@simplewebauthn/server
Version:
SimpleWebAuthn for Servers
74 lines (73 loc) • 3.17 kB
JavaScript
import { AsnParser } from '@peculiar/asn1-schema';
import { ECDSASigValue } from '@peculiar/asn1-ecc';
import { COSECRV } from '../../cose.js';
import { isoUint8Array } from '../index.js';
/**
* In WebAuthn, EC2 signatures are wrapped in ASN.1 structure so we need to peel r and s apart.
*
* See https://www.w3.org/TR/webauthn-2/#sctn-signature-attestation-types
*/
export function unwrapEC2Signature(signature, crv) {
const parsedSignature = AsnParser.parse(signature, ECDSASigValue);
const rBytes = new Uint8Array(parsedSignature.r);
const sBytes = new Uint8Array(parsedSignature.s);
const componentLength = getSignatureComponentLength(crv);
const rNormalizedBytes = toNormalizedBytes(rBytes, componentLength);
const sNormalizedBytes = toNormalizedBytes(sBytes, componentLength);
const finalSignature = isoUint8Array.concat([
rNormalizedBytes,
sNormalizedBytes,
]);
return finalSignature;
}
/**
* The SubtleCrypto Web Crypto API expects ECDSA signatures with `r` and `s` values to be encoded
* to a specific length depending on the order of the curve. This function returns the expected
* byte-length for each of the `r` and `s` signature components.
*
* See <https://www.w3.org/TR/WebCryptoAPI/#ecdsa-operations>
*/
function getSignatureComponentLength(crv) {
switch (crv) {
case COSECRV.P256:
return 32;
case COSECRV.P384:
return 48;
case COSECRV.P521:
return 66;
default:
throw new Error(`Unexpected COSE crv value of ${crv} (EC2)`);
}
}
/**
* Converts the ASN.1 integer representation to bytes of a specific length `n`.
*
* DER encodes integers as big-endian byte arrays, with as small as possible representation and
* requires a leading `0` byte to disambiguate between negative and positive numbers. This means
* that `r` and `s` can potentially not be the expected byte-length that is needed by the
* SubtleCrypto Web Crypto API: if there are leading `0`s it can be shorter than expected, and if
* it has a leading `1` bit, it can be one byte longer.
*
* See <https://www.itu.int/rec/T-REC-X.690-202102-I/en>
* See <https://www.w3.org/TR/WebCryptoAPI/#ecdsa-operations>
*/
function toNormalizedBytes(bytes, componentLength) {
let normalizedBytes;
if (bytes.length < componentLength) {
// In case the bytes are shorter than expected, we need to pad it with leading `0`s.
normalizedBytes = new Uint8Array(componentLength);
normalizedBytes.set(bytes, componentLength - bytes.length);
}
else if (bytes.length === componentLength) {
normalizedBytes = bytes;
}
else if (bytes.length === componentLength + 1 && bytes[0] === 0 && (bytes[1] & 0x80) === 0x80) {
// The bytes contain a leading `0` to encode that the integer is positive. This leading `0`
// needs to be removed for compatibility with the SubtleCrypto Web Crypto API.
normalizedBytes = bytes.subarray(1);
}
else {
throw new Error(`Invalid signature component length ${bytes.length}, expected ${componentLength}`);
}
return normalizedBytes;
}