Re: [Cfrg] Multi-recipient public key authenticated encryption

Robert Moskowitz <rgm-sec@htt-consult.com> Wed, 29 April 2020 22:16 UTC

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To: Neil Madden <neil.e.madden@gmail.com>, CFRG <cfrg@irtf.org>
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From: Robert Moskowitz <rgm-sec@htt-consult.com>
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Date: Wed, 29 Apr 2020 18:16:27 -0400
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Subject: Re: [Cfrg] Multi-recipient public key authenticated encryption
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I am probably missing something here,,,

but this sounds like the problem MLS is solving?

Multiple recipients of an encrypted message.
Proof of single sender.

They had to invent? the ratchet tree.

?

On 4/27/20 10:12 AM, Neil Madden wrote:
> Hi all,
>
> I am working on an enhancement to the JOSE standards and would like 
> feedback from members of CFRG about solutions to a particular issue if 
> any of you have time.
>
> In JOSE currently if you wish to create a message that has both 
> confidentiality and sender authentication using public key 
> cryptography then the only option is to both sign and then encrypt the 
> message. This is expensive because it involves multiple passes over 
> the message and results in a very bulky nested message structure with 
> two layers of base64-encoding.
>
> Given that many uses of this sign-then-encrypt pattern do not require 
> the strong security properties of signatures, I have proposed [1] a 
> public key authenticated encryption mode based on NIST’s one-pass 
> unified model from SP 800-56A. This avoids the nested structure and 
> means that you don’t need multiple cryptographic primitives. The 
> proposed algorithm uses two ECDH key agreements: one between the 
> sender’s ephemeral private key and the recipient’s long-term public 
> key; and a second between the two parties’ long term keys. The two 
> shared secrets are concatenated and passed through a KDF along with 
> some context arguments. For a single recipient this achieves sender 
> authentication (subject to replay), and the single recipient case is 
> what I am primarily concerned about.
>
> (If you squint this is also roughly similar to the Noise framework “K” 
> one-way pattern, but my hands are waving quite a lot here).
>
> To support multiple recipients I copied the existing pattern used in 
> JOSE’s ECDH-ES+A256KW algorithm family in which the message is 
> encrypted using a random Content Encryption Key (CEK) and then the CEK 
> is encrypted for each recipient using AES-KeyWrap with the 
> ECDH-derived key. As I then mention in the security considerations 
> this leads to any recipient being able to produce a forgery using that 
> CEK and claim it came from the original sender:
>
>     When Key Agreement with Key Wrapping is used, with the same Content
>     Encryption Key (CEK) reused for multiple recipients, any of those
>     recipients can produce a new message that appears to come from the
>     original sender.  The new message will be indistinguishable from a
>     genuine message from the original sender to any of the other
>     participants.  To avoid this attack, the content SHOULD be encrypted
>     separately to each recipient with a unique CEK or a nested signature
>     over the content SHOULD be used.
>
> Because I am primarily interested in single-recipient use cases, this 
> seemed like an acceptable trade-off. However, I have since been 
> contacted by people who would like to use this draft for 
> multi-recipient messages and would not like to fall back on a nested 
> signature structure.
>
> An initial proposal was to solve this by simply including the MAC tag 
> from the content encryption in either the per-recipient payload 
> (encrypted using AES-KeyWrap) or as an additional context field to the 
> KDF. But the MAC is computed using the CEK that is known to all 
> recipients, so for this to be secure would require second preimage 
> resistance of the MAC with a known key, which cannot be guaranteed for 
> JOSE because it supports content encryption using AES-GCM for which 
> second preimages can be trivially computed if you know the key.
>
> Assuming that a per-recipient MAC is too much overhead, an alternative 
> would be to include a collision-resistant hash of entire ciphertext 
> (and IV and associated data) in the KDF. This is unfortunate as it 
> requires another pass over the entire message when we’ve already 
> encrypted and MACed, but it appears to be a solution and at least is 
> no more inefficient than the original signed-then-encrypted approach 
> which also needs to hash the entire message.
>
> So two questions:
>
> 1. Is including a hash (e.g., SHA-512) of the ciphertext (assuming 
> symmetric AE) in the per-recipient KDF calculation sufficient to 
> prevent forgeries in the multi-recipient setting?
>
> 2. Are there more efficient alternatives that don’t assume 2nd 
> preimage resistance of the underlying symmetric MAC?
>
> [1]: https://tools.ietf.org/html/draft-madden-jose-ecdh-1pu-03
>
> Kind regards,
>
> Neil Madden
>
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