[CFRG] Request for feedback on Hierarchical Deterministic Keys

[Sander Dijkhuis <[e-mail]>]

Hi CFRG,

I would like to request your feedback on an Internet-Draft:

Hierarchical Deterministic Keys (HDKeys)
draft-dijkhuis-cfrg-hdkeys-01
https://datatracker.ietf.org/doc/draft-dijkhuis-cfrg-hdkeys/01/

The spec results from an informal collaboration among participants in 
large scale pilots for the EU Digital Identity, incorporating ideas from 
BIP-32 and ARKG. We aim to standardise the algorithm for use with common 
wallet protocols and widely available cryptographic mechanisms for 
EU-governed applications. The CFRG seems the best place to do so.

In the EU Digital Identity context, providers issue digital documents 
with proof-of-possession public keys bound to a secure cryptographic 
devices, preventing cloning and unauthorised use. To ensure 
unlinkability across relying parties, they should never see the same 
public key. HDKeys allows re-issuance of documents with unique public 
keys derived from a single device-bound key. The algorithm combines 
deterministic key derivation and blinded proof of possession. It can be 
distributed in such a way that enables binding to regular ECDSA or ECDH 
devices.

HDKeys can be applied with current certified provider hardware and 
wallet secure cryptographic devices. Therefore it enables a transitory 
solution for EU Digital Identity Wallet until more efficient or 
effective alternatives are widely available. Promising alternatives 
include multi-message signature schemes such as BBS, which requires 
development and certification of HSMs, and ZKP circuits for common 
digital document proofs, which are under development. EU Member States 
need a transitory solution to meet the December 2026 launch deadline 
without unnecessarily compromising privacy. For more details, see 
§4.4.4.2 of:

ETSI TR 119 476 v1.2.1
https://www.etsi.org/deliver/etsi_tr/119400_119499/119476/01.02.01_60/tr_119476v010201p.pdf

The European Commission considers referencing HDKeys in the context of 
two wallet architecture topics:

Topic A: Privacy risks and mitigations
https://github.com/eu-digital-identity-wallet/eudi-doc-architecture-and-reference-framework/issues/327

Topic C: Wallet Unit Attestation (WUA) and key attestation
https://github.com/eu-digital-identity-wallet/eudi-doc-architecture-and-reference-framework/issues/333

To enable adoption for EU Digital Identity, HDKeys needs further review 
and standardisation. I look forward to hearing your first feedback 
regarding the approach, initial draft, and feasibility to adopt.

Best regards,

Sander