Make it possible to include psa_crypto_random_impl.h with psa_util.h

There was some intentional duplication between
library/psa_crypto_random_impl.h and include/mbedtls/psa_util.h, with
the intent that the compiler would complain if one file was edited in
a way that's incompatible with the other file. However, the two files
were never included together, and in fact could not be included
together because some definitions can't be duplicated (type, static
variable).

Now library/psa_crypto_random_impl.h includes
include/mbedtls/psa_util.h, so the compiler will check what it can.
There is less redundancy since it isn't always possible to declare
something twice (specifically, types can't be declared).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/psa_util.h

@@ -420,8 +420,9 @@ static inline int mbedtls_psa_tls_ecpoint_to_psa_ec( unsigned char const *src,
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
 
 /* Expose whatever RNG the PSA subsystem uses to applications using the
- * mbedtls_xxx API. The declarations here need to be consistent with the
- * implementation in library/psa_crypto_random_impl.h. */
+ * mbedtls_xxx API. The declarations and definitions here need to be
+ * consistent with the implementation in library/psa_crypto_random_impl.h.
+ * See that file for implementation documentation. */
 #if defined(MBEDTLS_PSA_CRYPTO_C)
 
 /* The type of a `f_rng` random generator function that many library functions

library/psa_crypto_random_impl.h

@@ -3,7 +3,12 @@
  * \brief PSA crypto random generator implementation abstraction.
  *
  * The definitions here need to be consistent with the declarations
- * in include/mbedtls/psa_util.h.
+ * in include/mbedtls/psa_util.h. This file contains some redundant
+ * declarations to increase the chance that a compiler will detect
+ * inconsistencies if one file is changed without updating the other,
+ * but not all potential inconsistencies can be enforced, so make sure
+ * to check the public declarations and contracts in
+ * include/mbedtls/psa_util.h if you modify this file.
  */
 /*
  *  Copyright The Mbed TLS Contributors
@@ -25,6 +30,8 @@
 #ifndef PSA_CRYPTO_RANDOM_IMPL_H
 #define PSA_CRYPTO_RANDOM_IMPL_H
 
+#include <mbedtls/psa_util.h>
+
 #if defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)
 
 #include <string.h>
@@ -78,14 +85,6 @@ int mbedtls_psa_get_random( void *p_rng,
 
 #include "mbedtls/entropy.h"
 
-/** The type of the PSA DRBG context.
- */
-#if defined(MBEDTLS_CTR_DRBG_C)
-typedef mbedtls_ctr_drbg_context mbedtls_psa_drbg_context_t;
-#elif defined(MBEDTLS_HMAC_DRBG_C)
-typedef mbedtls_hmac_drbg_context mbedtls_psa_drbg_context_t;
-#endif
-
 /** Initialize the PSA DRBG.
  *
  * \param p_rng        Pointer to the Mbed TLS DRBG state.
@@ -125,34 +124,9 @@ typedef struct
     mbedtls_psa_drbg_context_t drbg;
 } mbedtls_psa_random_context_t;
 
-/* The type of an Mbed TLS random generator function. This should be
- * part of the public API instead of repeating the type everywhere.
- * For the time being, declare it here. Declaring a type is necessary
- * to define mbedtls_psa_get_random as a variable of a function pointer
- * type without incurring the wrath of check-names.sh. */
-typedef int mbedtls_f_rng_t( void *p_rng, unsigned char *output, size_t output_size );
-
-/** Return random data.
- *
- * This function is suitable as the \p f_rng parameter to Mbed TLS functions
- * that require a random generator. Use #MBEDTLS_PSA_RANDOM_STATE to
- * obtain the \p p_rng parameter.
- *
- * \param p_rng         The DRBG context. This must be
- *                      #MBEDTLS_PSA_RANDOM_STATE.
- * \param output        The buffer to fill.
- * \param output_len    The length of the buffer in bytes.
- *                      It must be at most #MBEDTLS_PSA_RANDOM_MAX_REQUEST.
- *
- * \retval              \c 0 on success.
- * \return              \c MBEDTLS_ERR_xxx_DRBG_xxx or
- *                      \c MBEDTLS_ERR_PLATFORM_xxx on failure.
- */
-#if defined(MBEDTLS_CTR_DRBG_C)
-static mbedtls_f_rng_t *const mbedtls_psa_get_random = mbedtls_ctr_drbg_random;
-#elif defined(MBEDTLS_HMAC_DRBG_C)
-static mbedtls_f_rng_t *const mbedtls_psa_get_random = mbedtls_hmac_drbg_random;
-#endif
+/* Defined in include/mbedtls/psa_util.h so that it's visible to
+ * application code. */
+static mbedtls_f_rng_t *const mbedtls_psa_get_random;
 
 /** The maximum number of bytes that mbedtls_psa_get_random() is expected to
  * return.
@@ -170,6 +144,9 @@ static mbedtls_f_rng_t *const mbedtls_psa_get_random = mbedtls_hmac_drbg_random;
  */
 /* psa_crypto.c sets this variable to a pointer to the DRBG state in the
  * global PSA crypto state. */
+/* The type `mbedtls_psa_drbg_context_t` is defined in
+ * include/mbedtls/psa_util.h so that `mbedtls_psa_random_state` can be
+ * declared there and be visible to application code. */
 extern mbedtls_psa_drbg_context_t *const mbedtls_psa_random_state;
 
 /** A pointer to the PSA DRBG state.