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Move mbedtls_cf_rsaes_pkcs1_v15_unpadding function to the constant-time module

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Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
gabor-mezei-arm 2021-09-27 16:11:12 +02:00 committed by Gabor Mezei
parent c2aee6fc0b
commit f52941ef73
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GPG key ID: 106F5A41ECC305BD
3 changed files with 161 additions and 145 deletions
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150
library/constant_time.c
View file

@ -29,6 +29,7 @@
#include "mbedtls/ssl_internal.h"
#endif
#include <string.h>
/* constant-time buffer comparison */
int mbedtls_ssl_safer_memcmp( const void *a, const void *b, size_t n )
@ -710,3 +711,152 @@ int mbedtls_mpi_lt_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
}
#endif /* MBEDTLS_BIGNUM_C */
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
int mbedtls_cf_rsaes_pkcs1_v15_unpadding( int mode,
size_t ilen,
size_t *olen,
unsigned char *output,
size_t output_max_len,
unsigned char *buf )
{
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
size_t i, plaintext_max_size;
/* The following variables take sensitive values: their value must
* not leak into the observable behavior of the function other than
* the designated outputs (output, olen, return value). Otherwise
* this would open the execution of the function to
* side-channel-based variants of the Bleichenbacher padding oracle
* attack. Potential side channels include overall timing, memory
* access patterns (especially visible to an adversary who has access
* to a shared memory cache), and branches (especially visible to
* an adversary who has access to a shared code cache or to a shared
* branch predictor). */
size_t pad_count = 0;
unsigned bad = 0;
unsigned char pad_done = 0;
size_t plaintext_size = 0;
unsigned output_too_large;
plaintext_max_size = mbedtls_cf_size_if( output_max_len > ilen - 11,
ilen - 11,
output_max_len );
/* Check and get padding length in constant time and constant
* memory trace. The first byte must be 0. */
bad |= buf[0];
if( mode == MBEDTLS_RSA_PRIVATE )
{
/* Decode EME-PKCS1-v1_5 padding: 0x00 || 0x02 || PS || 0x00
* where PS must be at least 8 nonzero bytes. */
bad |= buf[1] ^ MBEDTLS_RSA_CRYPT;
/* Read the whole buffer. Set pad_done to nonzero if we find
* the 0x00 byte and remember the padding length in pad_count. */
for( i = 2; i < ilen; i++ )
{
pad_done |= ((buf[i] | (unsigned char)-buf[i]) >> 7) ^ 1;
pad_count += ((pad_done | (unsigned char)-pad_done) >> 7) ^ 1;
}
}
else
{
/* Decode EMSA-PKCS1-v1_5 padding: 0x00 || 0x01 || PS || 0x00
* where PS must be at least 8 bytes with the value 0xFF. */
bad |= buf[1] ^ MBEDTLS_RSA_SIGN;
/* Read the whole buffer. Set pad_done to nonzero if we find
* the 0x00 byte and remember the padding length in pad_count.
* If there's a non-0xff byte in the padding, the padding is bad. */
for( i = 2; i < ilen; i++ )
{
pad_done |= mbedtls_cf_uint_if( buf[i], 0, 1 );
pad_count += mbedtls_cf_uint_if( pad_done, 0, 1 );
bad |= mbedtls_cf_uint_if( pad_done, 0, buf[i] ^ 0xFF );
}
}
/* If pad_done is still zero, there's no data, only unfinished padding. */
bad |= mbedtls_cf_uint_if( pad_done, 0, 1 );
/* There must be at least 8 bytes of padding. */
bad |= mbedtls_cf_size_gt( 8, pad_count );
/* If the padding is valid, set plaintext_size to the number of
* remaining bytes after stripping the padding. If the padding
* is invalid, avoid leaking this fact through the size of the
* output: use the maximum message size that fits in the output
* buffer. Do it without branches to avoid leaking the padding
* validity through timing. RSA keys are small enough that all the
* size_t values involved fit in unsigned int. */
plaintext_size = mbedtls_cf_uint_if(
bad, (unsigned) plaintext_max_size,
(unsigned) ( ilen - pad_count - 3 ) );
/* Set output_too_large to 0 if the plaintext fits in the output
* buffer and to 1 otherwise. */
output_too_large = mbedtls_cf_size_gt( plaintext_size,
plaintext_max_size );
/* Set ret without branches to avoid timing attacks. Return:
* - INVALID_PADDING if the padding is bad (bad != 0).
* - OUTPUT_TOO_LARGE if the padding is good but the decrypted
* plaintext does not fit in the output buffer.
* - 0 if the padding is correct. */
ret = - (int) mbedtls_cf_uint_if(
bad, - MBEDTLS_ERR_RSA_INVALID_PADDING,
mbedtls_cf_uint_if( output_too_large,
- MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE,
0 ) );
/* If the padding is bad or the plaintext is too large, zero the
* data that we're about to copy to the output buffer.
* We need to copy the same amount of data
* from the same buffer whether the padding is good or not to
* avoid leaking the padding validity through overall timing or
* through memory or cache access patterns. */
bad = mbedtls_cf_uint_mask( bad | output_too_large );
for( i = 11; i < ilen; i++ )
buf[i] &= ~bad;
/* If the plaintext is too large, truncate it to the buffer size.
* Copy anyway to avoid revealing the length through timing, because
* revealing the length is as bad as revealing the padding validity
* for a Bleichenbacher attack. */
plaintext_size = mbedtls_cf_uint_if( output_too_large,
(unsigned) plaintext_max_size,
(unsigned) plaintext_size );
/* Move the plaintext to the leftmost position where it can start in
* the working buffer, i.e. make it start plaintext_max_size from
* the end of the buffer. Do this with a memory access trace that
* does not depend on the plaintext size. After this move, the
* starting location of the plaintext is no longer sensitive
* information. */
mbedtls_cf_mem_move_to_left( buf + ilen - plaintext_max_size,
plaintext_max_size,
plaintext_max_size - plaintext_size );
/* Finally copy the decrypted plaintext plus trailing zeros into the output
* buffer. If output_max_len is 0, then output may be an invalid pointer
* and the result of memcpy() would be undefined; prevent undefined
* behavior making sure to depend only on output_max_len (the size of the
* user-provided output buffer), which is independent from plaintext
* length, validity of padding, success of the decryption, and other
* secrets. */
if( output_max_len != 0 )
memcpy( output, buf + ilen - plaintext_max_size, plaintext_max_size );
/* Report the amount of data we copied to the output buffer. In case
* of errors (bad padding or output too large), the value of *olen
* when this function returns is not specified. Making it equivalent
* to the good case limits the risks of leaking the padding validity. */
*olen = plaintext_size;
return( ret );
}
#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */

11
library/constant_time.h
View file

@ -148,3 +148,14 @@ int mbedtls_ssl_cf_hmac(
unsigned char *output );
#endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
int mbedtls_cf_rsaes_pkcs1_v15_unpadding( int mode,
size_t ilen,
size_t *olen,
unsigned char *output,
size_t output_max_len,
unsigned char *buf );
#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */

145
library/rsa.c
View file

@ -1479,151 +1479,6 @@ cleanup:
#endif /* MBEDTLS_PKCS1_V21 */
#if defined(MBEDTLS_PKCS1_V15)
int mbedtls_cf_rsaes_pkcs1_v15_unpadding( int mode,
size_t ilen,
size_t *olen,
unsigned char *output,
size_t output_max_len,
unsigned char *buf )
{
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
size_t i, plaintext_max_size;
/* The following variables take sensitive values: their value must
* not leak into the observable behavior of the function other than
* the designated outputs (output, olen, return value). Otherwise
* this would open the execution of the function to
* side-channel-based variants of the Bleichenbacher padding oracle
* attack. Potential side channels include overall timing, memory
* access patterns (especially visible to an adversary who has access
* to a shared memory cache), and branches (especially visible to
* an adversary who has access to a shared code cache or to a shared
* branch predictor). */
size_t pad_count = 0;
unsigned bad = 0;
unsigned char pad_done = 0;
size_t plaintext_size = 0;
unsigned output_too_large;
plaintext_max_size = mbedtls_cf_size_if( output_max_len > ilen - 11,
ilen - 11,
output_max_len );
/* Check and get padding length in constant time and constant
* memory trace. The first byte must be 0. */
bad |= buf[0];
if( mode == MBEDTLS_RSA_PRIVATE )
{
/* Decode EME-PKCS1-v1_5 padding: 0x00 || 0x02 || PS || 0x00
* where PS must be at least 8 nonzero bytes. */
bad |= buf[1] ^ MBEDTLS_RSA_CRYPT;
/* Read the whole buffer. Set pad_done to nonzero if we find
* the 0x00 byte and remember the padding length in pad_count. */
for( i = 2; i < ilen; i++ )
{
pad_done |= ((buf[i] | (unsigned char)-buf[i]) >> 7) ^ 1;
pad_count += ((pad_done | (unsigned char)-pad_done) >> 7) ^ 1;
}
}
else
{
/* Decode EMSA-PKCS1-v1_5 padding: 0x00 || 0x01 || PS || 0x00
* where PS must be at least 8 bytes with the value 0xFF. */
bad |= buf[1] ^ MBEDTLS_RSA_SIGN;
/* Read the whole buffer. Set pad_done to nonzero if we find
* the 0x00 byte and remember the padding length in pad_count.
* If there's a non-0xff byte in the padding, the padding is bad. */
for( i = 2; i < ilen; i++ )
{
pad_done |= mbedtls_cf_uint_if( buf[i], 0, 1 );
pad_count += mbedtls_cf_uint_if( pad_done, 0, 1 );
bad |= mbedtls_cf_uint_if( pad_done, 0, buf[i] ^ 0xFF );
}
}
/* If pad_done is still zero, there's no data, only unfinished padding. */
bad |= mbedtls_cf_uint_if( pad_done, 0, 1 );
/* There must be at least 8 bytes of padding. */
bad |= mbedtls_cf_size_gt( 8, pad_count );
/* If the padding is valid, set plaintext_size to the number of
* remaining bytes after stripping the padding. If the padding
* is invalid, avoid leaking this fact through the size of the
* output: use the maximum message size that fits in the output
* buffer. Do it without branches to avoid leaking the padding
* validity through timing. RSA keys are small enough that all the
* size_t values involved fit in unsigned int. */
plaintext_size = mbedtls_cf_uint_if(
bad, (unsigned) plaintext_max_size,
(unsigned) ( ilen - pad_count - 3 ) );
/* Set output_too_large to 0 if the plaintext fits in the output
* buffer and to 1 otherwise. */
output_too_large = mbedtls_cf_size_gt( plaintext_size,
plaintext_max_size );
/* Set ret without branches to avoid timing attacks. Return:
* - INVALID_PADDING if the padding is bad (bad != 0).
* - OUTPUT_TOO_LARGE if the padding is good but the decrypted
* plaintext does not fit in the output buffer.
* - 0 if the padding is correct. */
ret = - (int) mbedtls_cf_uint_if(
bad, - MBEDTLS_ERR_RSA_INVALID_PADDING,
mbedtls_cf_uint_if( output_too_large,
- MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE,
0 ) );
/* If the padding is bad or the plaintext is too large, zero the
* data that we're about to copy to the output buffer.
* We need to copy the same amount of data
* from the same buffer whether the padding is good or not to
* avoid leaking the padding validity through overall timing or
* through memory or cache access patterns. */
bad = mbedtls_cf_uint_mask( bad | output_too_large );
for( i = 11; i < ilen; i++ )
buf[i] &= ~bad;
/* If the plaintext is too large, truncate it to the buffer size.
* Copy anyway to avoid revealing the length through timing, because
* revealing the length is as bad as revealing the padding validity
* for a Bleichenbacher attack. */
plaintext_size = mbedtls_cf_uint_if( output_too_large,
(unsigned) plaintext_max_size,
(unsigned) plaintext_size );
/* Move the plaintext to the leftmost position where it can start in
* the working buffer, i.e. make it start plaintext_max_size from
* the end of the buffer. Do this with a memory access trace that
* does not depend on the plaintext size. After this move, the
* starting location of the plaintext is no longer sensitive
* information. */
mbedtls_cf_mem_move_to_left( buf + ilen - plaintext_max_size,
plaintext_max_size,
plaintext_max_size - plaintext_size );
/* Finally copy the decrypted plaintext plus trailing zeros into the output
* buffer. If output_max_len is 0, then output may be an invalid pointer
* and the result of memcpy() would be undefined; prevent undefined
* behavior making sure to depend only on output_max_len (the size of the
* user-provided output buffer), which is independent from plaintext
* length, validity of padding, success of the decryption, and other
* secrets. */
if( output_max_len != 0 )
memcpy( output, buf + ilen - plaintext_max_size, plaintext_max_size );
/* Report the amount of data we copied to the output buffer. In case
* of errors (bad padding or output too large), the value of *olen
* when this function returns is not specified. Making it equivalent
* to the good case limits the risks of leaking the padding validity. */
*olen = plaintext_size;
return( ret );
}
/*
* Implementation of the PKCS#1 v2.1 RSAES-PKCS1-V1_5-DECRYPT function
*/