Re: [TLS] What would make TLS cryptographically better for TLS 1.3

"Dan Harkins" <dharkins@lounge.org> Sat, 02 November 2013 02:13 UTC

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Date: Fri, 1 Nov 2013 19:13:44 -0700 (PDT)
From: "Dan Harkins" <dharkins@lounge.org>
To: "Robert Ransom" <rransom.8774@gmail.com>
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Subject: Re: [TLS] What would make TLS cryptographically better for TLS 1.3
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On Fri, November 1, 2013 5:38 pm, Robert Ransom wrote:
> On 11/1/13, Dan Harkins <dharkins@lounge.org> wrote:
>>
>>
>> On Fri, November 1, 2013 4:13 pm, Nico Williams wrote:
>
>>> Agreed.  A small nonce should suffice for key derivation if either
>>> party
>>> reuses their supposedly-ephemeral DH keys.  By "small" I mean "not
>>> nearly as large as 32 bytes, i.e., not enough to suffice for a
>>> Dual_EC-type backdoored RNG attack".  The party reusing a key should
>>> send some such small nonce, possibly a 64-bit nonce.  In any case, the
>>> client ought not be reusing ephemeral DH keys, and any server that does
>>> should be rotating them often (like SSHv1 used to).
>>
>>   It has nothing to do with key derivation, it's to prevent a small
>> sub-group
>> attack. Sending a nonce does not prevent this attack since it's just
>> going
>> to
>> be another known for the attacker to use when running through the small
>> sub-group looking the right secret.
>
> Small-subgroup attacks are entirely orthogonal to whether a server can
> be caused to reuse a session key.  (Reusing a session key destroys the
> security properties of many bulk encryption methods, including all
> stream ciphers, regardless of whether the server's DH secret key is
> disclosed.)

  "caused to reuse a session key"? Who said anything about that. My
statement was in response to yours that said:

   "If the server sends a nonce during DHE/ECDHE key exchange, the
    server can safely reuse its DH keypair for multiple clients WITH NO
    FURTHER DESIGN OR IMPLEMENTATION CONSIDERATIONS  (emphasis
    mine)".

Again, if the server decides to reuse its DH key pair with multiple
clients then it needs some implementation considerations, namely it
needs to protect itself against a small sub-group attack. And this is
regardless of whether it sends a nonce or not. It has nothing to do with
key derivation.

  Also, your statement that "[r]eusing a session key destroys the security
properties of...all stream ciphers" leaves a bit of quantification up to
the reader. It's reuse of a session key _and_ a counter/nonce, and that's
only if you are not using SIV mode.

>>>> regardless of whether a nonce is sent or not.
>>>
>>> For some curves there's no need to validate the client's public key.
>>
>>   Which curves would those be?
>
> Curve25519 ECDH implementations which follow the specification in Dr.
> Bernstein's paper do not leak any information about the server's
> secret key under any active attack.  They prevent small-subgroup
> attacks by using the Montgomery ladder on a curve with a secure
> quadratic twist, and by generating all secret keys to be divisible by
> 8 (the LCM of the orders of small subgroups).

  Thanks for the reference. I'll check it out.

  Dan.