CTR_DRBG: grab a nonce from the entropy source if needed

include/mbedtls/ctr_drbg.h

@@ -12,30 +12,14 @@
  * The Mbed TLS implementation of CTR_DRBG uses AES-256 (default) or AES-128
  * (if \c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is enabled at compile time)
  * as the underlying block cipher, with a derivation function.
- * The initial seeding grabs #MBEDTLS_CTR_DRBG_ENTROPY_LEN bytes of entropy.
- * See the documentation of mbedtls_ctr_drbg_seed() for more details.
- *
- * Based on NIST SP 800-90A §10.2.1 table 3 and NIST SP 800-57 part 1 table 2,
- * here are the security strengths achieved in typical configuration:
- * - 256 bits under the default configuration of the library, with AES-256
- *   and with #MBEDTLS_CTR_DRBG_ENTROPY_LEN set to 48 or more.
- * - 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().
- * - 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()).
- * - 128 bits if AES-128 is used (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY enabled)
- *   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).
- *
- * 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
- *   \c MBEDTLS_ENTROPY_FORCE_SHA256 is disabled at compile time.
- *   This is the default configuration of the library.
- * - \c 32 if the module \c MBEDTLS_SHA512_C is disabled at compile time.
- * - \c 32 if \c MBEDTLS_ENTROPY_FORCE_SHA256 is enabled at compile time.
+ *
+ * The security strength as defined in NIST SP 800-90A is
+ * 128 bits when AES-128 is used (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY enabled)
+ * and 256 bits otherwise, provided that #MBEDTLS_CTR_DRBG_ENTROPY_LEN is
+ * kept at its default value (and not overridden in config.h) and that the
+ * DRBG instance is set up with default parameters.
+ * See the documentation of mbedtls_ctr_drbg_seed() for more
+ * information.
  */
 /*
  *  Copyright (C) 2006-2019, Arm Limited (or its affiliates), All Rights Reserved
@@ -163,20 +147,47 @@
 extern "C" {
 #endif
 
+#if MBEDTLS_CTR_DRBG_ENTROPY_LEN >= MBEDTLS_CTR_DRBG_KEYSIZE * 3 / 2
+/** The default length of the nonce read from the entropy source.
+ *
+ * This is \c 0 because a single read from the entropy source is sufficient
+ * to include a nonce.
+ * See the documentation of mbedtls_ctr_drbg_seed() for more information.
+ */
+#define MBEDTLS_CTR_DRBG_ENTROPY_NONCE_LEN 0
+#else
+/** The default length of the nonce read from the entropy source.
+ *
+ * This is half of the default entropy length because a single read from
+ * the entropy source does not provide enough material to form a nonce.
+ * See the documentation of mbedtls_ctr_drbg_seed() for more information.
+ */
+#define MBEDTLS_CTR_DRBG_ENTROPY_NONCE_LEN ( MBEDTLS_CTR_DRBG_ENTROPY_LEN + 1 ) / 2
+#endif
+
 /**
  * \brief          The CTR_DRBG context structure.
  */
 typedef struct mbedtls_ctr_drbg_context
 {
     unsigned char counter[16];  /*!< The counter (V). */
-    int reseed_counter;         /*!< The reseed counter. */
+    int reseed_counter;         /*!< The reseed counter.
+                                 * This is the number of requests that have
+                                 * been made since the last (re)seeding,
+                                 * 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
                                      each random generation. */
     size_t entropy_len;         /*!< The amount of entropy grabbed on each
-                                     seed or reseed operation. */
-    int reseed_interval;        /*!< The reseed interval. */
+                                     seed or reseed operation, in bytes. */
+    int reseed_interval;        /*!< The reseed interval.
+                                 * This is the maximum number of requests
+                                 * that can be made between reseedings. */
 
     mbedtls_aes_context aes_ctx;        /*!< The AES context. */
 
@@ -217,43 +228,68 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
  * 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
+ * The entropy nonce length is:
+ * - \c 0 if the entropy length is at least 3/2 times the entropy length,
+ *   which guarantees that the security strength is the maximum permitted
+ *   by the key size and entropy length according to NIST SP 800-90A §10.2.1;
+ * - Half the entropy length otherwise.
+ * You can override it by calling mbedtls_ctr_drbg_set_nonce_len().
+ * With the default entropy length, the entropy nonce length is
+ * #MBEDTLS_CTR_DRBG_ENTROPY_NONCE_LEN.
+ *
+ * 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.
+ * 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.
  */
-#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.
+#if MBEDTLS_CTR_DRBG_ENTROPY_NONCE_LEN == 0
+/**
+ * - If mbedtls_ctr_drbg_set_nonce_len() has been called, a string
+ *   obtained by calling \p f_entropy function for the specified length.
+ */
+#else
+/**
+ * - A string obtained by calling \p f_entropy function for the entropy nonce
+ *   length. If the entropy nonce length is \c 0, this function does not
+ *   make a second call to \p f_entropy.
  */
 #endif
 /**
+ * - 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
+ *                      and the entropy nonce length 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.
+ *                      It must have been initialized with
+ *                      mbedtls_ctr_drbg_init().
+ *                      After a successful call to mbedtls_ctr_drbg_seed(),
+ *                      you may not call mbedtls_ctr_drbg_seed() again on
+ *                      the same context unless you call
+ *                      mbedtls_ctr_drbg_free() and mbedtls_ctr_drbg_init()
+ *                      again first.
  * \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
@@ -301,11 +337,6 @@ void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx,
  *
  * The default value is #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
  *
- * \note                mbedtls_ctr_drbg_seed() always sets the entropy length
- *                      to #MBEDTLS_CTR_DRBG_ENTROPY_LEN, so this function
- *                      only has an effect when it is called after
- *                      mbedtls_ctr_drbg_seed().
- *
  * \note                The security strength of CTR_DRBG is bounded by the
  *                      entropy length. Thus:
  *                      - When using AES-256
@@ -320,11 +351,35 @@ void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx,
  *
  * \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.
  *

include/mbedtls/hmac_drbg.h

@@ -139,7 +139,7 @@ void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
  *                      Note that SHA-256 is just as efficient as SHA-224.
  *                      The security strength can be reduced if a smaller
  *                      entropy length is set with
- *                      mbedtls_hmac_drbg_set_entropy_len() afterwards.
+ *                      mbedtls_hmac_drbg_set_entropy_len().
  *
  * \note                The default entropy length is the security strength
  *                      (converted from bits to bytes). You can override
@@ -222,14 +222,9 @@ void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx
 
 /**
  * \brief               This function sets the amount of entropy grabbed on each
- *                      reseed.
+ *                      seed or reseed.
  *
- * The default value is set by mbedtls_hmac_drbg_seed().
- *
- * \note                mbedtls_hmac_drbg_seed() always sets the entropy length
- *                      to the default value based on the chosen MD algorithm,
- *                      so this function only has an effect if it is called
- *                      after mbedtls_hmac_drbg_seed().
+ * See the documentation of mbedtls_hmac_drbg_seed() for the default value.
  *
  * \param ctx           The HMAC_DRBG context.
  * \param len           The amount of entropy to grab, in bytes.