At 11:33 AM 9/6/2013, Peter Fairbrother wrote:
However, while the case for forward secrecy is easy to make,
implementing it may be a little dangerous - if NSA have broken ECDH then
using it only gives them plaintext they maybe didn't have before.
I thought the normal operating mode for PFS is that there's an
initial session key exchange (typically RSA) and authentication,
which is used to set up an encrypted session, and within that session
there's a DH or ECDH key exchange to set up an ephemeral session key,
and then that session key is used for the rest of the session.
If so, even if the NSA has broken ECDH, they presumably need to see
both Alice and Bob's keyparts to use their break,
which they can only do if they've cracked the outer session (possibly
after the fact.)
So you're not going to leak any additional plaintext by doing ECDH
compared to sending the same plaintext without it.
One point which has been mentioned, but perhaps not emphasised
enough - if NSA have a secret backdoor into the main NIST ECC
curves, then even if the fact of the backdoor was exposed - the
method is pretty well known - without the secret constants no-one
_else_ could break ECC.
So NSA could advocate the widespread use of ECC while still
fulfilling their mission of protecting US gubbmint communications
from enemies foreign and domestic. Just not from themselves.
Yep. It's definitely the fun kind of backdoor to use.
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