Add an HKDF (RFC 5869) implementation
diff --git a/library/error.c b/library/error.c
index 222d85b..58c5a6f 100644
--- a/library/error.c
+++ b/library/error.c
@@ -101,6 +101,10 @@
 #include "mbedtls/gcm.h"
 #endif
 
+#if defined(MBEDTLS_HKDF_C)
+#include "mbedtls/hkdf.h"
+#endif
+
 #if defined(MBEDTLS_HMAC_DRBG_C)
 #include "mbedtls/hmac_drbg.h"
 #endif
@@ -698,6 +702,11 @@
         mbedtls_snprintf( buf, buflen, "GCM - Bad input parameters to function" );
 #endif /* MBEDTLS_GCM_C */
 
+#if defined(MBEDTLS_HKDF_C)
+    if( use_ret == -(MBEDTLS_ERR_HKDF_BAD_INPUT_DATA) )
+        mbedtls_snprintf( buf, buflen, "HKDF - Bad input parameters to function" );
+#endif /* MBEDTLS_HKDF_C */
+
 #if defined(MBEDTLS_HMAC_DRBG_C)
     if( use_ret == -(MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG) )
         mbedtls_snprintf( buf, buflen, "HMAC_DRBG - Too many random requested in single call" );