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import cbor from 'cbor';
import { decodeCborFirst } from './decodeCbor';
/**
* Make sense of the authData buffer contained in an Attestation
*/
export default function parseAuthenticatorData(authData: Buffer): ParsedAuthenticatorData {
if (authData.byteLength < 37) {
throw new Error(
`Authenticator data was ${authData.byteLength} bytes, expected at least 37 bytes`,
);
}
let pointer = 0;
const rpIdHash = authData.slice(pointer, (pointer += 32));
const flagsBuf = authData.slice(pointer, (pointer += 1));
const flagsInt = flagsBuf[0];
// Bit positions can be referenced here:
// https://www.w3.org/TR/webauthn-2/#flags
const flags = {
up: !!(flagsInt & 1 << 0), // User Presence
uv: !!(flagsInt & 1 << 2), // User Verified
be: !!(flagsInt & 1 << 3), // Backup Eligible
bs: !!(flagsInt & 1 << 4), // Backup State
at: !!(flagsInt & 1 << 6), // Attested Credential Data Present
ed: !!(flagsInt & 1 << 7), // Extension Data Present
flagsInt,
};
const counterBuf = authData.slice(pointer, (pointer += 4));
const counter = counterBuf.readUInt32BE(0);
let aaguid: Buffer | undefined = undefined;
let credentialID: Buffer | undefined = undefined;
let credentialPublicKey: Buffer | undefined = undefined;
if (flags.at) {
aaguid = authData.slice(pointer, (pointer += 16));
const credIDLenBuf = authData.slice(pointer, (pointer += 2));
const credIDLen = credIDLenBuf.readUInt16BE(0);
credentialID = authData.slice(pointer, (pointer += credIDLen));
// Decode the next CBOR item in the buffer, then re-encode it back to a Buffer
const firstDecoded = decodeCborFirst(authData.slice(pointer));
const firstEncoded = Buffer.from(cbor.encode(firstDecoded) as ArrayBuffer);
credentialPublicKey = firstEncoded;
pointer += firstEncoded.byteLength;
}
let extensionsDataBuffer: Buffer | undefined = undefined;
if (flags.ed) {
const firstDecoded = decodeCborFirst(authData.slice(pointer));
const firstEncoded = Buffer.from(cbor.encode(firstDecoded) as ArrayBuffer);
extensionsDataBuffer = firstEncoded;
pointer += firstEncoded.byteLength;
}
// Pointer should be at the end of the authenticator data, otherwise too much data was sent
if (authData.byteLength > pointer) {
throw new Error('Leftover bytes detected while parsing authenticator data');
}
return {
rpIdHash,
flagsBuf,
flags,
counter,
counterBuf,
aaguid,
credentialID,
credentialPublicKey,
extensionsDataBuffer,
};
}
export type ParsedAuthenticatorData = {
rpIdHash: Buffer;
flagsBuf: Buffer;
flags: {
up: boolean;
uv: boolean;
be: boolean;
bs: boolean;
at: boolean;
ed: boolean;
flagsInt: number;
};
counter: number;
counterBuf: Buffer;
aaguid?: Buffer;
credentialID?: Buffer;
credentialPublicKey?: Buffer;
extensionsDataBuffer?: Buffer;
};
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