diff --git a/library/constant_time.c b/library/constant_time.c index d84132749..3784365a6 100644 --- a/library/constant_time.c +++ b/library/constant_time.c @@ -29,6 +29,7 @@ #include "ssl_misc.h" #endif +#include /* constant-time buffer comparison */ int mbedtls_ssl_safer_memcmp( const void *a, const void *b, size_t n ) @@ -710,3 +711,134 @@ 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( 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]; + + + /* 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; + } + + + /* 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 */ diff --git a/library/constant_time.h b/library/constant_time.h index c7ca490d3..9a6db2807 100644 --- a/library/constant_time.h +++ b/library/constant_time.h @@ -148,3 +148,13 @@ 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( 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 */ diff --git a/library/rsa.c b/library/rsa.c index ff769bb11..f4131fd8f 100644 --- a/library/rsa.c +++ b/library/rsa.c @@ -1458,133 +1458,6 @@ cleanup: #endif /* MBEDTLS_PKCS1_V21 */ #if defined(MBEDTLS_PKCS1_V15) -int mbedtls_cf_rsaes_pkcs1_v15_unpadding( 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]; - - - /* 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; - } - - - /* 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 */