diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 5609f42..c552b53 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -629,6 +629,17 @@
                 ret = mbedtls_pk_write_key_der( &pk, data, data_size );
             if( ret < 0 )
                 return( mbedtls_to_psa_error( ret ) );
+            /* The mbedtls_pk_xxx functions write to the end of the buffer.
+             * Move the data to the beginning and erase remaining data
+             * at the original location. */
+            if( 2 * (size_t) ret <= data_size )
+            {
+                memcpy( data, data + data_size - ret, ret );
+            }
+            else if( (size_t) ret < data_size )
+            {
+                memmove( data, data + data_size - ret, ret );
+            }
             *data_length = ret;
             return( PSA_SUCCESS );
         }