diff --git a/include/mbedtls/pk_info.h b/include/mbedtls/pk_info.h
index fe2dbdf..a90e489 100644
--- a/include/mbedtls/pk_info.h
+++ b/include/mbedtls/pk_info.h
@@ -32,58 +32,196 @@
 
 #include "pk.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Methods that opaque key pair objects must implement.
+ *
+ * Engines that interface with external cryptographic processors must
+ * implement this interface. Platform-specific hardware accelerators
+ * that can be used for all keys of a given type should use alternative
+ * ("xxx_alt") interfaces instead. This interface allows using different
+ * engines for each key.
+ *
+ * An engine for asymmetric cryptography must implement the interface
+ * described in this structure. The interface for the engine may be
+ * exposed in one of two ways:
+ *
+ * - Declare the mbedtls_pk_info_t structure and instruct users to call
+ *   mbedtls_pk_setup with that structure.
+ * - Keep the mbedtls_pk_info_t structure hidden and declare a function
+ *   to call instead of mbedtls_pk_setup. This function should have an
+ *   interface of the form
+ *    `int mbedtls_pk_setup_myengine(mbedtls_pk_context *, ...)`
+ *   where the extra parameters depend on the engine, e.g. handles to keys
+ *   stored in an external cryptographic module.
+ *
+ * Unless otherwise indicated, functions returning int must return an
+ * Mbed TLS status code, either 0 for success or a negative value to indicate
+ * an error. It is recommended to use the MBEDTLS_ERR_PK_XXX error codes
+ * defined in pk.h.
+ *
+ * Some methods are optional; this is clearly indicated in their description.
+ * If a method is optional, then an opaque key implementation may put NULL
+ * in the corresponding field. The corresponding function in pk.h will
+ * return MBEDTLS_ERR_PK_TYPE_MISMATCH in this case.
+ *
+ * \note If you are using the PK interface to perform operations on
+ * keys, call the functions in pk.h. The interface in this file should only
+ * be used by implementers of opaque key engines.
+ */
 struct mbedtls_pk_info_t
 {
-    /** Key pair type with indication of supported algorithms */
+    /** Key pair type.
+     *
+     * mbedtls_pk_get_type() returns this value.
+     *
+     * For transparent keys, this contains an indication of supported
+     * algorithms. For opaque keys, this is \c MBEDTLS_PK_OPAQUE. */
     mbedtls_pk_type_t type;
 
-    /** Type name */
+    /** Type name.
+     *
+     * mbedtls_pk_get_name() returns this value. It must be a
+     * null-terminated string.
+     *
+     * For transparent keys, this reflects the key type. For opaque keys,
+     * this reflects the cryptographic module driver. */
     const char *name;
 
-    /** Get key size in bits */
+    /** Get key size in bits.
+     *
+     * mbedtls_pk_get_bitlen() returns this value.
+     *
+     * This function cannot fail. */
     size_t (*get_bitlen)( const void *ctx );
 
-    /** Tell if the context implements this type (e.g. ECKEY can do ECDSA) */
+    /** Tell if the context implements this type (e.g.\ ECKEY can do ECDSA).
+     *
+     * mbedtls_pk_can_do() calls this function.
+     *
+     * This function is only based on the key type. It does not take any
+     * usage restrictions into account. */
     int (*can_do)( const void * ctx, mbedtls_pk_type_t type );
 
-    /** Verify signature */
+    /** Verify signature
+     *
+     * mbedtls_pk_verify() calls this function.
+     *
+     * Opaque implementations may omit this method if they do not support
+     * signature verification. */
     int (*verify_func)( void *ctx, mbedtls_md_type_t md_alg,
                         const unsigned char *hash, size_t hash_len,
                         const unsigned char *sig, size_t sig_len );
 
-    /** Make signature */
+    /** Make signature
+     *
+     * mbedtls_pk_sign() calls this function.
+     *
+     * Assume that the buffer \c sig has room for
+     * \c signature_size_func(ctx) bytes.
+     *
+     * The arguments \c f_rng and \c p_rng are provided in case the
+     * algorithm requires randomization. Implementations are not
+     * required to use it if they have their own random source. If \c
+     * f_rng is null, the implementation should operate if it can, and
+     * return #MBEDTLS_ERR_PK_BAD_INPUT_DATA otherwise.
+     *
+     * Opaque implementations may omit this method if they do not support
+     * signature. */
     int (*sign_func)( void *ctx, mbedtls_md_type_t md_alg,
                       const unsigned char *hash, size_t hash_len,
                       unsigned char *sig, size_t *sig_len,
                       int (*f_rng)(void *, unsigned char *, size_t),
                       void *p_rng );
 
-    /** Decrypt message */
+    /** Decrypt message
+     *
+     * mbedtls_pk_decrypt() calls this function.
+     *
+     * The arguments \c f_rng and \c p_rng are provided in case the
+     * algorithm requires randomization. Implementations are not
+     * required to use it if they have their own random source. If \c
+     * f_rng is null, the implementation should operate if it can, and
+     * return #MBEDTLS_ERR_PK_BAD_INPUT_DATA otherwise.
+     *
+     * Opaque implementations may omit this method if they do not support
+     * decryption. */
     int (*decrypt_func)( void *ctx, const unsigned char *input, size_t ilen,
                          unsigned char *output, size_t *olen, size_t osize,
                          int (*f_rng)(void *, unsigned char *, size_t),
                          void *p_rng );
 
-    /** Encrypt message */
+    /** Encrypt message
+     *
+     * mbedtls_pk_decrypt() calls this function.
+     *
+     * The arguments \c f_rng and \c p_rng are provided in case the
+     * algorithm requires randomization. Implementations are not
+     * required to use it if they have their own random source. If \c
+     * f_rng is null, the implementation should operate if it can, and
+     * return #MBEDTLS_ERR_PK_BAD_INPUT_DATA otherwise.
+     *
+     * Opaque implementations may omit this method if they do not support
+     * encryption. */
     int (*encrypt_func)( void *ctx, const unsigned char *input, size_t ilen,
                          unsigned char *output, size_t *olen, size_t osize,
                          int (*f_rng)(void *, unsigned char *, size_t),
                          void *p_rng );
 
-    /** Check public-private key pair */
-    int (*check_pair_func)( const void *pub, const void *prv );
+    /** Check public-private key pair
+     *
+     * mbedtls_pk_check_pair() calls this function on the private key pair
+     * object \c prv. The other argument \c pub may be of any type, but it
+     * is guaranteed to be initialized.
+     *
+     * Opaque implementations may omit this method. */
+    int (*check_pair_func)( const mbedtls_pk_context *pub, const void *prv );
 
-    /** Allocate a new context */
+    /** Allocate a new context
+     *
+     * mbedtls_pk_setup() calls this function.
+     *
+     * If this function returns NULL, the allocation is considered to
+     * have failed and the the object remains uninitialized.
+     *
+     * Opaque implementations may omit this method. In this case,
+     * mbedtls_pk_setup will set the \c pk_ctx field of the mbedtls_pk_context
+     * object to NULL, and it is up to an engine-specific setup function to
+     * initialize the \c pk_ctx field. This is useful if the size of the
+     * memory depends on extra parameters passed to the engine-specific setup
+     * function. */
     void * (*ctx_alloc_func)( void );
 
-    /** Free the given context */
+    /** Free the given context
+     *
+     * mbedtls_pk_free() calls this function. It must free the data allocated
+     * by \b ctx_alloc_func as well as any other resource that belongs to
+     * the object.
+     * */
     void (*ctx_free_func)( void *ctx );
 
-    /** Interface with the debug module */
+    /** Interface with the debug module
+     *
+     * mbedtls_pk_debug() calls this function.
+     *
+     * Opaque implementations may omit this method. */
     void (*debug_func)( const void *ctx, mbedtls_pk_debug_item *items );
 
-    /** Signature size */
+    /** Signature size
+     *
+     * mbedtls_pk_signature_size() returns this value.
+     *
+     * Opaque implementations may omit this method. In this case, the value
+     * returned by \c get_bitlen (rounded up to a whole number of bytes)
+     * is used instead. */
     size_t (*signature_size_func)( const void *ctx );
 };
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* MBEDTLS_PK_INFO_H */