[bip151] slightly increase robustness of the re-keying

Author: jonasschnelli

bip-0151.mediawiki

@@ -148,7 +148,7 @@ If more data is present, another message must be deserialized. There is no expli
 
 A responding peer can inform the requesting peer over a re-keying with a <code>encack</code> message containing 33byte of zeros to indicate that all encrypted message following after this <code>encack</code> message will be encrypted with ''the next symmetric cipher key''.
 
-The new symmetric cipher key will be calculated by <code>SHA256(SHA256(old_symetric_cipher_key))</code>.
+The new symmetric cipher key will be calculated by <code>SHA256(SHA256(session_id || old_symmetric_cipher_key))</code>.
 
 Re-Keying interval is a peer policy with a minimum timespan of 10 seconds.