>>> The core problem posed by BIP151 is a MITM attack. The implied solution (BIP151 + authentication) requires that a peer trusts that another is not an attacker.
>>
>> BIP151 would increase the risks for MITM attackers.
>> What are the benefits for Mallory of he can't be sure Alice and Bob may
>> know that he is intercepting the channel?
> 
> It is not clear to me why you believe an attack on privacy by an anonymous peer is detectable.

If Mallory has substituted the ephemeral keys in both directions, at the
point where Alice and Bob will do an authentication, they can be sure
Mallory is listening.

Simple dummy example:
1.) Encryption setup with ECDH with ephemeral keys after BIP151
2.) Mallory is MITMling the connection. He is substituting both
direction with its own keys
3.) Connection is successfully MITMled
4.) Alice tells Bob "prove me that you are Bob, please sign the
session-ID with your identity key"
5.) Bob signs the sessionID (ECDH secret) with his identity key which
will be unusable for Mallory who has a substituted sessionID in both
directions.
6.) Alice has successfully detected the Mallory

Disclaimer: 4) and 5) are _not_ authentication proposals :-)

</jonas>