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Web Crypto API for OPRF

This is a specification of a Web Crypto API for OPRF base mode, which is specified in https://datatracker.ietf.org/doc/draft-irtf-cfrg-voprf.

Author: Armando Faz (armfazh@cloudflare.com)

License: BSD-3-Clause

Contents:

Parameters

Group namedCurve Size of scalar (n bytes) Size of points (n+1 bytes)
P-256 "P-256" 32 33
P-384 "P-384" 48 49
P-521 "P-521" 66 67

OPRF Server side support

This is the expected functionality for an OPRF server.

OPRF Method Spec WebCrypto Method Comments
Keygen Section 2.1 key = crypto.subtle.generateKey(algorithm, extractable, keyUsages) Use this function for generating an OPRF CryptoKey (private key). Internally, the key stores a scalar k.
Evaluate Section 3.4.1.1 sig = crypto.subtle.sign(key, msg) Behind the scenes, this is a scalar multiplication. key = a CryptoKey containing a scalar k. msg = a compressed point P on the curve (an ArrayBuffer of size n+1) sig = a compressed point kP on the curve (an ArrayBuffer of size n+1)
crypto.subtle.importKey Converts raw bytes into an OPRF CryptoKey. During importing, the key can be defined as exportable or not. If exportable, subtle.export can recover the raw bytes from the OPRF CryptoKey.
crypto.subtle.exportKey if exportable, converts a OPRF CryptoKey into raw bytes.
crypto.subtle.verify (not implemented) No method is mapped to subtle.verify

API description

Keygen

const result = crypto.subtle.generateKey(algorithm, extractable, keyUsages)

Parameters

algorithm: {
    name: "OPRF",
    namedCurve: "P-256" // one of "P-256", "P-384", "P-521"
}

extractable: true

keyUsages: [ "sign" ]

Returns

It returns a CryptoKey used only for OPRF signatures.

Evaluate

Parameters

const signature = crypto.subtle.sign(algorithm, key, data);
algorithm: {
    name: "OPRF"
}

key: key is a CryptoKey imported with subtle.importKey.

data: data is an ArrayBuffer containing a compressed point on the curve.

Returns

It returns a Promise to an ArrayBufer containing a compressed point on the curve.

importKey

Parameters

const result = crypto.subtle.importKey(
    format,
    keyData,
    algorithm,
    extractable,
    keyUsages
);

format: "raw"

keyData: keydata is an ArrayBuffer of length n containing the key.

algorithm: {
    name: "OPRF",
    namedCurve: "P-256" // one of "P-256", "P-384", "P-521"
}

extractable: true

keyUsages: [ "sign" ]

Returns

It returns a CryptoKey used only for OPRF signatures.

exportKey

Parameters

const result = crypto.subtle.exportKey(format, key);

format: "raw"

key: key is a CryptoKey used only for OPRF signatures.

Returns

It returns an ArrayBuffer of length n containing the key.