diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index e208da2..92f43ac 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -359,12 +359,18 @@
  * \param hash      Hash of the message to sign
  * \param hash_len  Hash length or 0 (see notes)
  * \param sig       Place to write the signature
- * \param sig_len   Number of bytes written
+ * \param sig_len   Number of bytes written to sig
  * \param f_rng     RNG function
  * \param p_rng     RNG parameter
  *
  * \return          0 on success, or a type-specific error code.
  *
+ * \note            The signature buffer \c sig must be of appropriate size
+ *                  which can be calculated with \c mbedtls_pk_signature_size.
+ *                  Depending on the algorithm, the value returned in
+ *                  \c sig_len may be less or equal to the value returned by
+ *                  \c mbedtls_pk_signature_size.
+ *
  * \note            For RSA keys, the default padding type is PKCS#1 v1.5.
  *                  There is no interface in the PK module to make RSASSA-PSS
  *                  signatures yet.
@@ -381,6 +387,15 @@
              int (*f_rng)(void *, unsigned char *, size_t), void *p_rng );
 
 /**
+ * \brief           Calculate the size of a signature made with this key.
+ *
+ * \param ctx       PK context to use
+ *
+ * \return          Maximum size in bytes of a signature made with this key.
+ */
+size_t mbedtls_pk_signature_size( const mbedtls_pk_context *ctx );
+
+/**
  * \brief           Decrypt message (including padding if relevant).
  *
  * \param ctx       PK context to use - must hold a private key