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Add documentation for the functions

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Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
gabor-mezei-arm 2021-08-11 16:40:35 +02:00 committed by Gabor Mezei
parent 1ffd0ccf02
commit 7e6a1eaf8f
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GPG key ID: 106F5A41ECC305BD
2 changed files with 243 additions and 132 deletions
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132
library/constant_time.c
View file

@ -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,

243
library/constant_time.h
View file

@ -30,53 +30,241 @@
#include <stddef.h>
/** Constant-time buffer comparison without branches.
*
* This is equivalent to the standard memncmp function, but is likely to be
* compiled to code using bitwise operation rather than a branch.
*
* 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.
*
* \param a Pointer to the first buffer.
* \param b Pointer to the second buffer.
* \param n The number of bytes to compare in the buffer.
*
* \return Zero if the content of the two buffer is the same,
* otherwise non-zero.
*/
int mbedtls_cf_memcmp( const void *a,
const void *b,
size_t n );
/** Turn a value into a mask:
* - if \p value == 0, return the all-bits 0 mask, aka 0
* - otherwise, return the all-bits 1 mask, aka (size_t) -1
*
* 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.
*
* \param value The value to analyze.
*
* \return Zero if \p value is zero, otherwise all-bits-one.
*/
unsigned mbedtls_cf_uint_mask( unsigned value );
/** Turn a value into a mask:
* - if \p value == 0, return the all-bits 0 mask, aka 0
* - otherwise, return the all-bits 1 mask, aka (size_t) -1
*
* 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.
*
* \param value The value to analyze.
*
* \return Zero if \p value is zero, otherwise all-bits-one.
*/
size_t mbedtls_cf_size_mask( size_t value );
#if defined(MBEDTLS_BIGNUM_C)
/** Turn a value into a mask:
* - if \p value == 0, return the all-bits 0 mask, aka 0
* - otherwise, return the all-bits 1 mask, aka (size_t) -1
*
* 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.
*
* \param value The value to analyze.
*
* \return Zero if \p value is zero, otherwise all-bits-one.
*/
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 \p x < \p 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.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return All-bits-one if \p x is less than \p y, otherwise zero.
*/
size_t mbedtls_cf_size_mask_lt( size_t x,
size_t y );
/** Constant-flow mask generation for "greater or equal" comparison:
* - if \p x >= \p 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.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return All-bits-one if \p x is greater or equal than \p y,
* otherwise zero.
*/
size_t mbedtls_cf_size_mask_ge( size_t x,
size_t y );
/** Constant-flow boolean "equal" comparison:
* return x == y
*
* This is equivalent to \p x == \p y, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return 1 if \p x equals to \p y, otherwise 0.
*/
unsigned mbedtls_cf_size_bool_eq( size_t x,
size_t y );
/** Constant-flow "greater than" comparison:
* return x > y
*
* This is equivalent to \p x > \p y, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return 1 if \p x greater than \p y, otherwise 0.
*/
unsigned mbedtls_cf_size_gt( size_t x,
size_t y );
#if defined(MBEDTLS_BIGNUM_C)
/** Decide if an integer is less than the other, without branches.
*
* This is equivalent to \p x < \p y, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return 1 if \p x is less than \p y, otherwise 0.
*/
unsigned mbedtls_cf_mpi_uint_lt( const mbedtls_mpi_uint x,
const mbedtls_mpi_uint y );
#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.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \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 );
/** 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.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \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.
*/
size_t mbedtls_cf_size_if( unsigned condition,
size_t if1,
size_t if0 );
/** Select between two sign values witout branches.
*
* This is functionally equivalent to `condition ? if1 : if0` but uses only bit
* operations in order to avoid branches.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param condition Condition to test.
* \param if1 The first sign; must be either +1 or -1.
* \param 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 );
#if defined(MBEDTLS_BIGNUM_C)
/** Conditionally assign a value without branches.
*
* This is equivalent to `if ( condition ) dest = src`, but is likely
* to be compiled to code using bitwise operation rather than a branch.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param n \p dest and \p src must be arrays of limbs of size n.
* \param dest The MPI to conditionally assign to. This must point
* to an initialized MPI.
* \param src The MPI to be assigned from. This must point to an
* initialized MPI.
* \param condition Condition to test, must be 0 or 1.
*/
void mbedtls_cf_mpi_uint_cond_assign( size_t n,
mbedtls_mpi_uint *dest,
const mbedtls_mpi_uint *src,
@ -84,10 +272,41 @@ void mbedtls_cf_mpi_uint_cond_assign( size_t n,
#endif /* MBEDTLS_BIGNUM_C */
/** Shift some data towards the left inside a buffer.
*
* `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 );
/** Conditional memcpy without branches.
*
* This is equivalent to `if ( c1 == c2 ) memcpy(dst, src, len)`, but is likely
* to be compiled to code using bitwise operation rather than a branch.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param dest The pointer to conditionally copy to.
* \param src The pointer to copy from.
* \param len The number of bytes to copy.
* \param c1 The first value to analyze in the condition.
* \param c2 The second value to analyze in the condition.
*/
void mbedtls_cf_memcpy_if_eq( unsigned char *dest,
const unsigned char *src,
size_t len,
@ -98,6 +317,7 @@ void mbedtls_cf_memcpy_if_eq( unsigned char *dest,
* This function copies \p len bytes from \p src_base + \p offset_secret to \p
* dst, with a code flow and memory access pattern that does not depend on \p
* offset_secret, but only on \p offset_min, \p offset_max and \p len.
* Functionally equivalent to memcpy(dst, src + offset_secret, len).
*
* \param dst The destination buffer. This must point to a writable
* buffer of at least \p len bytes.
@ -166,6 +386,29 @@ int mbedtls_cf_hmac( mbedtls_md_context_t *ctx,
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
/** This function performs the unpadding part of a PKCS#1 v1.5 decryption
* operation (RSAES-PKCS1-v1_5-DECRYPT).
*
* \note The output buffer length \c output_max_len should be
* as large as the size \p ctx->len of \p ctx->N, for example,
* 128 Bytes if RSA-1024 is used, to be able to hold an
* arbitrary decrypted message. If it is not large enough to
* hold the decryption of the particular ciphertext provided,
* the function returns #MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE.
*
* \param mode The mode of operation. This must be either
* #MBEDTLS_RSA_PRIVATE or #MBEDTLS_RSA_PUBLIC (deprecated).
* \param ilen The length of the ciphertext.
* \param olen The address at which to store the length of
* the plaintext. This must not be \c NULL.
* \param output The buffer used to hold the plaintext. This must
* be a writable buffer of length \p output_max_len Bytes.
* \param output_max_len The length in Bytes of the output buffer \p output.
* \param buf The input buffer for the unpadding operation.
*
* \return \c 0 on success.
* \return An \c MBEDTLS_ERR_RSA_XXX error code on failure.
*/
int mbedtls_cf_rsaes_pkcs1_v15_unpadding( int mode,
size_t ilen,
size_t *olen,