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Bleichenbacher fix: don't leak the plaintext length (step 1)

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mbedtls_rsa_rsaes_pkcs1_v15_decrypt takes care not to reveal whether
the padding is valid or not, even through timing or memory access
patterns. This is a defense against an attack published by
Bleichenbacher. The attacker can also obtain the same information by
observing the length of the plaintext. The current implementation
leaks the length of the plaintext through timing and memory access
patterns.

This commit is a first step towards fixing this leak. It reduces the
leak to a single memmove call inside the working buffer.
This commit is contained in:
Gilles Peskine 2018-10-04 19:13:43 +02:00
parent 331d80e162
commit 9265ff4ee6
1 changed files with 40 additions and 29 deletions
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69
library/rsa.c
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@ -1434,6 +1434,7 @@ int mbedtls_rsa_rsaes_pkcs1_v15_decrypt( mbedtls_rsa_context *ctx,
size_t plaintext_max_size = ( output_max_len > ilen - 11 ?
ilen - 11 :
output_max_len );
unsigned output_too_large;
unsigned char buf[MBEDTLS_MPI_MAX_SIZE];
unsigned char *p = buf;
@ -1490,11 +1491,6 @@ int mbedtls_rsa_rsaes_pkcs1_v15_decrypt( mbedtls_rsa_context *ctx,
/* There must be at least 8 bytes of padding. */
bad |= ( pad_count < 8 );
/* Set bad to zero if the padding is valid and
* all-bits-one otherwise. The whole calculation of bad
* is done in such a way to avoid branches. */
bad = all_or_nothing_int( bad );
/* 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
@ -1506,38 +1502,53 @@ int mbedtls_rsa_rsaes_pkcs1_v15_decrypt( mbedtls_rsa_context *ctx,
(unsigned) plaintext_max_size,
(unsigned) ( ilen - ( p - buf ) ) );
/* Check if the decrypted plaintext fits in the output buffer.
* If the padding is bad, this will always be the case,
* thus we don't leak the padding validity by trying to produce
* a larger output than what the caller expects. */
if( plaintext_size > output_max_len )
{
ret = MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE;
goto cleanup;
}
/* Set output_too_large to 0 if the plaintext fits in the output
* buffer and to 1 otherwise. This is the sign bit (1 for negative)
* of (output_max_len - plaintext_size). */
output_too_large = ( ( output_max_len - plaintext_size ) >>
( ( sizeof( output_max_len ) * 8 - 1 ) ) );
/* Set ret to INVALID_PADDING if the padding is bad and to 0
* otherwise. At this point, the variable bad is zero if
* the padding is good and can be any nonzero value otherwise.
* Do this without branches to avoid timing attacks. */
ret = - ( bad & ( - MBEDTLS_ERR_RSA_INVALID_PADDING ) );
/* 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 = - if_int( bad, - MBEDTLS_ERR_RSA_INVALID_PADDING,
if_int( output_too_large, - MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE,
0 ) );
/* If the padding is bad, zero the data that we're about to copy
* to the output buffer. We need to copy the same amount of data
/* 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. */
for( i = 11; i < ilen; i++ )
buf[i] &= ~bad;
buf[i] &= ~( bad | output_too_large );
/* Copy the decrypted plaintext from the end of the buffer. */
memcpy( output, buf + ilen - plaintext_size, plaintext_size );
/* 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 = if_int( output_too_large,
(unsigned) plaintext_max_size,
(unsigned) plaintext_size );
/* Report the amount of data we copied to the output buffer.
* When the padding is invalid, the value of *olen when this
* function returns is not specified. Making it equivalent to
* the good-padding case limits the risks of leaking the
* padding validity. */
/* Move the plaintext to the beginning of the working buffer so that
* its position no longer depends on the padding and we have enough
* room from the beginning of the plaintext to copy a number of bytes
* that does not depend on the padding. */
/* FIXME: we need a constant-time, constant-trace memmove here. */
memmove( buf, p, plaintext_size );
/* Finally copy the decrypted plaintext plus trailing data
* into the output buffer. */
memcpy( output, buf, 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;
cleanup: