What is a Blind RSA Signature (RFC 9474) and how does it guarantee unlinkability?

Blind RSA (standardized in RFC 9474) is a cryptographic protocol where an issuer signs a message without learning its content. A client blinds a message m using a random secret blinding factor r: m* = m * r^e mod N. The server signs the blinded message s* = (m*)^d mod N. The client then unblinds the signature by multiplying by r^(-1) mod N, producing a valid RSA signature s = m^d mod N. When the client later redeems (m, s), the server verifies the signature is valid, but cannot correlate m with any blinded request it signed previously.

How does Privacy Pass and Apple Private Relay utilize Blind RSA?

Privacy Pass allows users who have solved a CAPTCHA or authenticated via Apple Device Attestation to receive blind tokens. When browsing Cloudflare or Fastly-protected websites, the user presents the unblinded token to bypass CAPTCHAs. Because the token is mathematically unlinkable to the issuance session, the website knows the visitor is a verified human or trusted device without learning the user's identity or browsing history.

What is the difference between Randomized and Zeroizing PSS modes in RFC 9474?

RFC 9474 specifies two primary variants: RSABSSA-SHA384-PSS-Randomized (where a random salt is embedded into the PSS padding) and RSABSSA-SHA384-PSS-Zeroizing (where the salt length is zero, producing deterministic padding). The zeroizing variant is preferred in protocols where the client must prove to a third party that the signed message followed exact deterministic formatting without covert side-channels in the salt.

How are double-spending and replay attacks prevented in blind token redemption?

Because the server does not learn the token during issuance, replay protection cannot happen at signing time. Instead, when the client redeems (m, s), the verifier computes a cryptographic digest of the message (such as SHA-256(m)) and inserts it into a spent-token ledger or distributed bloom filter. Any subsequent redemption attempt with the same token digest is rejected as a double-spend.

Blind RSA Signatures Studio | RFC 9474 Anonymous Tokens & Privacy Pass

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IETF RFC 9474 Standard Privacy Pass Compatible

Blind RSA Signatures (RFC 9474) Studio

Simulate the complete RFC 9474 Blind RSA token lifecycle. Inspect mathematical blinding, server blind signing, client unblinding, verify cryptographic unlinkability, and test double-spending redemption defense.

1. RSA Key Pair & Ciphersuite

RFC 9474 Ciphersuite: Server RSA Modulus & Exponents:

            N = 3233 (p=61, q=53), Public e = 17, Private d = 2753
          

Client Raw Token Request (m):

2. Mathematical Blinding State

Client Blinding Factor r

-

Blinded Message m*

-

Server Blind Signature s*

-

Unblinded Signature s

-

3. Anonymous Token Redemption (Step 4)

Present unblinded token (m, s) to edge verifier. Verifier checks s^e ≡ m (mod N) and validates that m has never been spent.

AWAITING REDEMPTION

Complete Steps 1-3 to obtain unblinded token

4. Double-Spend Spent-Token Ledger

            [ledger] Spent-token registry initialized. 0 tokens redeemed.
          

5. Protocol Execution Trace & Unlinkability Proof

          Click "Step 1: Generate Blinding Factor" to begin RFC 9474 protocol execution...
        

6. Anonymous Credential Architectures Compared

Protocol	Standardization	Verification Overhead	Attribute Disclosure	Primary Production Deployment
Blind RSA (RFC 9474)	IETF RFC 9474	Microseconds (Single modular exp)	Single boolean redemption	Apple Private Relay, Cloudflare Privacy Pass
BBS+ Signatures	W3C / Decentralized Identity	Medium (Bilinear pairings)	Selective attribute disclosure	Verifiable Credentials, mDL
Zero-Knowledge SNARKs	Ad-hoc / Academic	High (Pairing verification)	Arbitrary Turing-complete claims	Zcash, Semaphore, Anon Aadhaar

Blind RSA Signatures (RFC 9474) Studio

1. RSA Key Pair & Ciphersuite

2. Mathematical Blinding State

3. Anonymous Token Redemption (Step 4)

4. Double-Spend Spent-Token Ledger

5. Protocol Execution Trace & Unlinkability Proof

6. Anonymous Credential Architectures Compared

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