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Use AES-GCM-256 for session ticket protection

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Manuel Pégourié-Gonnard 2015-05-20 19:59:39 +02:00
parent 8eff512274
commit 1041a39338
2 changed files with 74 additions and 107 deletions
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8
include/mbedtls/ssl_ticket.h
View file

@ -25,7 +25,7 @@
#define MBEDTLS_SSL_TICKET_H #define MBEDTLS_SSL_TICKET_H
#include "ssl.h" #include "ssl.h"
#include "aes.h" #include "cipher.h"
#if defined(MBEDTLS_THREADING_C) #if defined(MBEDTLS_THREADING_C)
#include "threading.h" #include "threading.h"
@ -40,10 +40,8 @@ extern "C" {
*/ */
typedef struct typedef struct
{ {
unsigned char key_name[16]; /*!< name to quickly reject bad tickets */ unsigned char key_name[4]; /*!< name to quickly reject bad tickets */
mbedtls_aes_context enc; /*!< encryption context */ mbedtls_cipher_context_t cipher;/*!< cipher context */
mbedtls_aes_context dec; /*!< decryption context */
unsigned char mac_key[16]; /*!< authentication key */
uint32_t ticket_lifetime; /*!< lifetime of tickets in seconds */ uint32_t ticket_lifetime; /*!< lifetime of tickets in seconds */

173
library/ssl_ticket.c
View file

@ -64,33 +64,39 @@ int mbedtls_ssl_ticket_setup( mbedtls_ssl_ticket_context *ctx,
uint32_t lifetime ) uint32_t lifetime )
{ {
int ret; int ret;
unsigned char buf[16]; unsigned char buf[32];
ctx->f_rng = f_rng; ctx->f_rng = f_rng;
ctx->p_rng = p_rng; ctx->p_rng = p_rng;
ctx->ticket_lifetime = lifetime; ctx->ticket_lifetime = lifetime;
mbedtls_aes_init( &ctx->enc ); if( ( ret = mbedtls_cipher_setup( &ctx->cipher,
mbedtls_aes_init( &ctx->dec ); mbedtls_cipher_info_from_type(
MBEDTLS_CIPHER_AES_256_GCM ) ) ) != 0 )
if( ( ret = f_rng( p_rng, ctx->key_name, 16 ) != 0 ) ||
( ret = f_rng( p_rng, ctx->mac_key, 16 ) != 0 ) ||
( ret = f_rng( p_rng, buf, 16 ) != 0 ) )
{ {
return( ret ); goto cleanup;
} }
if( ( ret = mbedtls_aes_setkey_enc( &ctx->enc, buf, 128 ) ) != 0 || if( ( ret = f_rng( p_rng, buf, sizeof( buf ) ) != 0 ) )
( ret = mbedtls_aes_setkey_dec( &ctx->dec, buf, 128 ) ) != 0 )
{ {
mbedtls_ssl_ticket_free( ctx ); goto cleanup;
return( ret );
} }
/* With GCM and CCM, same context can encrypt & decrypt */
if( ( ret = mbedtls_cipher_setkey( &ctx->cipher, buf, 256,
MBEDTLS_ENCRYPT ) ) != 0 )
{
goto cleanup;
}
cleanup:
mbedtls_zeroize( buf, sizeof( buf ) ); mbedtls_zeroize( buf, sizeof( buf ) );
return( 0 ); if( ret != 0 )
mbedtls_ssl_ticket_free( ctx );
return( ret );
} }
/* /*
@ -203,16 +209,17 @@ static int ssl_load_session( mbedtls_ssl_session *session,
} }
/* /*
* Create session ticket, secured as recommended in RFC 5077 section 4: * Create session ticket, with the following structure:
* *
* struct { * struct {
* opaque key_name[16]; * opaque key_name[4];
* opaque iv[16]; * opaque iv[12];
* opaque encrypted_state<0..2^16-1>; * opaque encrypted_state<0..2^16-1>;
* opaque mac[32]; * opaque tag[16];
* } ticket; * } ticket;
* *
* (the internal state structure differs, however). * The key_name, iv, and length of encrypted_state are the additional
* authenticated data.
*/ */
int mbedtls_ssl_ticket_write( void *p_ticket, int mbedtls_ssl_ticket_write( void *p_ticket,
const mbedtls_ssl_session *session, const mbedtls_ssl_session *session,
@ -223,20 +230,21 @@ int mbedtls_ssl_ticket_write( void *p_ticket,
{ {
int ret; int ret;
mbedtls_ssl_ticket_context *ctx = p_ticket; mbedtls_ssl_ticket_context *ctx = p_ticket;
unsigned char *p = start; unsigned char *key_name = start;
unsigned char *state; unsigned char *iv = start + 4;
unsigned char iv[16]; unsigned char *state_len_bytes = iv + 12;
size_t clear_len, enc_len, pad_len, i; unsigned char *state = state_len_bytes + 2;
unsigned char *tag;
size_t clear_len, ciph_len;
*tlen = 0; *tlen = 0;
if( ctx == NULL ) if( ctx == NULL || ctx->f_rng == NULL )
return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA ); return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
/* We need at least 16 bytes for key_name, 16 for IV, 2 for len /* We need at least 4 bytes for key_name, 12 for IV, 2 for len 16 for tag,
* 16 for padding, 32 for MAC, in addition to session itself, * in addition to session itself, that will be checked when writing it. */
* that will be checked when writing it. */ if( end - start < 4 + 12 + 2 + 16 )
if( end - start < 16 + 16 + 2 + 16 + 32 )
return( MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL ); return( MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL );
#if defined(MBEDTLS_THREADING_C) #if defined(MBEDTLS_THREADING_C)
@ -246,52 +254,36 @@ int mbedtls_ssl_ticket_write( void *p_ticket,
*ticket_lifetime = ctx->ticket_lifetime; *ticket_lifetime = ctx->ticket_lifetime;
/* Write key name */ memcpy( key_name, ctx->key_name, 4 );
memcpy( p, ctx->key_name, 16 );
p += 16;
/* Generate and write IV (with a copy for aes_crypt) */ if( ( ret = ctx->f_rng( ctx->p_rng, iv, 12 ) ) != 0 )
if( ( ret = ctx->f_rng( ctx->p_rng, p, 16 ) ) != 0 )
goto cleanup; goto cleanup;
memcpy( iv, p, 16 );
p += 16;
/* Dump session state */ /* Dump session state */
state = p + 2;
if( ( ret = ssl_save_session( session, if( ( ret = ssl_save_session( session,
state, end - state, &clear_len ) ) != 0 ) state, end - state, &clear_len ) ) != 0 ||
(unsigned long) clear_len > 65535 )
{ {
goto cleanup; goto cleanup;
} }
state_len_bytes[0] = ( clear_len >> 8 ) & 0xff;
state_len_bytes[1] = ( clear_len ) & 0xff;
/* Apply PKCS padding */ /* Encrypt and authenticate */
pad_len = 16 - clear_len % 16; tag = state + clear_len;
enc_len = clear_len + pad_len; if( ( ret = mbedtls_cipher_auth_encrypt( &ctx->cipher,
for( i = clear_len; i < enc_len; i++ ) iv, 12, key_name, 4 + 12 + 2,
state[i] = (unsigned char) pad_len; state, clear_len, state, &ciph_len, tag, 16 ) ) != 0 )
/* Encrypt */
if( ( ret = mbedtls_aes_crypt_cbc( &ctx->enc, MBEDTLS_AES_ENCRYPT,
enc_len, iv, state, state ) ) != 0 )
{ {
goto cleanup; goto cleanup;
} }
if( ciph_len != clear_len )
/* Write length */
*p++ = (unsigned char)( ( enc_len >> 8 ) & 0xFF );
*p++ = (unsigned char)( ( enc_len ) & 0xFF );
p = state + enc_len;
/* Compute and write MAC( key_name + iv + enc_state_len + enc_state ) */
if( ( ret = mbedtls_md_hmac( mbedtls_md_info_from_type( MBEDTLS_MD_SHA256 ),
ctx->mac_key, 16,
start, p - start, p ) ) != 0 )
{ {
ret = MBEDTLS_ERR_SSL_INTERNAL_ERROR;
goto cleanup; goto cleanup;
} }
p += 32;
*tlen = p - start; *tlen = 4 + 12 + 2 + 16 + ciph_len;
cleanup: cleanup:
#if defined(MBEDTLS_THREADING_C) #if defined(MBEDTLS_THREADING_C)
@ -313,16 +305,17 @@ int mbedtls_ssl_ticket_parse( void *p_ticket,
int ret; int ret;
mbedtls_ssl_ticket_context *ctx = p_ticket; mbedtls_ssl_ticket_context *ctx = p_ticket;
unsigned char *key_name = buf; unsigned char *key_name = buf;
unsigned char *iv = buf + 16; unsigned char *iv = buf + 4;
unsigned char *enc_len_p = iv + 16; unsigned char *enc_len_p = iv + 12;
unsigned char *ticket = enc_len_p + 2; unsigned char *ticket = enc_len_p + 2;
unsigned char *mac; unsigned char *tag;
unsigned char computed_mac[32]; size_t enc_len, clear_len;
size_t enc_len, clear_len, i;
unsigned char pad_len, diff;
if( len < 34 || ctx == NULL ) if( ctx == NULL || ctx->f_rng == NULL ||
len < 4 + 12 + 2 + 16 )
{
return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA ); return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
}
#if defined(MBEDTLS_THREADING_C) #if defined(MBEDTLS_THREADING_C)
if( ( ret = mbedtls_mutex_lock( &ctx->mutex ) ) != 0 ) if( ( ret = mbedtls_mutex_lock( &ctx->mutex ) ) != 0 )
@ -330,57 +323,34 @@ int mbedtls_ssl_ticket_parse( void *p_ticket,
#endif #endif
enc_len = ( enc_len_p[0] << 8 ) | enc_len_p[1]; enc_len = ( enc_len_p[0] << 8 ) | enc_len_p[1];
mac = ticket + enc_len; tag = ticket + enc_len;
if( len != enc_len + 66 ) if( len != 4 + 12 + 2 + enc_len + 16 )
{ {
ret = MBEDTLS_ERR_SSL_BAD_INPUT_DATA; ret = MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
goto cleanup; goto cleanup;
} }
/* Check name, in constant time though it's not a big secret */ /* Check name (public data) */
diff = 0; if( memcmp( key_name, ctx->key_name, 4 ) != 0 )
for( i = 0; i < 16; i++ )
diff |= key_name[i] ^ ctx->key_name[i];
/* don't return yet, check the MAC anyway */
/* Check mac, with constant-time buffer comparison */
if( ( ret = mbedtls_md_hmac( mbedtls_md_info_from_type( MBEDTLS_MD_SHA256 ),
ctx->mac_key, 16,
buf, len - 32, computed_mac ) ) != 0 )
{
goto cleanup;
}
for( i = 0; i < 32; i++ )
diff |= mac[i] ^ computed_mac[i];
/* Now return if ticket is not authentic, since we want to avoid
* decrypting arbitrary attacker-chosen data */
if( diff != 0 )
{ {
ret = MBEDTLS_ERR_SSL_INVALID_MAC; ret = MBEDTLS_ERR_SSL_INVALID_MAC;
goto cleanup; goto cleanup;
} }
/* Decrypt */ /* Decrypt and authenticate */
if( ( ret = mbedtls_aes_crypt_cbc( &ctx->dec, MBEDTLS_AES_DECRYPT, if( ( ret = mbedtls_cipher_auth_decrypt( &ctx->cipher, iv, 12,
enc_len, iv, ticket, ticket ) ) != 0 ) key_name, 4 + 12 + 2, ticket, enc_len,
ticket, &clear_len, tag, 16 ) ) != 0 )
{ {
/* TODO: convert AUTH_FAILED to INVALID_MAC */
goto cleanup; goto cleanup;
} }
if( clear_len != enc_len )
/* Check PKCS padding */ {
pad_len = ticket[enc_len - 1]; ret = MBEDTLS_ERR_SSL_INTERNAL_ERROR;
ret = 0;
for( i = 2; i < pad_len; i++ )
if( ticket[enc_len - i] != pad_len )
ret = MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
if( ret != 0 )
goto cleanup; goto cleanup;
}
clear_len = enc_len - pad_len;
/* Actually load session */ /* Actually load session */
if( ( ret = ssl_load_session( session, ticket, clear_len ) ) != 0 ) if( ( ret = ssl_load_session( session, ticket, clear_len ) ) != 0 )
@ -414,8 +384,7 @@ cleanup:
*/ */
void mbedtls_ssl_ticket_free( mbedtls_ssl_ticket_context *ctx ) void mbedtls_ssl_ticket_free( mbedtls_ssl_ticket_context *ctx )
{ {
mbedtls_aes_free( &ctx->enc ); mbedtls_cipher_free( &ctx->cipher );
mbedtls_aes_free( &ctx->dec );
#if defined(MBEDTLS_THREADING_C) #if defined(MBEDTLS_THREADING_C)
mbedtls_mutex_free( &ctx->mutex ); mbedtls_mutex_free( &ctx->mutex );