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https://github.com/yuzu-emu/mbedtls.git
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Implement cf_hmac() actually with constant flow
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard
parent
0dab12ec2c
commit
de02b580c8
@ -1809,6 +1809,48 @@ static int ssl_encrypt_buf( mbedtls_ssl_context *ssl )
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( defined(MBEDTLS_SSL_PROTO_TLS1) || \
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defined(MBEDTLS_SSL_PROTO_TLS1_1) || \
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defined(MBEDTLS_SSL_PROTO_TLS1_2) )
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/*
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* Constant-flow conditional memcpy:
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* - if c1 == c2, equivalent to memcpy(dst, src, len),
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* - otherwise, a no-op,
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* but with execution flow independent of the values of c1 and c2.
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*
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* Use only bit operations to avoid branches that could be used by some
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* compilers on some platforms to translate comparison operators.
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*/
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static void mbedtls_ssl_cf_memcpy_if_eq(unsigned char *dst,
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const unsigned char *src,
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size_t len,
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size_t c1, size_t c2 )
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{
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/* diff = 0 if c1 == c2, non-zero otherwise */
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const size_t diff = c1 ^ c2;
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/* MSVC has a warning about unary minus on unsigned integer types,
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* but this is well-defined and precisely what we want to do here. */
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#if defined(_MSC_VER)
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#pragma warning( push )
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#pragma warning( disable : 4146 )
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#endif
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/* diff_msb's most significant bit is bit equal to c1 != c2 */
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const size_t diff_msb = ( diff | -diff );
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/* diff1 = c1 != c2 */
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const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
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/* mask = c1 != c2 ? 0xff : 0x00 */
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unsigned char mask = (unsigned char) -diff1;
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#if defined(_MSC_VER)
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#pragma warning( pop )
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#endif
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/* dst[i] = c1 != c2 ? dst[i] : src[i] */
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for( size_t i = 0; i < len; i++ )
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dst[i] = ( dst[i] & mask ) | ( src[i] & ~mask );
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}
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/*
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* Compute HMAC of variable-length data with constant flow.
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*/
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@ -1819,85 +1861,61 @@ int mbedtls_ssl_cf_hmac(
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size_t min_data_len, size_t max_data_len,
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unsigned char *output )
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{
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/* WORK IN PROGRESS - THIS IS ONLY PSEUDO-CONTANT-TIME */
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/*
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* Process MAC and always update for padlen afterwards to make
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* total time independent of padlen.
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* This function breaks the HMAC abstraction and uses the md_clone()
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* extension to the MD API in order to get constant-flow behaviour.
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*
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* Known timing attacks:
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* - Lucky Thirteen (http://www.isg.rhul.ac.uk/tls/TLStiming.pdf)
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* HMAC(msg) is defined as HASH(okey + HASH(ikey + msg)) where + means
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* concatenation, and okey/ikey is the XOR of the key with some fix bit
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* patterns (see RFC 2104, sec. 2), which are stored in ctx->hmac_ctx.
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*
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* To compensate for different timings for the MAC calculation
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* depending on how much padding was removed (which is determined
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* by padlen), process extra_run more blocks through the hash
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* function.
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* We'll first compute inner_hash = HASH(ikey + msg) by hashing up to
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* minlen, then cloning the context, and for each byte up to maxlen
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* finishing up the hash computation, keeping only the correct result.
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*
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* The formula in the paper is
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* extra_run = ceil( (L1-55) / 64 ) - ceil( (L2-55) / 64 )
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* where L1 is the size of the header plus the decrypted message
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* plus CBC padding and L2 is the size of the header plus the
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* decrypted message. This is for an underlying hash function
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* with 64-byte blocks.
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* We use ( (Lx+8) / 64 ) to handle 'negative Lx' values
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* correctly. We round down instead of up, so -56 is the correct
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* value for our calculations instead of -55.
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*
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* Repeat the formula rather than defining a block_size variable.
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* This avoids requiring division by a variable at runtime
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* (which would be marginally less efficient and would require
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* linking an extra division function in some builds).
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* Then we only need to compute HASH(okey + inner_hash) and we're done.
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*/
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size_t j, extra_run = 0;
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/* This size is enough to server either as input to
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* md_process() or as output to md_finish() */
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unsigned char tmp[128];
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const mbedtls_md_type_t md_alg = mbedtls_md_get_type( ctx->md_info );
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const size_t block_size = md_alg == MBEDTLS_MD_SHA384 ? 128 : 64;
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const unsigned char * const ikey = (unsigned char *) ctx->hmac_ctx;
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const unsigned char * const okey = ikey + block_size;
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const size_t hash_size = mbedtls_md_get_size( ctx->md_info );
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memset( tmp, 0, sizeof( tmp ) );
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unsigned char aux_out[MBEDTLS_MD_MAX_SIZE];
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mbedtls_md_context_t aux;
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size_t offset;
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switch( mbedtls_md_get_type( ctx->md_info ) )
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mbedtls_md_init( &aux );
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mbedtls_md_setup( &aux, ctx->md_info, 0 );
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/* After hmac_start() of hmac_reset(), ikey has already been hashed,
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* so we can start directly with the message */
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mbedtls_md_update( ctx, add_data, add_data_len );
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mbedtls_md_update( ctx, data, min_data_len );
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/* For each possible length, compute the hash up to that point */
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for( offset = min_data_len; offset <= max_data_len; offset++ )
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{
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#if defined(MBEDTLS_MD5_C) || defined(MBEDTLS_SHA1_C) || \
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defined(MBEDTLS_SHA256_C)
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case MBEDTLS_MD_MD5:
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case MBEDTLS_MD_SHA1:
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case MBEDTLS_MD_SHA256:
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/* 8 bytes of message size, 64-byte compression blocks */
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extra_run = ( add_data_len + max_data_len + 8 ) / 64 -
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( add_data_len + data_len_secret + 8 ) / 64;
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break;
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#endif
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#if defined(MBEDTLS_SHA512_C)
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case MBEDTLS_MD_SHA384:
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/* 16 bytes of message size, 128-byte compression blocks */
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extra_run = ( add_data_len + max_data_len + 16 ) / 128 -
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( add_data_len + data_len_secret + 16 ) / 128;
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break;
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#endif
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default:
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return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
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mbedtls_md_clone( &aux, ctx );
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mbedtls_md_finish( &aux, aux_out );
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/* Keep only the correct inner_hash in the output buffer */
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mbedtls_ssl_cf_memcpy_if_eq( output, aux_out, hash_size,
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offset, data_len_secret );
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if( offset < max_data_len )
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mbedtls_md_update( ctx, data + offset, 1 );
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}
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mbedtls_md_hmac_update( ctx, add_data, add_data_len );
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mbedtls_md_hmac_update( ctx, data, data_len_secret );
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/* Make sure we access everything even when padlen > 0. This
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* makes the synchronisation requirements for just-in-time
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* Prime+Probe attacks much tighter and hopefully impractical. */
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ssl_read_memory( data + min_data_len, max_data_len - min_data_len );
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mbedtls_md_hmac_finish( ctx, output );
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/* Dummy calls to compression function.
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* Call mbedtls_md_process at least once due to cache attacks
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* that observe whether md_process() was called of not.
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* Respect the usual start-(process|update)-finish sequence for
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* the sake of hardware accelerators that might require it. */
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/* Now compute HASH(okey + inner_hash) */
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mbedtls_md_starts( ctx );
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for( j = 0; j < extra_run + 1; j++ )
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mbedtls_md_process( ctx, tmp );
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mbedtls_md_finish( ctx, tmp );
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mbedtls_md_update( ctx, okey, block_size );
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mbedtls_md_update( ctx, output, hash_size );
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mbedtls_md_finish( ctx, output );
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/* Done, get ready for next time */
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mbedtls_md_hmac_reset( ctx );
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mbedtls_md_free( &aux );
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return( 0 );
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}
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#endif /* MBEDTLS_SSL_SOME_SUITES_USE_CBC && TLS 1.0-1.2 */
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