[dhcwg] sedhcpv6 data size issue (Re: WGLC on draft-ietf-dhc-sedhcpv6-21 - Respond by March 29th)
神明達哉 <jinmei@wide.ad.jp> Thu, 20 April 2017 17:53 UTC
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Date: Thu, 20 Apr 2017 10:53:22 -0700
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To: Timothy Carlin <tjcarlin@iol.unh.edu>
Cc: dhcwg <dhcwg@ietf.org>, draft-ietf-dhc-sedhcpv6 authors <draft-ietf-dhc-sedhcpv6@ietf.org>
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Subject: [dhcwg] sedhcpv6 data size issue (Re: WGLC on draft-ietf-dhc-sedhcpv6-21 - Respond by March 29th)
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At Wed, 5 Apr 2017 13:13:38 -0400, Timothy Carlin <tjcarlin@iol.unh.edu> wrote: > I'm also wondering about using RSA for encryption, as the data size (and > thus message size) is limited to the key size that the client or server > provides. This could cause problems with large DHCP messages. Could a > symmetric key algorithm (e.g. AES) be used? The keys for this would have > to be negotiated during the initial Information-request/Reply. I agree the data size limitation of RSA can be critical. At least data for the very initial Encrypted-Query message is quite unlikely to be encrypted at once, since the data contains a certificate option (which contains the client's public key). Assuming a 2048-bit key, we can only encrypt 256 - (overhead for padding etc) octets of data at once. I'm afraid this is too limited even for subsequent messages that don't contain the certificate option. One straightforward workaround is to divide the data and encrypt each chunk separately, but this may not be acceptable due to the cost, especially for the server. (Even if we adopt this approach I guess the current draft text is too unclear about it and should be revised to describe specifically how to do it). I suspect using symmetric session keys can't be an (easy) option, either. At least I don't think we can do this negotiation in the initial Information-Request/Reply exchange, since to do so the client would have to include its certificate in the Information-Request message but we intentionally avoid that due to a privacy concern. In any case, I'm afraid this issue will require some substantial changes to the specification. Right now I don't have a clear idea about what's the best. -- JINMEI, Tatuya