ch-13: mention second preimage hash function security req

15_security_privacy_ln.asciidoc

@@ -20,7 +20,7 @@ Then, they would describe the relevant properties of the system and check whethe
 A security model is based on a set of underlying _security assumptions_.
 In cryptographic systems, these assumptions are often centered around the mathematical properties of the cryptographic primitives such as ciphers, signatures, and hash functions.
 The security assumptions of the Lightning Network are that the ECDSA signatures, SHA-256 hash function, and other cryptographic functions used in the protocol behave within their security definitions.
-For example, we assume that it is practically impossible to find a preimage of a hash function.
+For example, we assume that it is practically impossible to find a preimage (and second preimage) of a hash function.
 This allows the LN to rely on the HTLC mechanism (which uses the preimage of a hash function) for the atomicity of multi-hop payments: nobody except the final recipient can reveal the payment secret and resolve the HTLC.
 We also assume a degree of connectivity in the network, namely, that Lightning channels form a connected graph. Therefore, it is possible to find a path from any sender to any receiver. Finally, we assume network messages are propagated within certain timeouts.