OPAQUE Implementation in Rust

Alex Rivera•2025-01-10•8 min read
authenticationopaquepasswordcryptographyrust

The OPAQUE protocol represents a paradigm shift in authentication: it enables password-based authentication without the server ever seeing the password in any form. This isn’t just salting and hashing—it’s a complete reimagining of how authentication can work.

Why OPAQUE Matters

Traditional password authentication has fundamental flaws:

  1. Server breaches expose password hashes that can be cracked
  2. Weak passwords remain vulnerable even with good hashing
  3. Phishing attacks can capture plaintext passwords
  4. Password reuse amplifies breach impact

OPAQUE solves these by ensuring the server never learns anything about the password, not even a hash.

The OPAQUE Protocol

OPAQUE combines several cryptographic primitives:

  • Oblivious Pseudorandom Functions (OPRF): Server computes a function on client input without learning the input
  • Key Exchange: Establishes a shared session key
  • Envelope Encryption: Client’s private key is encrypted with password-derived key

Here’s the flow:

use opaque_ke::{
    ClientLogin, ClientLoginFinishParameters, ClientRegistration,
    ClientRegistrationFinishParameters, ServerLogin, ServerLoginStartParameters,
    ServerRegistration, ServerSetup,
};
use rand::rngs::OsRng;

pub struct OpaqueServer {
    server_setup: ServerSetup<DefaultSuite>,
}

impl OpaqueServer {
    pub fn new() -> Result<Self, OpaqueError> {
        let mut rng = OsRng;
        let server_setup = ServerSetup::<DefaultSuite>::new(&mut rng)?;
        
        Ok(Self { server_setup })
    }
    
    pub fn start_registration(
        &self,
        username: &str,
    ) -> Result<(ServerRegistration<DefaultSuite>, RegistrationResponse), OpaqueError> {
        let mut rng = OsRng;
        let (server_registration, registration_response) = 
            ServerRegistration::start(&self.server_setup, username.as_bytes(), &mut rng)?;
        
        Ok((server_registration, registration_response))
    }
}

Client-Side Implementation

The client handles password-based operations without revealing the password:

pub struct OpaqueClient {
    client_registration: Option<ClientRegistration<DefaultSuite>>,
}

impl OpaqueClient {
    pub fn new() -> Self {
        Self {
            client_registration: None,
        }
    }
    
    pub fn start_registration(
        &mut self,
        password: &str,
    ) -> Result<RegistrationRequest, OpaqueError> {
        let mut rng = OsRng;
        let (client_registration, registration_request) = 
            ClientRegistration::start(password.as_bytes(), &mut rng)?;
        
        self.client_registration = Some(client_registration);
        Ok(registration_request)
    }
    
    pub fn finish_registration(
        &mut self,
        response: RegistrationResponse,
    ) -> Result<RegistrationUpload, OpaqueError> {
        let client_registration = self.client_registration
            .take()
            .ok_or(OpaqueError::InvalidState)?;
        
        let mut rng = OsRng;
        let registration_upload = client_registration.finish(
            response,
            ClientRegistrationFinishParameters::default(),
            &mut rng,
        )?;
        
        Ok(registration_upload)
    }
}

The Magic: OPRF in Detail

The core of OPAQUE is the Oblivious Pseudorandom Function. Here’s what happens:

  1. Client blinds password: blinded_password = blind(password, r) where r is random
  2. Server evaluates OPRF: evaluated = OPRF(server_key, blinded_password)
  3. Client unblinds: oprf_output = unblind(evaluated, r)
use curve25519_dalek::{RistrettoPoint, Scalar};
use sha2::{Sha512, Digest};

pub struct BlindedPassword {
    point: RistrettoPoint,
    blinding_factor: Scalar,
}

impl BlindedPassword {
    pub fn new(password: &[u8], mut rng: impl RngCore) -> Self {
        // Hash password to curve point
        let mut hasher = Sha512::new();
        hasher.update(b"OPAQUE-HashToGroup");
        hasher.update(password);
        let hash = hasher.finalize();
        
        let point = RistrettoPoint::hash_from_bytes::<Sha512>(&hash);
        let blinding_factor = Scalar::random(&mut rng);
        
        Self {
            point: point * blinding_factor,
            blinding_factor,
        }
    }
    
    pub fn unblind(&self, server_evaluation: RistrettoPoint) -> RistrettoPoint {
        server_evaluation * self.blinding_factor.invert()
    }
}

Security Properties

OPAQUE provides several strong security guarantees:

Forward Secrecy

Even if the server is completely compromised, past sessions remain secure.

Pre-computation Resistance

Attackers can’t pre-compute password hashes because each authentication uses fresh randomness.

Strong Password Security

Even weak passwords are protected against offline attacks because the server never sees password-related data.

Integration Example

Here’s how you might integrate OPAQUE into a web application:

use axum::{
    extract::{State, Json},
    http::StatusCode,
    response::Json as ResponseJson,
};
use serde::{Deserialize, Serialize};

#[derive(Deserialize)]
pub struct RegisterRequest {
    username: String,
    registration_request: Vec<u8>, // Serialized OPAQUE data
}

#[derive(Serialize)]
pub struct RegisterResponse {
    registration_response: Vec<u8>,
}

pub async fn register_start(
    State(server): State<OpaqueServer>,
    Json(request): Json<RegisterRequest>,
) -> Result<ResponseJson<RegisterResponse>, StatusCode> {
    let (server_registration, response) = server
        .start_registration(&request.username)
        .map_err(|_| StatusCode::INTERNAL_SERVER_ERROR)?;
    
    // Store server_registration state associated with username
    // In production, use a proper session store
    
    Ok(ResponseJson(RegisterResponse {
        registration_response: response.serialize(),
    }))
}

Performance Considerations

OPAQUE operations are more expensive than traditional password hashing:

  • Registration: ~10ms (includes key generation)
  • Login: ~15ms (includes key exchange)
  • Memory: Minimal additional overhead

For most applications, this overhead is acceptable given the security benefits.

Common Implementation Pitfalls

  1. State Management: OPAQUE requires careful handling of intermediate state between protocol rounds
  2. Randomness: Poor RNG can completely break security
  3. Side Channels: Timing attacks can leak information about password attempts
  4. Serialization: Protocol messages must be serialized/deserialized correctly

Migration Strategy

Moving from traditional passwords to OPAQUE:

pub enum AuthMethod {
    Legacy(LegacyPassword),
    Opaque(OpaqueCredentials),
}

pub async fn authenticate(
    username: &str,
    auth_data: AuthData,
) -> Result<AuthResult, AuthError> {
    match get_user_auth_method(username).await? {
        AuthMethod::Legacy(legacy) => {
            // Authenticate with legacy method
            let result = legacy_authenticate(&legacy, &auth_data).await?;
            
            // Optionally upgrade to OPAQUE on successful login
            if result.success {
                prompt_opaque_upgrade(username).await?;
            }
            
            Ok(result)
        }
        AuthMethod::Opaque(opaque) => {
            opaque_authenticate(&opaque, &auth_data).await
        }
    }
}

Conclusion

OPAQUE represents the future of password-based authentication. While implementation requires careful attention to cryptographic details, the security benefits are substantial.

The Rust ecosystem provides excellent OPAQUE implementations like opaque-ke, making it practical to deploy this protocol today. As password breaches continue to plague the industry, protocols like OPAQUE offer a path toward truly secure authentication.

For applications handling sensitive data, the small performance overhead of OPAQUE is a worthwhile trade-off for eliminating password-related security risks entirely.

Alex Rivera is a security engineer specializing in cryptographic protocol implementation. He has contributed to several Rust cryptography libraries and regularly speaks at security conferences about practical cryptography.

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