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Simplify AEAD nonce derivation

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This commit simplifies nonce derivation for AEAD based record protection
routines in the following way.

So far, code distinguished between the cases of GCM+CCM and ChachaPoly:

- In the case of GCM+CCM, the AEAD nonce is the concatentation
  of a 4-byte Fixed IV and a dynamically chosen 8-byte IV which is prepended
  to the record. In Mbed TLS, this is always chosen to be the record sequence
  number, but it need not to.

- In the case of ChaChaPoly, the AEAD nonce is derived as

    `( 12-byte Fixed IV ) XOR ( 0 || 8-byte dynamic IV == record seq nr )`

  and the dynamically chosen IV is no longer prepended to the record.

This commit removes this distinction by always computing the record nonce
via the formula

  `IV == ( Fixed IV || 0 ) XOR ( 0 || Dynamic IV )`

The ChaChaPoly case is recovered in case `Len(Fixed IV) == Len(IV)`, and
GCM+CCM is recovered when `Len(IV) == Len(Fixed IV) + Len(Dynamic IV)`.

Moreover, a getter stub `ssl_transform_aead_dynamic_iv_is_explicit()`
is introduced which infers from a transform whether the dynamically
chosen part of the IV is explicit, which in the current implementation
of `mbedtls_ssl_transform` can be derived from the helper field
`mbedtls_ssl_transform::fixed_ivlen`.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker 2020-05-28 07:05:48 +01:00
parent df8be226ba
commit 17263803aa
1 changed files with 44 additions and 91 deletions
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113
library/ssl_msg.c
View file

@ -536,76 +536,39 @@ static void ssl_mac( mbedtls_md_context_t *md_ctx,
}
#endif /* MBEDTLS_SSL_PROTO_SSL3 */
#define SSL_RECORD_AEAD_NONCE_UNKNOWN 0u
#define SSL_RECORD_AEAD_NONCE_CONCAT 1u
#define SSL_RECORD_AEAD_NONCE_XOR 2u
static int ssl_transform_get_nonce_mode( mbedtls_ssl_transform const *transform )
static int ssl_transform_aead_dynamic_iv_is_explicit(
mbedtls_ssl_transform const *transform )
{
#if defined(MBEDTLS_CHACHAPOLY_C)
if( transform->ivlen == 12 && transform->fixed_ivlen == 12 )
{
return( SSL_RECORD_AEAD_NONCE_XOR );
}
#endif /* MBEDTLS_CHACHAPOLY_C */
#if defined(MBEDTLS_GCM_C) || defined(MBEDTLS_CCM_C)
if( transform->ivlen == 12 && transform->fixed_ivlen == 4 )
{
return( SSL_RECORD_AEAD_NONCE_CONCAT );
}
#endif /* MBEDTLS_GCM_C || MBEDTLS_CCM_C */
return( SSL_RECORD_AEAD_NONCE_UNKNOWN );
return( transform->ivlen != transform->fixed_ivlen );
}
/* Preconditions:
* - If mode == SSL_RECORD_AEAD_NONCE_CONCAT, then
* dst_nonce_len == fixed_iv_len + dynamic_iv_len
* - If mode == SSL_RECORD_AEAD_NONCE_XOR, then
* dst_nonce_len == fixed_iv_len &&
* dynamic_iv_len < dst_nonce
/* Compute IV := ( fixed_iv || 0 ) XOR ( 0 || dynamic_IV )
*
* Concretely, this occurs in two variants:
*
* a) Fixed and dynamic IV lengths add up to total IV length, giving
* IV = fixed_iv || dynamic_iv
*
* b) Fixed IV lengths matches total IV length, giving
* IV = fixed_iv XOR ( 0 || dynamic_iv )
*/
static int ssl_build_record_nonce( unsigned char *dst_nonce,
size_t dst_nonce_len,
static void ssl_build_record_nonce( unsigned char *dst_iv,
size_t dst_iv_len,
unsigned char const *fixed_iv,
size_t fixed_iv_len,
unsigned char const *dynamic_iv,
size_t dynamic_iv_len,
unsigned mode )
size_t dynamic_iv_len )
{
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
((void) dst_nonce_len);
size_t i;
/* Start with Fixed IV || 0 */
memcpy( dst_nonce, fixed_iv, fixed_iv_len );
dst_nonce += fixed_iv_len;
memset( dst_iv, 0, dst_iv_len );
memcpy( dst_iv, fixed_iv, fixed_iv_len );
if( mode == SSL_RECORD_AEAD_NONCE_CONCAT )
{
/* Nonce := Fixed IV || Dynamic IV */
memcpy( dst_nonce, dynamic_iv, dynamic_iv_len );
ret = 0;
}
else if( mode == SSL_RECORD_AEAD_NONCE_XOR )
{
/* Nonce := Fixed IV XOR ( 0 || Dynamic IV ) */
unsigned char i;
/* This is safe by the second precondition above. */
dst_nonce -= dynamic_iv_len;
dst_iv += dst_iv_len - dynamic_iv_len;
for( i = 0; i < dynamic_iv_len; i++ )
dst_nonce[i] ^= dynamic_iv[i];
ret = 0;
}
else
{
ret = MBEDTLS_ERR_SSL_INTERNAL_ERROR;
}
return( ret );
dst_iv[i] ^= dynamic_iv[i];
}
int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
@ -833,11 +796,8 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
unsigned char iv[12];
unsigned char *dynamic_iv;
size_t dynamic_iv_len;
unsigned const nonce_mode
= ssl_transform_get_nonce_mode( transform );
unsigned const dynamic_iv_is_explicit
= nonce_mode == SSL_RECORD_AEAD_NONCE_CONCAT;
int dynamic_iv_is_explicit =
ssl_transform_aead_dynamic_iv_is_explicit( transform );
/* Check that there's space for the authentication tag. */
if( post_avail < transform->taglen )
@ -861,14 +821,11 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
dynamic_iv = rec->ctr;
dynamic_iv_len = sizeof( rec->ctr );
ret = ssl_build_record_nonce( iv, sizeof( iv ),
ssl_build_record_nonce( iv, sizeof( iv ),
transform->iv_enc,
transform->fixed_ivlen,
dynamic_iv,
dynamic_iv_len,
nonce_mode );
if( ret != 0 )
return( ret );
dynamic_iv_len );
/*
* Build additional data for AEAD encryption.
@ -911,7 +868,7 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
/*
* Prefix record content with dynamic IV in case it is explicit.
*/
if( dynamic_iv_is_explicit == 1 )
if( dynamic_iv_is_explicit )
{
if( rec->data_offset < dynamic_iv_len )
{
@ -1160,7 +1117,6 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
mode == MBEDTLS_MODE_CHACHAPOLY )
{
unsigned char iv[12];
unsigned const nonce_mode = ssl_transform_get_nonce_mode( transform );
unsigned char *dynamic_iv;
size_t dynamic_iv_len;
@ -1173,11 +1129,7 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
* agree with the record sequence number.
*/
dynamic_iv_len = sizeof( rec->ctr );
if( nonce_mode == SSL_RECORD_AEAD_NONCE_XOR )
{
dynamic_iv = rec->ctr;
}
else
if( ssl_transform_aead_dynamic_iv_is_explicit( transform ) == 1 )
{
if( rec->data_len < dynamic_iv_len )
{
@ -1192,6 +1144,10 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
rec->data_offset += dynamic_iv_len;
rec->data_len -= dynamic_iv_len;
}
else
{
dynamic_iv = rec->ctr;
}
/* Check that there's space for the authentication tag. */
if( rec->data_len < transform->taglen )
@ -1204,14 +1160,11 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
/*
* Prepare nonce from dynamic and static parts.
*/
ret = ssl_build_record_nonce( iv, sizeof( iv ),
ssl_build_record_nonce( iv, sizeof( iv ),
transform->iv_dec,
transform->fixed_ivlen,
dynamic_iv,
dynamic_iv_len,
nonce_mode );
if( ret != 0 )
return( ret );
dynamic_iv_len );
/*
* Build additional data for AEAD encryption.