Hi Chris,
Hi all,
I'm reviewing draft-ietf-ipsecme-ikev2-mlkem-00 [1] and had a few questions
about its hybrid security. Forgive me if this concern has already been raised
and addressed, as I'm new to this mailing list. I briefly searched the archive
and didn't find a related thread.
Suppose we do ECDH for the initial key exchange and ML-KEM for the first
intermediate key exchange. I understand the key exchange to work roughly as
follows.
The key exchange involves the following values:
- Ni // Initiator's nonce
- Nr // Responder's nonce
- SPIi // Initiator's SPI
- SPIr // Responder's SPI
- KEi(0) // Initiator's ECDH key share
- KEr(0) // Responder's ECH key share
- KEi(1) // ML-KEM public key
- KEr(1) // ML-KEM ciphertext
The key schedule is as follows:
1. KEi(0) and KEr(0) are combined to form shared secret SK(0)
2. SKEYSEED(0) is derived from prf(Ni | Nr, SK(0))
3. SK_d(0) is derived from prf+ (SKEYSEED(0), Ni | Nr | SPIi | SPIr )
4. KEi(1) and KEr(1) are combined to form shared secret SK(1)
5. SKEYSEED(1) is derived from prf(SK_d(0), SK(1) | Ni | Nr)
Finally, SKEYSEED(1) is used to derive session keys or to carry out another
intermediate key exchange. Do I understand this right?
Yes.
This is similar to what TLS 1.3 does [2]: session keys are derived by mixing
the shared secrets SK(0), SK(1) and binding them to some protocol context Ni,
Nr, SPIi, SPIr. However, there is an important difference: in TLS 1.3, the
protocol context includes the ECDH key shares and the ML-KEM public key and
ciphertext; in IKEv2, the protocol context does not include these values.
This difference is interesting when we think of the key schedule as a "KEM
combiner" [3]. In TLS 1.3, the combiner binds the key to the ECDH key shares
and ML-KEM public key and ciphertext; in IKEv2, the combiner does not. This
means the combiner is not robust [4], meaning a weakness in ECDH or ML-KEM
could imply a weakness in the hybrid KEM.
Of course, whether this is a problem for IKEv2 depends on what properties of
the combiner are needed for the security of the protocol. The draft cites a
proof of IND-CPA security for the combiner, thus we'd need to be able to prove
IKEv2 secure based on the assumption that one of ECDH or ML-KEM is IND-CPA. Do
I understand that right?
Assuming I've got this all correct, I'd be curious to know if this working
group considered whether or not to bind the key to the key exchange messages.
On the one hand, it seems like doing so would require changing the IKEv2 key
schedule, which is probably undesirable. On the other hand, it might be useful
for proving stronger-than-usual security properties of IKEv2, even if it's not
strictly necessary for authenticated key exchange.
Unless I’m missing your point, I believe that the binding of shared
secrets to the protocol context
in IKEv2 is done via the way the content of the AUTH payload is
calculated.
For pure IKEv2 (RFC 7296 Section 2.15) for initiator:
BLOBi = MSGi | Nr | prf(SKpi, IDi)
where MSGi – initiator’s IKE_SA_INIT message (includes initiator’s
ECDH key share)
SKpi is derived from SKEYSEED
In the case of hybrid key exchange ECDN+ML-KEM (RFC 9242, Section
3.3.2) for initiator:
BLOBi = MSGi | Nr | prf(SKpi(1), IDi) | prf(SKpi(0), INTi) |
prf(SKpr(0), INTr) | 2
where MSGi - initiator’s IKE_SA_INIT message (includes initiator’s
ECDH key share)
SKpi(1) – derived from SKEYSEED(1)
INTi – initiator’s IKE_INTERMEDIATE message before its encryption
(includes initiator’s ML-KEM public key),
INTr – responder’s IKE_INTERMEDIATE message before its
encryption (includes ML-KEM ciphertext),
SKpi(0), SKpr(0) – derived from SKEYSEED(0)
BLOBi is then signed or MACed, which in my understanding provides the
necessary binding of the keys to the IKEv2 context.
Regards,
Valery.
On an unrelated note, I'm curious about the language around input validation in
<https://www.ietf.org/archive/id/draft-ietf-ipsecme-ikev2-mlkem-00.html#section-2.3>
https://www.ietf.org/archive/id/draft-ietf-ipsecme-ikev2-mlkem-00.html#section-2.3.
Namely, why use SHOULD instead of MUST for validating inputs?
Thanks,
Chris P.
[1] https://datatracker.ietf.org/doc/draft-ietf-ipsecme-ikev2-mlkem/
[2] https://datatracker.ietf.org/doc/draft-ietf-tls-ecdhe-mlkem/
[3] https://datatracker.ietf.org/doc/draft-irtf-cfrg-hybrid-kems/
[4] https://datatracker.ietf.org/doc/html/draft-ietf-tls-hybrid-design/
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