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

Paul Grubbs <> Thu, 07 May 2020 20:38 UTC

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From: Paul Grubbs <>
Date: Thu, 07 May 2020 16:37:53 -0400
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To: Neil Madden <>
Cc: Mike Jones <>, CFRG <>
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Subject: Re: [Cfrg] Multi-recipient public key authenticated encryption
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Oops - Neil, I interpreted your earlier message to mean that JOSE does
indeed only support GCM. (My apologies for not checking this before

On Thu, May 7, 2020 at 11:16 AM Neil Madden <> wrote:

> The issue is not that JOSE only supports GCM (it doesn’t). The issue is
> that JOSE supports GCM and allows that choice independently of the key
> agreement algorithm. Hence if GCM is not appropriate for this new algorithm
> then I need to add text forbidding it in this context - i.e. that
> implementations MUST reject a JWE with “alg”:”ECDH-1PU+A128KW” and
> “enc”:”A256GCM” for example.
> On 7 May 2020, at 16:10, Mike Jones <> wrote:
> I’m confused by the statement “JOSE only supports GCM”.  There’s an IANA
> registry for JOSE algorithms at
>  and any additional desired algorithms can be added there.
>                                                        -- Mike
> *From:* Cfrg <> *On Behalf Of *Neil Madden
> *Sent:* Thursday, May 7, 2020 7:27 AM
> *To:*
> *Cc:* CFRG <>
> *Subject:* : [Cfrg] Multi-recipient public key authenticated encryption
> That is interesting, thanks. JOSE does also support AES-CBC + HMAC (EtM),
> so it may perhaps then be simpler to forbid the use of GCM for
> multi-recipient messages in this spec and save myself a lot of trouble. It
> seems from your papers that CBC+HMAC is one of the schemes you looked at
> and it would be compactly committing, so this is fortunate.
> On 7 May 2020, at 14:39, wrote:
> Oh ok, it’s too bad JOSE only supports GCM. You should be careful with
> using plain GCM in these multi-receiver settings - because it’s
> non-committing, it’s possible to craft a single ciphertext that different
> receivers decrypt to different plaintexts.
> On May 7, 2020, at 7:28 AM, Neil Madden <> wrote:
> Apologies, I forgot to respond to this. I think the ccAE approach is a
> nice way of solving the problem. Unfortunately, JOSE allows specifying the
> key exchange algorithm and AEAD cipher entirely independently so I have to
> have a solution that works with the AES-GCM based AEADs that are already
> part of the standard. But I will read these papers closely, because I do
> think the security notions are very similar to what I’m trying to achieve.
> — Neil
> On 28 Apr 2020, at 16:14, Paul Grubbs <> wrote:
> I think you can get away without the additional hash of the ciphertext
> entirely if you use a compactly committing AEAD (ccAE) scheme [1]. A ccAE
> has a two-part ciphertext C = (C1, C2) with the property that C2 is a
> binding commitment to the plaintext and key. At least intuitively, then,
> using C2 in the KDF prevents a receiver from modifying the message. (This
> is morally similar to including the MAC tag in the KDF, as suggested, but
> sidesteps questions about MAC security and may be faster if optimal-rate
> schemes like HFC [2] are used.) I haven't analyzed this construction
> formally, though, so take this advice with a bit of salt.
> [1]
> [2]
> On Tue, Apr 28, 2020 at 8:30 AM Neil Madden <>
> wrote:
> Thanks, section 5 of the eprint is very useful. In that you discuss the
> solution of including the MAC tag in the KDF and you say that this requires
> collision resistance of the MAC, where I had previously assumed only 2nd
> preimage resistance would be enough. As I understand it, the potential
> attack is that if Charlie has access to an Alice-oracle by which he can get
> Alice to authenticate arbitrary messages to himself and Bob then he can use
> this to perform a collision search to find two messages with the same MAC
> tag (if the MAC is not collision resistant).
> I think this active attack would also apply to my proposed solution using
> a hash of the authenticated ciphertext as input to the KDF. So my assertion
> that 2nd preimage is enough is only valid for passive attacks, and full
> collision resistance would be needed in general. That also means that if I
> do pick SHA-256 to be consistent with existing use in JOSE then the
> security against such attacks would be limited to ~128 bit level. Perhaps
> that’s an incentive for me to also change the KDF in the scheme from JOSE’s
> existing Concat-KDF to HKDF with SHA-512.
> I think the same attack would also apply to saltpack [1], if I understand
> correctly? In saltpack a per-recipient MAC is calculated over a SHA-512 of
> the ciphertext.
> [1] (scroll down to Payload
> Packets)
> Best,
> Neil
> On 27 Apr 2020, at 16:19, Dan Brown <> wrote:
> Hi Neil,
> I too have encountered this interesting and well-known (perhaps not
> widely-known) problem.
> See (Section 5).
> See the very short
> “note”.
> I forget the details of the various claimed past solutions, but will try
> to remember, maybe in a couple weeks.  Meanwhile, since the problem is
> fresh in your mind, and you might try to make sense the couple of old
> suggestions above, though I expect your knowledge has already advance past
> this old stuff.
> Best regards,
> Dan
> *From:* Cfrg <> *On Behalf Of *Neil Madden
> *Sent:* Monday, April 27, 2020 10:12 AM
> *To:* CFRG <>
> *Subject:* [Cfrg] Multi-recipient public key authenticated encryption
> 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]:
> <>
> Kind regards,
> Neil Madden
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