### File openssl-One_and_Done.patch of Package openssl.11276

From 848113a30b431c2fe21ae8de2a366b9b6146fb92 Mon Sep 17 00:00:00 2001 From: User <milosprv@gmail.com> Date: Wed, 16 May 2018 13:59:36 -0400 Subject: [PATCH] bn/bn_exp.c: mitigation of the One-and-Done side-channel attack. The One&Done attack, which is described in a paper to appear in the USENIX Security'18 conference, uses EM emanations to recover the values of the bits that are obtained using BN_is_bit_set while constructing the value of the window in BN_mod_exp_consttime. The EM signal changes slightly depending on the value of the bit, and since the lookup of a bit is surrounded by highly regular execution (constant-time Montgomery multiplications) the attack is able to isolate the (very brief) part of the signal that changes depending on the bit. Although the change is slight, the attack recovers it successfully >90% of the time on several phones and IoT devices (all with ARM processors with clock rates around 1GHz), so after only one RSA decryption more than 90% of the bits in d_p and d_q are recovered correctly, which enables rapid recovery of the full RSA key using an algorithm (also described in the paper) that modifies the branch-and-prune approach for a situation in which the exponents' bits are recovered with errors, i.e. where we do not know a priori which bits are correctly recovered. The mitigation for the attack is relatively simple - all the bits of the window are obtained at once, along with other bits so that an entire integer's worth of bits are obtained together using masking and shifts, without unnecessarily considering each bit in isolation. This improves performance somewhat (one call to bn_get_bits is faster than several calls to BN_is_bit_set), so the attacker now gets one signal snippet per window (rather than one per bit) in which the signal is affected by all bits in the integer (rather than just the one bit). Reviewed-by: Andy Polyakov <appro@openssl.org> Reviewed-by: Rich Salz <rsalz@openssl.org> (Merged from https://github.com/openssl/openssl/pull/6276) --- crypto/bn/bn_exp.c | 39 +++++++++++++++++++++++++++------------ 1 file changed, 27 insertions(+), 12 deletions(-) Index: openssl-1.0.1i/crypto/bn/bn_exp.c =================================================================== --- openssl-1.0.1i.orig/crypto/bn/bn_exp.c 2018-11-15 12:51:59.095177140 +0100 +++ openssl-1.0.1i/crypto/bn/bn_exp.c 2018-11-15 13:54:16.378315901 +0100 @@ -529,6 +529,24 @@ err: return(ret); } +static BN_ULONG bn_get_bits(const BIGNUM *a, int bitpos) +{ + BN_ULONG ret = 0; + int wordpos; + + wordpos = bitpos / BN_BITS2; + bitpos %= BN_BITS2; + if (wordpos >= 0 && wordpos < a->top) { + ret = a->d[wordpos] & BN_MASK2; + if (bitpos) { + ret >>= bitpos; + if (++wordpos < a->top) + ret |= a->d[wordpos] << (BN_BITS2 - bitpos); + } + } + + return ret & BN_MASK2; +} /* BN_mod_exp_mont_consttime() stores the precomputed powers in a specific layout * so that accessing any of these table values shows the same access pattern as far @@ -616,7 +634,7 @@ static int MOD_EXP_CTIME_COPY_FROM_PREBU int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *in_mont) { - int i,bits,ret=0,window,wvalue; + int i,bits,ret=0,window,wvalue,wmask,window0; int top; BN_MONT_CTX *mont=NULL; @@ -828,34 +846,48 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr } } - bits--; - for (wvalue = 0, i = bits % window; i >= 0; i--, bits--) - wvalue = (wvalue << 1) + BN_is_bit_set(p, bits); + /* + * The exponent may not have a whole number of fixed-size windows. + * To simplify the main loop, the initial window has between 1 and + * full-window-size bits such that what remains is always a whole + * number of windows + */ + window0 = (bits - 1) % window + 1; + wmask = (1 << window0) - 1; + bits -= window0; + wvalue = bn_get_bits(p, bits) & wmask; if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&tmp, top, powerbuf, wvalue, window)) goto err; + wmask = (1 << window) - 1; /* Scan the exponent one window at a time starting from the most * significant bits. */ - while (bits >= 0) - { - wvalue=0; /* The 'value' of the window */ - - /* Scan the window, squaring the result as we go */ - for (i=0; i<window; i++,bits--) - { + while (bits > 0) { + /* Square the result window-size times */ + for (i = 0; i < window; i++) if (!BN_mod_mul_montgomery(&tmp,&tmp,&tmp,mont,ctx)) goto err; - wvalue = (wvalue<<1)+BN_is_bit_set(p,bits); - } + /* + * Get a window's worth of bits from the exponent + * This avoids calling BN_is_bit_set for each bit, which + * is not only slower but also makes each bit vulnerable to + * EM (and likely other) side-channel attacks like One&Done + * (for details see "One&Done: A Single-Decryption EM-Based + * Attack on OpenSSLâ€™s Constant-Time Blinded RSA" by M. Alam, + * H. Khan, M. Dey, N. Sinha, R. Callan, A. Zajic, and + * M. Prvulovic, in USENIX Security'18) + */ + bits -= window; + wvalue = bn_get_bits(p, bits) & wmask; /* Fetch the appropriate pre-computed value from the pre-buf */ if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&am, top, powerbuf, wvalue, window)) goto err; /* Multiply the result into the intermediate result */ if (!BN_mod_mul_montgomery(&tmp,&tmp,&am,mont,ctx)) goto err; - } + } } /* Convert the final result from montgomery to standard format */