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Merge pull request #828 from mpg/rsa-lookup-restricted

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Use constant-time look-up in modular exponentiation
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Manuel Pégourié-Gonnard 2021-06-22 09:33:20 +02:00 committed by GitHub
commit da1eab3c3f
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2 changed files with 158 additions and 11 deletions
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6
ChangeLog.d/fix-rsa-leak.txt Normal file
View file

@ -0,0 +1,6 @@
Security
* An adversary with access to precise enough information about memory
accesses (typically, an untrusted operating system attacking a secure
enclave) could recover an RSA private key after observing the victim
performing a single private-key operation. Found and reported by
Zili KOU, Wenjian HE, Sharad Sinha, and Wei ZHANG.

163
library/bignum.c
View file

@ -258,6 +258,36 @@ void mbedtls_mpi_swap( mbedtls_mpi *X, mbedtls_mpi *Y )
memcpy( Y, &T, sizeof( mbedtls_mpi ) );
}
/**
* Select between two sign values in constant-time.
*
* This is functionally equivalent to second ? a : b but uses only bit
* operations in order to avoid branches.
*
* \param[in] a The first sign; must be either +1 or -1.
* \param[in] b The second sign; must be either +1 or -1.
* \param[in] second Must be either 1 (return b) or 0 (return a).
*
* \return The selected sign value.
*/
static int mpi_safe_cond_select_sign( int a, int b, unsigned char second )
{
/* In order to avoid questions about what we can reasonnably assume about
* the representations of signed integers, move everything to unsigned
* by taking advantage of the fact that a and b are either +1 or -1. */
unsigned ua = a + 1;
unsigned ub = b + 1;
/* second was 0 or 1, mask is 0 or 2 as are ua and ub */
const unsigned mask = second << 1;
/* select ua or ub */
unsigned ur = ( ua & ~mask ) | ( ub & mask );
/* ur is now 0 or 2, convert back to -1 or +1 */
return( (int) ur - 1 );
}
/*
* Conditionally assign dest = src, without leaking information
* about whether the assignment was made or not.
@ -270,8 +300,23 @@ static void mpi_safe_cond_assign( size_t n,
unsigned char assign )
{
size_t i;
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
const mbedtls_mpi_uint mask = -assign;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
for( i = 0; i < n; i++ )
dest[i] = dest[i] * ( 1 - assign ) + src[i] * assign;
dest[i] = ( src[i] & mask ) | ( dest[i] & ~mask );
}
/*
@ -283,20 +328,34 @@ int mbedtls_mpi_safe_cond_assign( mbedtls_mpi *X, const mbedtls_mpi *Y, unsigned
{
int ret = 0;
size_t i;
mbedtls_mpi_uint limb_mask;
MPI_VALIDATE_RET( X != NULL );
MPI_VALIDATE_RET( Y != NULL );
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
/* make sure assign is 0 or 1 in a time-constant manner */
assign = (assign | (unsigned char)-assign) >> 7;
assign = (assign | (unsigned char)-assign) >> (sizeof( assign ) * 8 - 1);
/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
limb_mask = -assign;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) );
X->s = X->s * ( 1 - assign ) + Y->s * assign;
X->s = mpi_safe_cond_select_sign( X->s, Y->s, assign );
mpi_safe_cond_assign( Y->n, X->p, Y->p, assign );
for( i = Y->n; i < X->n; i++ )
X->p[i] *= ( 1 - assign );
X->p[i] &= ~limb_mask;
cleanup:
return( ret );
@ -312,6 +371,7 @@ int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X, mbedtls_mpi *Y, unsigned char sw
{
int ret, s;
size_t i;
mbedtls_mpi_uint limb_mask;
mbedtls_mpi_uint tmp;
MPI_VALIDATE_RET( X != NULL );
MPI_VALIDATE_RET( Y != NULL );
@ -319,22 +379,35 @@ int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X, mbedtls_mpi *Y, unsigned char sw
if( X == Y )
return( 0 );
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
/* make sure swap is 0 or 1 in a time-constant manner */
swap = (swap | (unsigned char)-swap) >> 7;
swap = (swap | (unsigned char)-swap) >> (sizeof( swap ) * 8 - 1);
/* all-bits 1 if swap is 1, all-bits 0 if swap is 0 */
limb_mask = -swap;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( Y, X->n ) );
s = X->s;
X->s = X->s * ( 1 - swap ) + Y->s * swap;
Y->s = Y->s * ( 1 - swap ) + s * swap;
X->s = mpi_safe_cond_select_sign( X->s, Y->s, swap );
Y->s = mpi_safe_cond_select_sign( Y->s, s, swap );
for( i = 0; i < X->n; i++ )
{
tmp = X->p[i];
X->p[i] = X->p[i] * ( 1 - swap ) + Y->p[i] * swap;
Y->p[i] = Y->p[i] * ( 1 - swap ) + tmp * swap;
X->p[i] = ( X->p[i] & ~limb_mask ) | ( Y->p[i] & limb_mask );
Y->p[i] = ( Y->p[i] & ~limb_mask ) | ( tmp & limb_mask );
}
cleanup:
@ -2126,6 +2199,71 @@ static void mpi_montred( mbedtls_mpi *A, const mbedtls_mpi *N,
mpi_montmul( A, &U, N, mm, T );
}
/*
* Constant-flow boolean "equal" comparison:
* return x == y
*
* This function can be used to write constant-time code by replacing branches
* with bit operations - it can be used in conjunction with
* mbedtls_ssl_cf_mask_from_bit().
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
static size_t mbedtls_mpi_cf_bool_eq( size_t x, size_t y )
{
/* diff = 0 if x == y, non-zero otherwise */
const size_t diff = x ^ y;
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
/* diff_msb's most significant bit is equal to x != y */
const size_t diff_msb = ( diff | (size_t) -diff );
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
/* diff1 = (x != y) ? 1 : 0 */
const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
return( 1 ^ diff1 );
}
/**
* Select an MPI from a table without leaking the index.
*
* This is functionally equivalent to mbedtls_mpi_copy(R, T[idx]) except it
* reads the entire table in order to avoid leaking the value of idx to an
* attacker able to observe memory access patterns.
*
* \param[out] R Where to write the selected MPI.
* \param[in] T The table to read from.
* \param[in] T_size The number of elements in the table.
* \param[in] idx The index of the element to select;
* this must satisfy 0 <= idx < T_size.
*
* \return \c 0 on success, or a negative error code.
*/
static int mpi_select( mbedtls_mpi *R, const mbedtls_mpi *T, size_t T_size, size_t idx )
{
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
for( size_t i = 0; i < T_size; i++ )
{
MBEDTLS_MPI_CHK( mbedtls_mpi_safe_cond_assign( R, &T[i],
(unsigned char) mbedtls_mpi_cf_bool_eq( i, idx ) ) );
}
cleanup:
return( ret );
}
/*
* Sliding-window exponentiation: X = A^E mod N (HAC 14.85)
*/
@ -2138,7 +2276,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
size_t i, j, nblimbs;
size_t bufsize, nbits;
mbedtls_mpi_uint ei, mm, state;
mbedtls_mpi RR, T, W[ 1 << MBEDTLS_MPI_WINDOW_SIZE ], Apos;
mbedtls_mpi RR, T, W[ 1 << MBEDTLS_MPI_WINDOW_SIZE ], WW, Apos;
int neg;
MPI_VALIDATE_RET( X != NULL );
@ -2162,6 +2300,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
mpi_montg_init( &mm, N );
mbedtls_mpi_init( &RR ); mbedtls_mpi_init( &T );
mbedtls_mpi_init( &Apos );
mbedtls_mpi_init( &WW );
memset( W, 0, sizeof( W ) );
i = mbedtls_mpi_bitlen( E );
@ -2302,7 +2441,8 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
/*
* X = X * W[wbits] R^-1 mod N
*/
mpi_montmul( X, &W[wbits], N, mm, &T );
MBEDTLS_MPI_CHK( mpi_select( &WW, W, (size_t) 1 << wsize, wbits ) );
mpi_montmul( X, &WW, N, mm, &T );
state--;
nbits = 0;
@ -2340,6 +2480,7 @@ cleanup:
mbedtls_mpi_free( &W[i] );
mbedtls_mpi_free( &W[1] ); mbedtls_mpi_free( &T ); mbedtls_mpi_free( &Apos );
mbedtls_mpi_free( &WW );
if( _RR == NULL || _RR->p == NULL )
mbedtls_mpi_free( &RR );