A seekable (random-access) XSALSA20 xor stream.
npm install raxor
const RAXOR = require('raxor')
// nonce and key should be buffers of the ... nonce and key
const r = new RAXOR(nonce, key)
r.seek(10000) // seek to byte offset 10k
r.update(out, input) // xor the input buffer as if it was as this offset
r.final() // destroy the instance
MIT