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Add documentation for the functions

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Signed-off-by: gabor-mezei-arm <gabor.mezei@arm.com>
This commit is contained in:
gabor-mezei-arm
2021-08-11 16:40:35 +02:00
parent b11a56e34c
commit 90d96cc741
2 changed files with 241 additions and 132 deletions
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132
library/constant_time.c
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@ -52,11 +52,6 @@ int mbedtls_cf_memcmp( const void *a,
return( (int)diff );
}
/** Turn zero-or-nonzero into zero-or-all-bits-one, without branches.
*
* \param value The value to analyze.
* \return Zero if \p value is zero, otherwise all-bits-one.
*/
unsigned mbedtls_cf_uint_mask( unsigned value )
{
/* MSVC has a warning about unary minus on unsigned, but this is
@ -71,17 +66,6 @@ unsigned mbedtls_cf_uint_mask( unsigned value )
#endif
}
/*
* Turn a value into a mask:
* - if value != 0, return the all-bits 1 mask, aka (size_t) -1
* - if value == 0, return the all-bits 0 mask, aka 0
*
* This function can be used to write constant-time code by replacing branches
* with bit operations using masks.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
size_t mbedtls_cf_size_mask( size_t value )
{
/* MSVC has a warning about unary minus on unsigned integer types,
@ -114,17 +98,6 @@ mbedtls_mpi_uint mbedtls_cf_mpi_uint_mask( mbedtls_mpi_uint value )
#endif /* MBEDTLS_BIGNUM_C */
/*
* Constant-flow mask generation for "less than" comparison:
* - if x < y, return all bits 1, that is (size_t) -1
* - otherwise, return all bits 0, that is 0
*
* This function can be used to write constant-time code by replacing branches
* with bit operations using masks.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
size_t mbedtls_cf_size_mask_lt( size_t x,
size_t y )
{
@ -140,34 +113,12 @@ size_t mbedtls_cf_size_mask_lt( size_t x,
return( mask );
}
/*
* Constant-flow mask generation for "greater or equal" comparison:
* - if x >= y, return all bits 1, that is (size_t) -1
* - otherwise, return all bits 0, that is 0
*
* This function can be used to write constant-time code by replacing branches
* with bit operations using masks.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
size_t mbedtls_cf_size_mask_ge( size_t x,
size_t y )
{
return( ~mbedtls_cf_size_mask_lt( x, y ) );
}
/*
* 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.
*/
unsigned mbedtls_cf_size_bool_eq( size_t x,
size_t y )
{
@ -194,16 +145,6 @@ unsigned mbedtls_cf_size_bool_eq( size_t x,
return( 1 ^ diff1 );
}
/** Check whether a size is out of bounds, without branches.
*
* This is equivalent to `x > y`, but is likely to be compiled to
* to code using bitwise operation rather than a branch.
*
* \param x Size to check.
* \param y Maximum desired value for \p size.
* \return \c 0 if `x <= y`.
* \return \c 1 if `x > y`.
*/
unsigned mbedtls_cf_size_gt( size_t x,
size_t y )
{
@ -213,13 +154,6 @@ unsigned mbedtls_cf_size_gt( size_t x,
#if defined(MBEDTLS_BIGNUM_C)
/** Decide if an integer is less than the other, without branches.
*
* \param x First integer.
* \param y Second integer.
*
* \return 1 if \p x is less than \p y, 0 otherwise
*/
unsigned mbedtls_cf_mpi_uint_lt( const mbedtls_mpi_uint x,
const mbedtls_mpi_uint y )
{
@ -250,17 +184,6 @@ unsigned mbedtls_cf_mpi_uint_lt( const mbedtls_mpi_uint x,
#endif /* MBEDTLS_BIGNUM_C */
/** Choose between two integer values, without branches.
*
* This is equivalent to `condition ? if1 : if0`, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* \param condition Condition to test.
* \param if1 Value to use if \p condition is nonzero.
* \param if0 Value to use if \p condition is zero.
* \return \c if1 if \p condition is nonzero, otherwise \c if0.
*/
unsigned mbedtls_cf_uint_if( unsigned condition,
unsigned if1,
unsigned if0 )
@ -277,18 +200,6 @@ size_t mbedtls_cf_size_if( unsigned condition,
return( ( mask & if1 ) | (~mask & if0 ) );
}
/**
* Select between two sign values in constant-time.
*
* This is functionally equivalent to condition ? if1 : if0 but uses only bit
* operations in order to avoid branches.
*
* \param[in] condition Must be either 1 (return \p if1) or 0 (return \pp if0).
* \param[in] if1 The first sign; must be either +1 or -1.
* \param[in] if0 The second sign; must be either +1 or -1.
*
* \return \c if1 if \p condition is nonzero, otherwise \c if0.
*/
int mbedtls_cf_cond_select_sign( unsigned char condition,
int if1,
int if0 )
@ -311,12 +222,6 @@ int mbedtls_cf_cond_select_sign( unsigned char condition,
#if defined(MBEDTLS_BIGNUM_C)
/*
* Conditionally assign dest = src, without leaking information
* about whether the assignment was made or not.
* dest and src must be arrays of limbs of size n.
* condition must be 0 or 1.
*/
void mbedtls_cf_mpi_uint_cond_assign( size_t n,
mbedtls_mpi_uint *dest,
const mbedtls_mpi_uint *src,
@ -344,23 +249,6 @@ void mbedtls_cf_mpi_uint_cond_assign( size_t n,
#endif /* MBEDTLS_BIGNUM_C */
/** Shift some data towards the left inside a buffer without leaking
* the length of the data through side channels.
*
* `mbedtls_cf_mem_move_to_left(start, total, offset)` is functionally
* equivalent to
* ```
* memmove(start, start + offset, total - offset);
* memset(start + offset, 0, total - offset);
* ```
* but it strives to use a memory access pattern (and thus total timing)
* that does not depend on \p offset. This timing independence comes at
* the expense of performance.
*
* \param start Pointer to the start of the buffer.
* \param total Total size of the buffer.
* \param offset Offset from which to copy \p total - \p offset bytes.
*/
void mbedtls_cf_mem_move_to_left( void *start,
size_t total,
size_t offset )
@ -385,15 +273,6 @@ void mbedtls_cf_mem_move_to_left( void *start,
}
}
/*
* Constant-flow conditional memcpy:
* - if c1 == c2, equivalent to memcpy(dst, src, len),
* - otherwise, a no-op,
* but with execution flow independent of the values of c1 and c2.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
void mbedtls_cf_memcpy_if_eq( unsigned char *dest,
const unsigned char *src,
size_t len,
@ -409,11 +288,6 @@ void mbedtls_cf_memcpy_if_eq( unsigned char *dest,
dest[i] = ( src[i] & mask ) | ( dest[i] & ~mask );
}
/*
* Constant-flow memcpy from variable position in buffer.
* - functionally equivalent to memcpy(dst, src + offset_secret, len)
* - but with execution flow independent from the value of offset_secret.
*/
void mbedtls_cf_memcpy_offset( unsigned char *dst,
const unsigned char *src_base,
size_t offset_secret,
@ -432,12 +306,6 @@ void mbedtls_cf_memcpy_offset( unsigned char *dst,
#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
/*
* Compute HMAC of variable-length data with constant flow.
*
* Only works with MD-5, SHA-1, SHA-256 and SHA-384.
* (Otherwise, computation of block_size needs to be adapted.)
*/
int mbedtls_cf_hmac( mbedtls_md_context_t *ctx,
const unsigned char *add_data,
size_t add_data_len,