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TLS Shim Layer DRBG (#2274)
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* TLS Shim Layer DRBG

Use PKCS #11 to seed a DRBG instead of calling PKCS #11 for all random numbers needed by TLS.
This reduces the load on secure elements significantly when creating many random numbers.
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@lundinc2
lundinc2 authored Jul 15, 2020
1 parent cdfd000 commit 68e1584
Showing 1 changed file with 73 additions and 48 deletions.
121 changes: 73 additions & 48 deletions libraries/freertos_plus/standard/tls/src/iot_tls.c
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Original file line number Diff line number Diff line change
Expand Up @@ -125,6 +125,7 @@ typedef struct TLSContext
mbedtls_x509_crt xMbedX509Cli;
mbedtls_pk_context xMbedPkCtx;
mbedtls_pk_info_t xMbedPkInfo;
mbedtls_ctr_drbg_context xMbedDrbgCtx;

/* PKCS#11. */
CK_FUNCTION_LIST_PTR pxP11FunctionList;
Expand Down Expand Up @@ -158,6 +159,7 @@ static void prvFreeContext( TLSContext_t * pxCtx )
mbedtls_ssl_close_notify( &pxCtx->xMbedSslCtx ); /*lint !e534 The error is already taken care of inside mbedtls_ssl_close_notify*/
mbedtls_ssl_free( &pxCtx->xMbedSslCtx );
mbedtls_ssl_config_free( &pxCtx->xMbedSslConfig );
mbedtls_ctr_drbg_free( &pxCtx->xMbedDrbgCtx );

/* Cleanup PKCS11 only if the handshake was started. */
if( ( TLS_HANDSHAKE_NOT_STARTED != pxCtx->xTLSHandshakeState ) &&
Expand Down Expand Up @@ -228,11 +230,11 @@ static int prvGenerateRandomBytes( void * pvCtx,
size_t xRandomLength )
{
TLSContext_t * pxCtx = ( TLSContext_t * ) pvCtx; /*lint !e9087 !e9079 Allow casting void* to other types. */
BaseType_t xResult;
int xResult = 0;

xResult = pxCtx->pxP11FunctionList->C_GenerateRandom( pxCtx->xP11Session, pucRandom, xRandomLength );
xResult = mbedtls_ctr_drbg_random( &pxCtx->xMbedDrbgCtx, pucRandom, xRandomLength );

if( xResult != CKR_OK )
if( xResult != 0 )
{
TLS_PRINT( ( "ERROR: Failed to generate random bytes %s : %s \r\n",
mbedtlsHighLevelCodeOrDefault( xResult ),
Expand Down Expand Up @@ -524,7 +526,6 @@ static int prvReadCertificateIntoContext( TLSContext_t * pxTlsContext,
static int prvInitializeClientCredential( TLSContext_t * pxCtx )
{
BaseType_t xResult = CKR_OK;
CK_SLOT_ID * pxSlotIds = NULL;
CK_ULONG xCount = 0;
CK_ATTRIBUTE xTemplate[ 2 ];
mbedtls_pk_type_t xKeyAlgo = ( mbedtls_pk_type_t ) ~0;
Expand All @@ -533,42 +534,10 @@ static int prvInitializeClientCredential( TLSContext_t * pxCtx )
/* Initialize the mbed contexts. */
mbedtls_x509_crt_init( &pxCtx->xMbedX509Cli );

/* Get the PKCS #11 module/token slot count. */
if( CKR_OK == xResult )
{
xResult = ( BaseType_t ) pxCtx->pxP11FunctionList->C_GetSlotList( CK_TRUE,
NULL,
&xCount );
}

/* Allocate memory to store the token slots. */
if( CKR_OK == xResult )
{
pxSlotIds = ( CK_SLOT_ID * ) pvPortMalloc( sizeof( CK_SLOT_ID ) * xCount );

if( NULL == pxSlotIds )
{
xResult = CKR_HOST_MEMORY;
}
}

/* Get all of the available private key slot identities. */
if( CKR_OK == xResult )
if( pxCtx->xP11Session == CK_INVALID_HANDLE )
{
xResult = ( BaseType_t ) pxCtx->pxP11FunctionList->C_GetSlotList( CK_TRUE,
pxSlotIds,
&xCount );
}

/* Start a private session with the P#11 module using the first
* enumerated slot. */
if( CKR_OK == xResult )
{
xResult = ( BaseType_t ) pxCtx->pxP11FunctionList->C_OpenSession( pxSlotIds[ 0 ],
CKF_SERIAL_SESSION,
NULL,
NULL,
&pxCtx->xP11Session );
xResult = CKR_SESSION_HANDLE_INVALID;
TLS_PRINT( ( "Error: PKCS #11 session was not initialized.\r\n" ) );
}

/* Put the module in authenticated mode. */
Expand Down Expand Up @@ -682,17 +651,54 @@ static int prvInitializeClientCredential( TLSContext_t * pxCtx )
&pxCtx->xMbedPkCtx );
}

/* Free memory. */
if( NULL != pxSlotIds )
{
vPortFree( pxSlotIds );
}

return xResult;
}

/*-----------------------------------------------------------*/

/**
* @brief Helper to seed the entropy module used by the DRBG. Periodically this
* this function will be called to get more random data from the TRNG.
*
* @param[in] tlsContext The TLS context.
* @param[out] outputBuffer The output buffer to return the generated random data.
* @param[in] outputBufferLength Length of the output buffer.
*
* @return Zero on success, otherwise a negative error code telling the cause of the error.
*/
static int prvEntropyCallback( void * tlsContext,
unsigned char * outputBuffer,
size_t outputBufferLength )
{
int ret = MBEDTLS_ERR_ENTROPY_SOURCE_FAILED;
CK_RV xResult = CKR_OK;
TLSContext_t * pxCtx = ( TLSContext_t * ) tlsContext; /*lint !e9087 !e9079 Allow casting void* to other types. */

if( pxCtx->xP11Session != CK_INVALID_HANDLE )
{
xResult = C_GenerateRandom( pxCtx->xP11Session,
outputBuffer,
outputBufferLength );
}
else
{
xResult = CKR_SESSION_HANDLE_INVALID;
TLS_PRINT( ( "Error: PKCS #11 session was not initialized.\r\n" ) );
}

if( xResult == CKR_OK )
{
ret = 0;
}
else
{
TLS_PRINT( ( "Error: PKCS #11 C_GenerateRandom failed with error code:" \
"%d\r\n", xResult ) );
}

return ret;
}

/*
* Interface routines.
*/
Expand All @@ -701,6 +707,7 @@ BaseType_t TLS_Init( void ** ppvContext,
TLSParams_t * pxParams )
{
BaseType_t xResult = CKR_OK;
int mbedTLSResult = 0;
TLSContext_t * pxCtx = NULL;
CK_C_GetFunctionList xCkGetFunctionList = NULL;

Expand All @@ -726,17 +733,35 @@ BaseType_t TLS_Init( void ** ppvContext,
xCkGetFunctionList = C_GetFunctionList;
xResult = ( BaseType_t ) xCkGetFunctionList( &pxCtx->pxP11FunctionList );

/* Ensure that the PKCS #11 module is initialized. */
if( CKR_OK == xResult )
/* Ensure that the PKCS #11 module is initialized and create a session. */
if( xResult == CKR_OK )
{
xResult = ( BaseType_t ) xInitializePKCS11();
xResult = xInitializePkcs11Session( &pxCtx->xP11Session );

/* It is ok if the module was previously initialized. */
if( xResult == CKR_CRYPTOKI_ALREADY_INITIALIZED )
{
xResult = CKR_OK;
}
}

if( xResult == CKR_OK )
{
mbedtls_ctr_drbg_init( &pxCtx->xMbedDrbgCtx );
mbedTLSResult = mbedtls_ctr_drbg_seed( &pxCtx->xMbedDrbgCtx,
prvEntropyCallback,
pxCtx,
NULL,
0 );

if( 0 != mbedTLSResult )
{
TLS_PRINT( ( "ERROR: Failed to setup DRBG seed %s : %s \r\n",
mbedtlsHighLevelCodeOrDefault( mbedTLSResult ),
mbedtlsLowLevelCodeOrDefault( mbedTLSResult ) ) );
xResult = CKR_FUNCTION_FAILED;
}
}
}
else
{
Expand Down

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