@freedomofpress/crypto-browser

crypto-browser

Browser-compatible cryptography utilities for Sigstore and TUF verification.

Used by tuf-browser and sigstore-browser.

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

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

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)
• 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

Installation

npm install @freedomofpress/crypto-browser

Usage

Import a public key and verify a signature

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

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

Parse ASN.1 structures

import { ASN1Obj } from '@freedomofpress/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 '@freedomofpress/crypto-browser';

const bytes = base64ToUint8Array('SGVsbG8=');
const hex = uint8ArrayToHex(bytes);
const der = toDER(pemString);
const pem = fromDER(derBytes, 'PUBLIC KEY');

Dependencies

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

Structure

src/
  asn1/           # ASN.1 parsing and encoding
  encoding.ts     # Base64, hex, UTF-8 encoding
  pem.ts          # PEM format conversion
  stream.ts       # ByteStream utility
  crypto.ts       # Cryptographic operations
  canonicalize.ts # Canonical JSON

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 - ASN.1 parsing, PEM encoding, and stream utilities
• tuf-js - Canonical JSON implementation

All files maintain their original copyright headers.