Re: [Detnet] I-D Action: draft-ietf-detnet-security-14.txt

[Ethan Grossman <[e-mail]>]

Hi Yaron and Ben,
Thank you so much for your help with this. First of all to Yaron I'm sorry
I wasn't clearer in my wording below - my intent with "leaving the text as
it is below" was not to use the draft 14 text (marked OLD below) but to use
my proposed text (marked NEW below); however, as Ben has pointed out, that
text is also flawed. Secondly, to Ben, my apologies - I somehow didn't
notice your additional inline replies beyond that first line, so I totally
missed that you had proposed new text there. So now we have two proposals:

Yaron:
If crypto keys are to be regenerated over the duration of the flow then the
time required to accomplish this must be accounted for in the latency
calculations.  Implementers should take care to use constant-time
operations for key generation. This is normally afforded by most common key
exchange protocols (e.g. those based on ECDH exchanges). Implementers
should avoid the rare cases (RSA ref and specific SSH kex method here) that
make constant time implementation difficult or impossible.

Ben:
In the general case cryptographic hygeine requires the generation of new
keys during the lifetime of an encrypted flow [ref ssh], and key generation
(or key exchange) takes additional time that must be accounted for in the
latency calculations for that flow.  For modern ECDH key-exchange
operations (such as x25519 [ref]), the operation can be performed in
(predictable) constant time, though this is not universally true (such as
for legacy RSA key exchange [ref 4432]).

At the risk of being slightly more verbose, I propose a hybrid of the two:

In the general case, cryptographic hygiene requires the generation of new
keys during the lifetime of an encrypted flow [ref ssh], and key generation
(or key exchange) requires additional computing time which must be
accounted for in the latency calculations for that flow.  For modern ECDH
(Elliptical Curve Diffie-Hellman) key-exchange operations (such as x25519
[ref]), these operations can be performed in (predictable) constant time,
though this is not universally true (such as for legacy RSA key exchange
[ref 4432]). Thus implementers should be aware of the time properties of
these algorithms and avoid those that make constant time implementation
difficult or impossible.

Is this OK with y'all?
Thanks again,
Ethan.




On Thu, Feb 11, 2021 at 11:59 PM Yaron Sheffer <[e-mail]>
wrote:

> Hi Ethan,
>
> On the contrary. The current text is incorrect. It currently stands:
>
>
>    If crypto keys are to be regenerated over the duration of the flow
>    then the time required to accomplish this must be accounted for in
>    the latency calculations.  Unfortunately, key generation is a
>    cryptographic operation that is frequently not possible to implement
>    in constant time, most notably (though not exclusively) for RSA key
>    pairs.
>
> I suggest to change to:
>
>
>    If crypto keys are to be regenerated over the duration of the flow
>    then the time required to accomplish this must be accounted for in
>    the latency calculations.  Implementers should take care to use
> constant-time operations for key generation. This is normally afforded by
> most common key exchange protocols (e.g. those based on ECDH exchanges).
> Implementers should avoid the rare cases (RSA ref and specific SSH kex
> method here) that make constant time implementation difficult or impossible.
>
> Thanks,
>         Yaron
>
> On 2/12/21, 01:42, "Ethan Grossman" <[e-mail]> wrote:
>
>     Thanks Ben, I did find RFC 4432, but all I can see that it says around
> this
>     is "This may be a transient key generated solely for
>        this SSH connection, or it may be re-used for several connections."
> It
>     doesn't say anything about being "safest to generate a key each time"
> so I
>     figured that wasn't the right reference. Anyway I don't mean to be
> pedantic
>     about this, I'm just trying to make the most out of the material that
> you've
>     given me. If it isn't worth pursuing, I'm OK with leaving the text as
> it is
>     below, if you and Yaron are OK with that.
>     Thanks again for your help...
>     Ethan.
>
>     -----Original Message-----
>     From: Benjamin Kaduk <[e-mail]>
>     Sent: Thursday, February 11, 2021 3:29 PM
>     To: Ethan Grossman <[e-mail]>
>     Cc: 'Yaron Sheffer' <[e-mail]>; [e-mail];
>     [e-mail]; [e-mail]; 'Magnus Westerlund'
>     <[e-mail]>; 'Murray Kucherawy' <
> [e-mail]>;
>     'Roman Danyliw' <[e-mail]>; 'Robert Wilton' <[e-mail]>;
> 'Barry
>     Leiba' <[e-mail]>; 'Eric Vyncke (evyncke)'
>     <[e-mail]>
>     Subject: Re: [Detnet] I-D Action: draft-ietf-detnet-security-14.txt
>
>     Hi Ethan,
>
>     On Thu, Feb 11, 2021 at 02:39:59PM -0800, Ethan Grossman wrote:
>     > Hi Ben and Yaron,
>     > Thanks for this info. Does my proposed text below correctly
> incorporate
>     your points?
>     >
>     > Regarding Ben's statement "SSH provides an "rsa2048-sha256" key
> exchange
>     method wherein the server sends an RSA public key and the client
> encrypts
>     some material to that key.  The guidance in that document is a little
> vague,
>     saying that it's safest to generate a new key each time, but allowing
> for
>     some unspecified level of reuse."  I can't seem to find the RFC for
> that
>     reference - I got as close as
>     https://tools.ietf.org/id/draft-ietf-curdle-ssh-kex-sha2-09.html
>     (section 3.29) but no closer. If you could point me in the right
> direction I
>     could update my text.
>
>     Sorry!  The spec for rsa2048-sha256 is RFC 4432.
>
>
>     > Thanks,
>     > Ethan.
>     >
>     > OLD:
>     > If crypto keys are to be regenerated over the duration of the flow
>     >    then the time required to accomplish this must be accounted for in
>     >    the latency calculations.  Unfortunately, key generation is a
>     >    cryptographic operation that is frequently not possible to
> implement
>     >    in constant time, most notably (though not exclusively) for RSA
> key
>     >    pairs.
>     >
>     > NEW:
>     > When protocols such as IPsec/IKEv2 and MACsec are used, keys are
>     > typically only generated when a flow is instantiated, so the time
>
>     I think I may have written sloppily to give you this impression, but
> it's
>     not the case (at least for IPsec/IKEv2) -- IKEv2 will periodically
> re-key a
>     connection after some amount of traffic.  It's just that IKEv2 only has
>     Diffie-Hellman or similar types of key exchange, so unpredictable RSA
> key
>     generation times are not a factor for IKEv2 re-key.
>
>     > required for key generation does not impact the latency of packets
>     > within a flow. However, depending on the use case, it may be
> desirable
>     > to regenerate fresh (ephemeral) RSA keys during the flow, for
> example
>     > based on a time period or amount of data sent. For example, for SSH,
>     > [ref to
>     > https://tools.ietf.org/html/rfc4253#section-9 ]) recommends that
> "keys
>     > be changed after each gigabyte of transmitted data or after each
> hour
>     > of connection time, whichever  comes sooner". That same reference
>     > describes the SSH process for key re-exchange, which is essentially
>     > the same mechanism as for initial key exchange.
>
>     It also seems like this paragraph is starting to get pretty long and
> it's
>     not clear that it's adding enough value to justify that length.
>     IIUC the key point is just that rekeying during a connection is a
> normal and
>     recommended operation, with the need to account for the delay
> introduced by
>     key generation being pushed off to the following paragraph.
>     Generating RSA keys is only noteworthy for its non-constant-time-ness,
> which
>     is not actually mentioned here, so mentioning RSA keygen seems to be
> not
>     doing much for us.
>
>     > If the keys are to be regenerated over the duration of a flow then
> the
>     > time required to accomplish this must be accounted for in the
> latency
>     > calculations for that flow.  Fortunately, modern ECDH (Elliptical
>     > Curve
>     > Diffie-Hellman) operations (such as x25519, e.g. see [ref to
>     > https://tools.ietf.org/html/rfc7748] ) can be performed in constant
> time.
>
>     (Maybe say explicitly that since it's constant time that it can be
> reliably
>     corrected for?)
>
>     > END
>
>     I think my proposal would be to stick closer to the original, with
> something
>     like:
>
>     In the general case cryptographic hygeine requires the generation of
> new
>     keys during the lifetime of an encrypted flow [ref ssh], and key
> generation
>     (or key exchange) takes additional time that must be accounted for in
> the
>     latency calculations for that flow.  For modern ECDH key-exchange
> operations
>     (such as x25519 [ref]), the operation can be performed in (predictable)
>     constant time, though this is not universally true (such as for legacy
> RSA
>     key exchange [ref 4432]).
>
>     -Ben
>
>     > -----Original Message-----
>     > From: Benjamin Kaduk <[e-mail]>
>     > Sent: Wednesday, February 10, 2021 8:13 PM
>     > To: Ethan Grossman <[e-mail]>
>     > Cc: 'Yaron Sheffer' <[e-mail]>; [e-mail];
>     > [e-mail]; [e-mail]; 'Magnus Westerlund'
>     > <[e-mail]>; 'Murray Kucherawy'
>     > <[e-mail]>; 'Roman Danyliw' <[e-mail]>; 'Robert
> Wilton'
>     > <[e-mail]>; 'Barry Leiba' <[e-mail]>; 'Eric
>     > Vyncke (evyncke)' <[e-mail]>
>     > Subject: Re: [Detnet] I-D Action: draft-ietf-detnet-security-14.txt
>     >
>     > Interestingly enough, I can, but only as a result of doing AD review
> of a
>     different document yesterday!
>     >
>     > The point is a good one to raise, as it is definitely unusual to be
> doing
>     ephemeral RSA key generation for key update on a live connection.
>     >
>     > That said, I believe that SSH provides an "rsa2048-sha256" key
> exchange
>     method wherein the server sends an RSA public key and the client
> encrypts
>     some material to that key.  The guidance in that document is a little
> vague,
>     saying that it's safest to generate a new key each time, but allowing
> for
>     some unspecified level of reuse.  Per
>     https://tools.ietf.org/html/rfc4253#section-9, key update of a live
>     connection basically just uses the same mechanisms as initial key
> exchange,
>     which thus might include generating a fresh ephemeral RSA key.
>     >
>     > (I don't believe that IPsec/IKEv2 has ever supported such a
> mechanism,
>     > and I'm not very familiar with the details of what MACsec does, but
> I
>     > don't expect it to have anything like this either.)
>     >
>     > My understanding matches Yaron's that one of the myriad benefits of
>     > (e.g.)
>     > x25519 ECDH key exchange is that it's super-easy to implement in
> constant
>     time.
>     >
>     > -Ben
>     >
>     > On Wed, Feb 03, 2021 at 11:17:21AM -0800, Ethan Grossman wrote:
>     > > Thanks Yaron, glad to hear it. Regarding the RSA key pair question,
>     perhaps Benjamin Kaduk could provide us some thoughts on this, since
> he did
>     have comments around that section? Benjamin?
>     > > Ethan.
>     > >
>     > > -----Original Message-----
>     > > From: Yaron Sheffer <[e-mail]>
>     > > Sent: Wednesday, February 3, 2021 10:58 AM
>     > > To: [e-mail]; [e-mail]; [e-mail];
>     > > [e-mail]
>     > > Cc: 'Magnus Westerlund' <[e-mail]>; 'Murray
>     > > Kucherawy' <[e-mail]>; 'Roman Danyliw' <[e-mail]>;
>     > > 'Benjamin Kaduk' <[e-mail]>; 'Robert Wilton'
>     > > <[e-mail]>; 'Barry Leiba' <[e-mail]>;
> 'Eric
>     Vyncke (evyncke)'
>     > > <[e-mail]>
>     > > Subject: Re: [Detnet] I-D Action: draft-ietf-detnet-security-14.txt
>     > >
>     > > Thank you, this is a lot better!
>     > >
>     > > A quick comment: in Sec. 7.5.1 you mention RSA key pairs. I'm not
>     familiar with cases where RSA key pairs are generated on the fly as
> part of
>     a key exchange protocol. And AFAIK (someone might want to correct me
> here)
>     the more common, more modern ECDH operations can be performed in
> constant
>     time.
>     > >
>     > > Thanks,
>     > >         Yaron
>     > >
>     > > On 2/2/21, 01:55, "Ethan Grossman" <[e-mail]> wrote:
>     > >
>     > >     Hi All,
>     > >     We have submitted draft 14 of the DetNet Security
> Considerations
>     draft,
>     > >     which includes resolutions for all remaining AD review
> comments (as
>     noted in
>     > >     my previous emails with the per-reviewer dispositions).
>     > >     Specifically, this version addresses comments from Yaron
> Sheffer,
>     Magnus
>     > >     Westerlund, Murray Kucherawy, Eric Vyncke, Roman Danyliw,
> Benjamin
>     Kaduk,
>     > >     Robert Wilton, and Barry Leiba.
>     > >
>     > >     Thank you all for your reviews, and I hope we have addressed
> each of
>     your
>     > >     comments to your satisfaction - if you have any further
> comments,
>     > >     suggestions or corrections please don't hesitate to let us
> know.
>     > >
>     > >     Sincerely,
>     > >     Ethan (as Editor, DetNet Security Considerations draft)
>     > >
>     > >     -----Original Message-----
>     > >     From: detnet <[e-mail]> On Behalf Of
>     [e-mail]
>     > >     Sent: Monday, February 1, 2021 3:44 PM
>     > >     To: [e-mail]
>     > >     Cc: [e-mail]
>     > >     Subject: [Detnet] I-D Action: draft-ietf-detnet-security-14.txt
>     > >
>     > >
>     > >     A New Internet-Draft is available from the on-line
> Internet-Drafts
>     > >     directories.
>     > >     This draft is a work item of the Deterministic Networking WG
> of the
>     IETF.
>     > >
>     > >             Title           : Deterministic Networking (DetNet)
> Security
>     > >     Considerations
>     > >             Authors         : Ethan Grossman
>     > >                               Tal Mizrahi
>     > >                               Andrew  J. Hacker
>     > >         Filename        : draft-ietf-detnet-security-14.txt
>     > >         Pages           : 59
>     > >         Date            : 2021-02-01
>     > >
>     > >     Abstract:
>     > >        A DetNet (deterministic network) provides specific
> performance
>     > >        guarantees to its data flows, such as extremely low data
> loss
>     rates
>     > >        and bounded latency (including bounded latency variation,
> i.e.
>     > >        "jitter").  As a result, securing a DetNet requires that in
>     addition
>     > >        to the best practice security measures taken for any
>     mission-critical
>     > >        network, additional security measures may be needed to
> secure the
>     > >        intended operation of these novel service properties.
>     > >
>     > >        This document addresses DetNet-specific security
> considerations
>     from
>     > >        the perspectives of both the DetNet system-level designer
> and
>     > >        component designer.  System considerations include a
> taxonomy of
>     > >        relevant threats and attacks, and associations of threats
> versus
>     use
>     > >        cases and service properties.  Component-level
> considerations
>     include
>     > >        ingress filtering and packet arrival time violation
> detection.
>     > >
>     > >        This document also addresses security considerations
> specific to
>     the
>     > >        IP and MPLS data plane technologies, thereby complementing
> the
>     > >        Security Considerations sections of those documents.
>     > >
>     > >
>     > >     The IETF datatracker status page for this draft is:
>     > >     https://datatracker.ietf.org/doc/draft-ietf-detnet-security/
>     > >
>     > >     There are also htmlized versions available at:
>     > >     https://tools.ietf.org/html/draft-ietf-detnet-security-14
>     > >
>     > >
> https://datatracker.ietf.org/doc/html/draft-ietf-detnet-security-14
>     > >
>     > >     A diff from the previous version is available at:
>     > >
> https://www.ietf.org/rfcdiff?url2=draft-ietf-detnet-security-14
>     > >
>     > >
>     > >     Please note that it may take a couple of minutes from the time
> of
>     submission
>     > >     until the htmlized version and diff are available at
> tools.ietf.org.
>     > >
>     > >     Internet-Drafts are also available by anonymous FTP at:
>     > >     ftp://ftp.ietf.org/internet-drafts/
>     > >
>     > >
>     > >     _______________________________________________
>     > >     detnet mailing list
>     > >     [e-mail]
>     > >     https://www.ietf.org/mailman/listinfo/detnet
>     > >
>     >
>     >
>
>
>
>