Implementing Privacy-Preserving Authentication

1. Understanding Privacy Concepts

Principle	Detail
Data minimization	Collect only necessary
Unlinkability	Sessions/identities not correlatable
Selective disclosure	Reveal only required attributes
Pseudonymity	Identity protected by alias

2. Implementing Anonymous Credentials

Scheme	Detail
U-Prove	Microsoft
Idemix	IBM
BBS+ signatures	Foundation for selective disclosure

3. Using Zero-Knowledge Proofs

Scheme	Use
zk-SNARK	Succinct proofs (Groth16, PLONK)
zk-STARK	Transparent setup
Bulletproofs	Range proofs
Use case	Prove age ≥ 18 without revealing DOB

4. Implementing Blind Signatures

Aspect	Detail
Concept	Issuer signs message it cannot see
Use	Anonymous tokens (Privacy Pass, RFC 9577)
Property	Unlinkable issuance to redemption

5. Using Privacy-Preserving ID Systems

System	Detail
Apple Private Relay	iCloud, hides IP
Private Access Tokens	RFC 9577 anonymous attestation
FedCM	Browser-mediated federation (less tracking)
EU Digital Identity Wallet	Selective disclosure (mDL, eIDAS 2.0)

6. Implementing Pseudonymous Authentication

Sign-in with Apple — generates per-app email alias. Users authenticate without revealing primary email.

7. Handling Data Minimization

Practice	Detail
Request only needed scopes	OAuth principle of least privilege
Avoid PII in tokens	Use opaque IDs
Default-off telemetry	Opt-in

8. Using Attribute-Based Credentials

Aspect	Detail
Idea	Prove attributes (over-18, EU-citizen) without identity
Tech	BBS+ signatures, AnonCreds
Standard	W3C Verifiable Credentials 2.0

9. Implementing Selective Disclosure

Format	Detail
SD-JWT	Selective Disclosure JWT (IETF draft)
mDL (ISO 18013-5)	Mobile driver's license with claim selection
VC 2.0	W3C Verifiable Credentials

10. Handling Privacy vs Security Trade-Offs