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Move mbedtls_ct_rsaes_pkcs1_v15_unpadding into rsa.c

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2023-05-09 11:10:21 +01:00
parent 0afe001871
commit 19e8cd06fe
3 changed files with 162 additions and 171 deletions
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135
library/constant_time.c
View File

@ -334,8 +334,8 @@ void mbedtls_ct_mpi_uint_cond_assign(size_t n,
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
void mbedtls_ct_mem_move_to_left(void *start,
size_t total,
size_t offset)
size_t total,
size_t offset)
{
volatile unsigned char *buf = start;
size_t i, n;
@ -590,134 +590,3 @@ int mbedtls_mpi_lt_mpi_ct(const mbedtls_mpi *X,
}
#endif /* MBEDTLS_BIGNUM_C */
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
int mbedtls_ct_rsaes_pkcs1_v15_unpadding(unsigned char *input,
size_t ilen,
unsigned char *output,
size_t output_max_len,
size_t *olen)
{
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 = (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 |= input[0];
/* Decode EME-PKCS1-v1_5 padding: 0x00 || 0x02 || PS || 0x00
* where PS must be at least 8 nonzero bytes. */
bad |= input[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 |= ((input[i] | (unsigned char) -input[i]) >> 7) ^ 1;
pad_count += ((pad_done | (unsigned char) -pad_done) >> 7) ^ 1;
}
/* If pad_done is still zero, there's no data, only unfinished padding. */
bad |= mbedtls_ct_uint_if(pad_done, 0, 1);
/* There must be at least 8 bytes of padding. */
bad |= mbedtls_ct_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_ct_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_ct_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_ct_uint_if(
bad, -MBEDTLS_ERR_RSA_INVALID_PADDING,
mbedtls_ct_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_ct_uint_mask(bad | output_too_large);
for (i = 11; i < ilen; i++) {
input[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_ct_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_ct_mem_move_to_left(input + 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, input + 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 */

40
library/constant_time_internal.h
View File

@ -238,42 +238,6 @@ void mbedtls_ct_memcpy_offset(unsigned char *dest,
#endif /* MBEDTLS_SSL_SOME_SUITES_USE_MAC */
#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 (EME-PKCS1-v1_5 decoding).
*
* \note The return value from this function is a sensitive value
* (this is unusual). #MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE shouldn't happen
* in a well-written application, but 0 vs #MBEDTLS_ERR_RSA_INVALID_PADDING
* is often a situation that an attacker can provoke and leaking which
* one is the result is precisely the information the attacker wants.
*
* \param input The input buffer which is the payload inside PKCS#1v1.5
* encryption padding, called the "encoded message EM"
* by the terminology.
* \param ilen The length of the payload in the \p input buffer.
* \param output The buffer for the payload, called "message M" by the
* PKCS#1 terminology. This must be a writable buffer of
* length \p output_max_len bytes.
* \param olen The address at which to store the length of
* the payload. This must not be \c NULL.
* \param output_max_len The length in bytes of the output buffer \p output.
*
* \return \c 0 on success.
* \return #MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE
* The output buffer is too small for the unpadded payload.
* \return #MBEDTLS_ERR_RSA_INVALID_PADDING
* The input doesn't contain properly formatted padding.
*/
int mbedtls_ct_rsaes_pkcs1_v15_unpadding(unsigned char *input,
size_t ilen,
unsigned char *output,
size_t output_max_len,
size_t *olen);
#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
#if defined(MBEDTLS_BASE64_C)
/** Constant-flow char selection
@ -333,8 +297,8 @@ unsigned mbedtls_ct_size_gt(size_t x, size_t y);
* \param offset Offset from which to copy \p total - \p offset bytes.
*/
void mbedtls_ct_mem_move_to_left(void *start,
size_t total,
size_t offset);
size_t total,
size_t offset);
#endif /* defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) */

158
library/rsa.c
View File

@ -56,6 +56,164 @@
#include "mbedtls/platform.h"
#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 (EME-PKCS1-v1_5 decoding).
*
* \note The return value from this function is a sensitive value
* (this is unusual). #MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE shouldn't happen
* in a well-written application, but 0 vs #MBEDTLS_ERR_RSA_INVALID_PADDING
* is often a situation that an attacker can provoke and leaking which
* one is the result is precisely the information the attacker wants.
*
* \param input The input buffer which is the payload inside PKCS#1v1.5
* encryption padding, called the "encoded message EM"
* by the terminology.
* \param ilen The length of the payload in the \p input buffer.
* \param output The buffer for the payload, called "message M" by the
* PKCS#1 terminology. This must be a writable buffer of
* length \p output_max_len bytes.
* \param olen The address at which to store the length of
* the payload. This must not be \c NULL.
* \param output_max_len The length in bytes of the output buffer \p output.
*
* \return \c 0 on success.
* \return #MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE
* The output buffer is too small for the unpadded payload.
* \return #MBEDTLS_ERR_RSA_INVALID_PADDING
* The input doesn't contain properly formatted padding.
*/
static int mbedtls_ct_rsaes_pkcs1_v15_unpadding(unsigned char *input,
size_t ilen,
unsigned char *output,
size_t output_max_len,
size_t *olen)
{
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 = (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 |= input[0];
/* Decode EME-PKCS1-v1_5 padding: 0x00 || 0x02 || PS || 0x00
* where PS must be at least 8 nonzero bytes. */
bad |= input[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 |= ((input[i] | (unsigned char) -input[i]) >> 7) ^ 1;
pad_count += ((pad_done | (unsigned char) -pad_done) >> 7) ^ 1;
}
/* If pad_done is still zero, there's no data, only unfinished padding. */
bad |= mbedtls_ct_uint_if(pad_done, 0, 1);
/* There must be at least 8 bytes of padding. */
bad |= mbedtls_ct_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_ct_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_ct_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_ct_uint_if(
bad, -MBEDTLS_ERR_RSA_INVALID_PADDING,
mbedtls_ct_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_ct_uint_mask(bad | output_too_large);
for (i = 11; i < ilen; i++) {
input[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_ct_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_ct_mem_move_to_left(input + 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, input + 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 */
#if !defined(MBEDTLS_RSA_ALT)
int mbedtls_rsa_import(mbedtls_rsa_context *ctx,