CTR_DRBG: add the possibility of grabbing entropy for a nonce
Add a new function mbedtls_ctr_drbg_set_nonce_len() which configures
the DRBG instance to call f_entropy a second time during the initial
seeding to grab a nonce.
The default nonce length is 0, so there is no behavior change unless
the user calls the new function.
diff --git a/include/mbedtls/ctr_drbg.h b/include/mbedtls/ctr_drbg.h
index 86d546a..2b0c617 100644
--- a/include/mbedtls/ctr_drbg.h
+++ b/include/mbedtls/ctr_drbg.h
@@ -22,6 +22,9 @@
* - 256 bits if AES-256 is used, #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set
* to 32 or more, and the DRBG is initialized with an explicit
* nonce in the \c custom parameter to mbedtls_ctr_drbg_seed().
+ * - 256 bits if AES-256 is used, #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set
+ * to 32 or more, and mbedtls_ctr_drbg_set_nonce_len() is called to set
+ * an entropy nonce length of 16 bytes or more.
* - 128 bits if AES-256 is used but #MBEDTLS_CTR_DRBG_ENTROPY_LEN is
* between 24 and 47 and the DRBG is not initialized with an explicit
* nonce (see mbedtls_ctr_drbg_seed()).
@@ -29,6 +32,9 @@
* and #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set to 24 or more (which is
* always the case unless it is explicitly set to a different value
* in config.h).
+ * - 128 bits if AES-128 is used (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY enabled)
+ * to 16 or more, and mbedtls_ctr_drbg_set_nonce_len() is called to set
+ * an entropy nonce length of 8 bytes or more.
*
* Note that the value of #MBEDTLS_CTR_DRBG_ENTROPY_LEN defaults to:
* - \c 48 if the module \c MBEDTLS_SHA512_C is enabled and the symbol
@@ -172,7 +178,11 @@
int reseed_counter; /*!< The reseed counter.
* This is the number of requests that have
* been made since the last (re)seeding,
- * minus one. */
+ * minus one.
+ * Before the initial seeding, this field
+ * contains the amount of entropy in bytes
+ * to use as a nonce for the initial seeding.
+ */
int prediction_resistance; /*!< This determines whether prediction
resistance is enabled, that is
whether to systematically reseed before
@@ -222,43 +232,45 @@
* The entropy length is #MBEDTLS_CTR_DRBG_ENTROPY_LEN by default.
* You can override it by calling mbedtls_ctr_drbg_set_entropy_len().
*
- * You can provide a personalization string in addition to the
+ * You can provide a nonce and personalization string in addition to the
* entropy source, to make this instantiation as unique as possible.
+ * See SP 800-90A §8.6.7 for more details about nonces.
*
- * \note The _seed_material_ value passed to the derivation
- * function in the CTR_DRBG Instantiate Process
- * described in NIST SP 800-90A §10.2.1.3.2
- * is the concatenation of the string obtained from
- * calling \p f_entropy and the \p custom string.
- * The origin of the nonce depends on the value of
- * the entropy length relative to the security strength.
- * - If the entropy length is at least 1.5 times the
- * security strength then the nonce is taken from the
- * string obtained with \p f_entropy.
- * - If the entropy length is less than the security
- * strength, then the nonce is taken from \p custom.
- * In this case, for compliance with SP 800-90A,
- * you must pass a unique value of \p custom at
- * each invocation. See SP 800-90A §8.6.7 for more
- * details.
- */
-#if MBEDTLS_CTR_DRBG_ENTROPY_LEN < MBEDTLS_CTR_DRBG_KEYSIZE * 3 / 2
-/** \warning When #MBEDTLS_CTR_DRBG_ENTROPY_LEN is less than
- * #MBEDTLS_CTR_DRBG_KEYSIZE * 3 / 2, to achieve the
- * maximum security strength permitted by CTR_DRBG,
- * you must pass a value of \p custom that is a nonce:
- * this value must never be repeated in subsequent
- * runs of the same application or on a different
- * device.
- */
-#endif
-/**
+ * The _seed_material_ value passed to the derivation function in
+ * the CTR_DRBG Instantiate Process described in NIST SP 800-90A §10.2.1.3.2
+ * is the concatenation of the following strings:
+ * - A string obtained by calling \p f_entropy function for the entropy
+ * length.
+ * - A string obtained by calling \p f_entropy function for the nonce
+ * length set with mbedtls_ctr_drbg_set_nonce_len(). If the entropy
+ * nonce length is \c 0, this function does not make a second call
+ * to \p f_entropy.
+ * - The \p custom string.
+ *
+ * \note To achieve the nominal security strength permitted
+ * by CTR_DRBG, the entropy length must be:
+ * - at least 16 bytes for a 128-bit strength
+ * (maximum achievable strength when using AES-128);
+ * - at least 32 bytes for a 256-bit strength
+ * (maximum achievable strength when using AES-256).
+ *
+ * In addition, if you do not pass a nonce in \p custom,
+ * the sum of the entropy length
+ * (#MBEDTLS_CTR_DRBG_ENTROPY_LEN unless overridden with
+ * mbedtls_ctr_drbg_set_entropy_len())
+ * and the entropy nonce length (\c 0 unless overridden
+ * with mbedtls_ctr_drbg_set_nonce_len()) must be:
+ * - at least 24 bytes for a 128-bit strength
+ * (maximum achievable strength when using AES-128);
+ * - at least 48 bytes for a 256-bit strength
+ * (maximum achievable strength when using AES-256).
+ *
* \param ctx The CTR_DRBG context to seed.
* \param f_entropy The entropy callback, taking as arguments the
* \p p_entropy context, the buffer to fill, and the
* length of the buffer.
* \p f_entropy is always called with a buffer size
- * equal to the entropy length.
+ * less than or equal to the entropy length.
* \param p_entropy The entropy context to pass to \p f_entropy.
* \param custom The personalization string.
* This can be \c NULL, in which case the personalization
@@ -320,12 +332,36 @@
*
* \param ctx The CTR_DRBG context.
* \param len The amount of entropy to grab, in bytes.
- * This must be at most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
+ * This must be at most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT
+ * and at most the maximum length accepted by the
+ * entropy function that is set in the context.
*/
void mbedtls_ctr_drbg_set_entropy_len( mbedtls_ctr_drbg_context *ctx,
size_t len );
/**
+ * \brief This function sets the amount of entropy grabbed
+ * as a nonce for the initial seeding.
+ *
+ * Call this function before calling mbedtls_ctr_drbg_seed() to read
+ * a nonce from the entropy source during the initial seeding.
+ *
+ * \param ctx The CTR_DRBG context.
+ * \param len The amount of entropy to grab for the nonce, in bytes.
+ * This must be at most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT
+ * and at most the maximum length accepted by the
+ * entropy function that is set in the context.
+ *
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG if \p len is
+ * more than #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
+ * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED
+ * if the initial seeding has already taken place.
+ */
+int mbedtls_ctr_drbg_set_nonce_len( mbedtls_ctr_drbg_context *ctx,
+ size_t len );
+
+/**
* \brief This function sets the reseed interval.
*
* The reseed interval is the number of calls to mbedtls_ctr_drbg_random()
diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index 92316da..85bd04f 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -86,6 +86,32 @@
ctx->entropy_len = len;
}
+int mbedtls_ctr_drbg_set_nonce_len( mbedtls_ctr_drbg_context *ctx,
+ size_t len )
+{
+ /* If mbedtls_ctr_drbg_seed() has already been called, it's
+ * too late. Return the error code that's closest to making sense. */
+ if( ctx->f_entropy != NULL )
+ return( MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED );
+
+ if( len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT )
+ return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
+#if SIZE_MAX > INT_MAX
+ /* This shouldn't be an issue because
+ * MBEDTLS_CTR_DRBG_MAX_SEED_INPUT < INT_MAX in any sensible
+ * configuration, but make sure anyway. */
+ if( len > INT_MAX )
+ return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
+#endif
+
+ /* For backward compatibility with Mbed TLS <= 2.19, store the
+ * entropy nonce length in a field that already exists, but isn't
+ * used until after the initial seeding. */
+ /* Due to the capping of len above, the value fits in an int. */
+ ctx->reseed_counter = (int) len;
+ return( 0 );
+}
+
void mbedtls_ctr_drbg_set_reseed_interval( mbedtls_ctr_drbg_context *ctx,
int interval )
{
@@ -319,7 +345,7 @@
#endif /* MBEDTLS_DEPRECATED_REMOVED */
/* CTR_DRBG_Reseed with derivation function (SP 800-90A §10.2.1.4.2)
- * mbedtls_ctr_drbg_reseed(ctx, additional, len)
+ * mbedtls_ctr_drbg_reseed(ctx, additional, len, nonce_len)
* implements
* CTR_DRBG_Reseed(working_state, entropy_input, additional_input)
* -> new_working_state
@@ -327,11 +353,14 @@
* ctx contains working_state
* additional[:len] = additional_input
* and entropy_input comes from calling ctx->f_entropy
+ * for (ctx->entropy_len + nonce_len) bytes
* and with output
* ctx contains new_working_state
*/
-int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx,
- const unsigned char *additional, size_t len )
+int mbedtls_ctr_drbg_reseed_internal( mbedtls_ctr_drbg_context *ctx,
+ const unsigned char *additional,
+ size_t len,
+ size_t nonce_len )
{
unsigned char seed[MBEDTLS_CTR_DRBG_MAX_SEED_INPUT];
size_t seedlen = 0;
@@ -339,7 +368,9 @@
if( ctx->entropy_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT )
return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
- if( len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len )
+ if( nonce_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len )
+ return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
+ if( len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len - nonce_len )
return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
memset( seed, 0, MBEDTLS_CTR_DRBG_MAX_SEED_INPUT );
@@ -351,6 +382,16 @@
}
seedlen += ctx->entropy_len;
+ /* Gather entropy for a nonce if requested. */
+ if( nonce_len != 0 )
+ {
+ if( 0 != ctx->f_entropy( ctx->p_entropy, seed, nonce_len ) )
+ {
+ return( MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED );
+ }
+ seedlen += nonce_len;
+ }
+
/* Add additional data if provided. */
if( additional != NULL && len != 0 )
{
@@ -372,6 +413,12 @@
return( ret );
}
+int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx,
+ const unsigned char *additional, size_t len )
+{
+ return( mbedtls_ctr_drbg_reseed_internal( ctx, additional, len, 0 ) );
+}
+
/* CTR_DRBG_Instantiate with derivation function (SP 800-90A §10.2.1.3.2)
* mbedtls_ctr_drbg_seed(ctx, f_entropy, p_entropy, custom, len)
* implements
@@ -403,16 +450,18 @@
ctx->entropy_len = MBEDTLS_CTR_DRBG_ENTROPY_LEN;
ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;
- /*
- * Initialize with an empty key
- */
+ /* Initialize with an empty key. */
if( ( ret = mbedtls_aes_setkey_enc( &ctx->aes_ctx, key,
MBEDTLS_CTR_DRBG_KEYBITS ) ) != 0 )
{
return( ret );
}
- if( ( ret = mbedtls_ctr_drbg_reseed( ctx, custom, len ) ) != 0 )
+ /* Do the initial seeding.
+ * ctx->reseed_counter contains the desired amount of entropy to
+ * grab for a nonce (see mbedtls_ctr_drbg_set_nonce_len()). */
+ if( ( ret = mbedtls_ctr_drbg_reseed_internal( ctx, custom, len,
+ ctx->reseed_counter ) ) != 0 )
{
return( ret );
}