crypto-browser

Browser-compatible cryptography utilities for Sigstore and TUF verification.

Used by tuf-browser and sigstore-browser.

Caution

This library has not received an independent security audit. Maintenance is performed by volunteers, and the project is not officially supported or endorsed by the Freedom of the Press Foundation.

Features

This package provides shared cryptographic and encoding utilities used across multiple Sigstore browser implementations:

ASN.1 Parsing & Encoding

• ASN1Obj: Parse and encode DER-encoded ASN.1 structures
• ASN1Tag: Handle ASN.1 tags and type checking
• Length encoding/decoding: DER length format support
• Type parsers: Extract integers, OIDs, dates, bit strings, booleans from ASN.1

X.509 Certificate Support

• X509Certificate: Parse and verify X.509 certificates (PEM or DER)
• X.509 Extensions: Support for standard and Fulcio-specific extensions
  • Basic Constraints
  • Key Usage
  • Subject Alternative Name
  • Authority/Subject Key Identifiers
  • Signed Certificate Timestamps (SCT)
  • Fulcio Issuer extensions (v1 & v2)

Encoding Utilities

• Base64: Encode/decode with standard and URL-safe variants
• Hex: Encode/decode hexadecimal strings
• PEM: Convert between PEM and DER formats
• UTF-8: String to Uint8Array conversion
• Constant-time comparison: Timing-attack resistant byte comparison

Cryptographic Operations

• Key import: Support for ECDSA (P-256/P-384/P-521), Ed25519, and RSA keys
• Signature verification: Verify signatures using Web Crypto API
  • ECDSA with SHA-256/384/512
  • Ed25519
  • RSA-PSS and RSASSA-PKCS1-v1_5
• Digest verification: ECDSA signature verification over pre-computed digests (using @noble/curves)
• Certificate chain verification: Verify certificate chains against trusted CAs with timestamp validation
• PKCS#1 to SPKI conversion: Convert RSA keys for Web Crypto compatibility

Additional Utilities

• ByteStream: Efficient byte buffer with auto-growing allocation
• Canonical JSON: TUF-compliant canonical JSON encoding
• OID mappings: Mappings for signature algorithms, hash algorithms, and ECDSA curves

Installation

npm install crypto-browser

Usage

Parse and verify an X.509 certificate

import { X509Certificate } from 'crypto-browser';

const cert = X509Certificate.parse(pemOrDerBytes);
console.log(cert.subject);
console.log(cert.notBefore, cert.notAfter);

const isValid = await cert.verify(issuerCert);

Import a public key and verify a signature

import { importKey, verifySignature } from 'crypto-browser';

const publicKey = await importKey('ECDSA', 'P-256', pemKey);
const isValid = await verifySignature(publicKey, message, signature, 'sha256');

Verify a certificate chain

import { verifyCertificateChain, X509Certificate } from 'crypto-browser';

const leafCert = X509Certificate.parse(certBytes);
const verifiedChain = await verifyCertificateChain(
  new Date(),
  leafCert,
  certificateAuthorities
);

Parse ASN.1 structures

import { ASN1Obj } from 'crypto-browser';

const asn1 = ASN1Obj.parseBuffer(derBytes);
const oid = asn1.subs[0].toOID();
const integer = asn1.subs[1].toInteger();

Encode/decode utilities

import {
  base64ToUint8Array,
  Uint8ArrayToHex,
  toDER,
  fromDER,
} from 'crypto-browser';

const bytes = base64ToUint8Array('SGVsbG8=');
const hex = Uint8ArrayToHex(bytes);
const der = toDER(pemCertificate);
const pem = fromDER(derBytes, 'CERTIFICATE');

Dependencies

• @noble/curves: Used for low-level ECDSA verification over pre-computed digests

Structure

crypto-browser/
    asn1/           # ASN.1 parsing and encoding
    x509/           # X.509 certificate and extension support
    encoding.ts     # Base64, hex, UTF-8 encoding
    pem.ts          # PEM format conversion
    stream.ts       # ByteStream utility
    crypto.ts       # Cryptographic operations
    oid.ts          # OID mappings
    canonicalize.ts # Canonical JSON
    interfaces.ts   # TypeScript interfaces and enums

Development

# Install dependencies
npm install

# Build the project
npm run build

License

Apache-2.0 - see LICENSE file for details.

Acknowledgments

This package provides browser-compatible versions of cryptographic utilities from the Sigstore and TUF ecosystems. All code is licensed under Apache-2.0.

Primary Sources:

• sigstore-js - Core ASN.1, X.509, and crypto utilities
• tuf-js - Canonical JSON implementation

All files maintain their original copyright headers.