Improve documentation of PKCS1 decryption functions

Document the preconditions on the input and output buffers for
the PKCS1 decryption functions
- mbedtls_rsa_pkcs1_decrypt,
- mbedtls_rsa_rsaes_pkcs1_v15_decrypt
- mbedtls_rsa_rsaes_oaep_decrypt

include/mbedtls/rsa.h

@@ -329,9 +329,15 @@ int mbedtls_rsa_rsaes_oaep_encrypt( mbedtls_rsa_context *ctx,
  *
  * \return         0 if successful, or an MBEDTLS_ERR_RSA_XXX error code
  *
- * \note           The output buffer must be as large as the size
+ * \note           The output buffer length \c output_max_len should be
- *                 of ctx->N (eg. 128 bytes if RSA-1024 is used) otherwise
+ *                 as large as the size ctx->len of ctx->N (eg. 128 bytes
- *                 an error is thrown.
+ *                 if RSA-1024 is used) to be able to hold an arbitrary
+ *                 decrypted message. If it is not large enough to hold
+ *                 the decryption of the particular ciphertext provided, 
+ *                 the function will return MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE.
+ *
+ * \note           The input buffer must be as large as the size
+ *                 of ctx->N (eg. 128 bytes if RSA-1024 is used).
  */
 int mbedtls_rsa_pkcs1_decrypt( mbedtls_rsa_context *ctx,
                        int (*f_rng)(void *, unsigned char *, size_t),
@@ -355,9 +361,15 @@ int mbedtls_rsa_pkcs1_decrypt( mbedtls_rsa_context *ctx,
  *
  * \return         0 if successful, or an MBEDTLS_ERR_RSA_XXX error code
  *
- * \note           The output buffer must be as large as the size
+ * \note           The output buffer length \c output_max_len should be
- *                 of ctx->N (eg. 128 bytes if RSA-1024 is used) otherwise
+ *                 as large as the size ctx->len of ctx->N (eg. 128 bytes
- *                 an error is thrown.
+ *                 if RSA-1024 is used) to be able to hold an arbitrary
+ *                 decrypted message. If it is not large enough to hold
+ *                 the decryption of the particular ciphertext provided, 
+ *                 the function will return MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE.
+ *
+ * \note           The input buffer must be as large as the size
+ *                 of ctx->N (eg. 128 bytes if RSA-1024 is used).
  */
 int mbedtls_rsa_rsaes_pkcs1_v15_decrypt( mbedtls_rsa_context *ctx,
                                  int (*f_rng)(void *, unsigned char *, size_t),
@@ -383,9 +395,15 @@ int mbedtls_rsa_rsaes_pkcs1_v15_decrypt( mbedtls_rsa_context *ctx,
  *
  * \return         0 if successful, or an MBEDTLS_ERR_RSA_XXX error code
  *
- * \note           The output buffer must be as large as the size
+ * \note           The output buffer length \c output_max_len should be
- *                 of ctx->N (eg. 128 bytes if RSA-1024 is used) otherwise
+ *                 as large as the size ctx->len of ctx->N (eg. 128 bytes
- *                 an error is thrown.
+ *                 if RSA-1024 is used) to be able to hold an arbitrary
+ *                 decrypted message. If it is not large enough to hold
+ *                 the decryption of the particular ciphertext provided, 
+ *                 the function will return MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE.
+ *
+ * \note           The input buffer must be as large as the size 
+ *                 of ctx->N (eg. 128 bytes if RSA-1024 is used).
  */
 int mbedtls_rsa_rsaes_oaep_decrypt( mbedtls_rsa_context *ctx,
                             int (*f_rng)(void *, unsigned char *, size_t),