the EC module the `BN_FLG_CONSTTIME` is always treated + * correctly and should not generate unintended consequences. + * + * Setting the BN_FLG_CONSTTIME flag alone is never enough, we also have + * to preallocate the BIGNUM internal buffer to a fixed public size big + * enough that operations performed during the processing never trigger + * a realloc which would leak the size of the scalar through memory + * accesses. + * + * Fixed Length + * ------------ + * + * The order of the large prime subgroup of the curve is our choice for + * a fixed public size, as that is generally the upper bound for + * generating a private key in EC cryptosystems and should fit all valid + * secret scalars. + * + * For preallocating the BIGNUM storage we look at the number of "words" + * required for the internal representation of the order, and we + * preallocate 2 extra "words" in case any of the subsequent processing + * might temporarily overflow the order length. + */ + tmp_key = BN_dup(priv_key); + if (tmp_key == NULL) + return 0; + + BN_set_flags(tmp_key, BN_FLG_CONSTTIME); + + fixed_top = bn_get_top(order) + 2; + if (bn_wexpand(tmp_key, fixed_top) == NULL) { + BN_clear_free(tmp_key); + return 0; + } + BN_clear_free(key->priv_key); - key->priv_key = BN_dup(priv_key); - return (key->priv_key == NULL) ? 0 : 1; + key->priv_key = tmp_key; + + return 1; } const EC_POINT *EC_KEY_get0_public_key(const EC_KEY *key) |