[ippm] draft-ietf-ippm-ioam-data-integrity

Justin Iurman <justin.iurman@uliege.be> Wed, 31 May 2023 14:33 UTC

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Date: Wed, 31 May 2023 16:32:51 +0200
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To: IETF IPPM WG <ippm@ietf.org>
Cc: IPPM Chairs <ippm-chairs@ietf.org>, draft-ietf-ippm-ioam-data-integrity@ietf.org
From: Justin Iurman <justin.iurman@uliege.be>
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Subject: [ippm] draft-ietf-ippm-ioam-data-integrity
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Hi ippm, chairs,

Today, we (the authors) discussed an important section of the draft and 
found an issue with our asymmetric key-based signature algorithm (i.e., 
ECDSA p256 with SHA256). Indeed, it looks like it is way too complicated 
to end up with a working solution, not to say maybe even impossible for 
the use case. This observation is also based on the feedback received 
from an implementer. Very briefly, the issue comes from the signature 
chain. A node's signature would be over (a) the previous signature and 
(b) the (hashed) inserted data by the node. We *cannot* afford to hash 
the whole thing (which is usually the case for a signature), since we 
need a way to "go back" (i.e., to check the signature chain in the 
reverse order, with public keys). Therefore, we need the previous 
signature to stay outside of the hash. But if we do so, the input size 
(provided to the signature algorithm) would be too big. As a 
consequence, we don't believe there is a good solution for this. Note 
that, on second thought, ECDSA is not used for encryption and is "only" 
a signature algorithm, which might not be the perfect fit for what we 
want (i.e., to decrypt/retrieve data). A possibility would be to use 
something else (e.g., RSA), without the certainty that it would even 
work (and the signature size would be much bigger in that case).


What we propose here is to not over complicate things unnecessarily and 
have only the symmetric key-based signature algorithm defined in the 
draft (i.e., remove the problematic asymmetric key-based signature 
algorithm). After all, asymmetric algorithms are usually used to safely 
exchange (symmetric/shared) keys, which makes us think that it's 
probably not worth it. If anyone thinks otherwise, please say so. If 
you're a security expert and think we're wrong, please speak up. We hope 
for feedback from the WG.

The authors