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knock-knock

Description

Knock knock? Who's there? Another pastebin!!
index.js
Dockerfile
const crypto = require('crypto');
class Database {
constructor() {
this.notes = [];
this.secret = `secret-${crypto.randomUUID}`;
}
createNote({ data }) {
const id = this.notes.length;
this.notes.push(data);
return {
id,
token: this.generateToken(id),
};
}
getNote({ id, token }) {
if (token !== this.generateToken(id)) return { error: 'invalid token' };
if (id >= this.notes.length) return { error: 'note not found' };
return { data: this.notes[id] };
}
generateToken(id) {
return crypto
.createHmac('sha256', this.secret)
.update(id.toString())
.digest('hex');
}
}
const db = new Database();
db.createNote({ data: process.env.FLAG });
const express = require('express');
const app = express();
app.use(express.urlencoded({ extended: false }));
app.use(express.static('public'));
app.post('/create', (req, res) => {
const data = req.body.data ?? 'no data provided.';
const { id, token } = db.createNote({ data: data.toString() });
res.redirect(`/note?id=${id}&token=${token}`);
});
app.get('/note', (req, res) => {
const { id, token } = req.query;
const note = db.getNote({
id: parseInt(id ?? '-1'),
token: (token ?? '').toString(),
});
if (note.error) {
res.send(note.error);
} else {
res.send(note.data);
}
});
app.listen(3000, () => {
console.log('listening on port 3000');
});
FROM node:17.4.0-buster-slim
RUN mkdir -p /app
WORKDIR /app
COPY package.json .
RUN yarn
COPY . .
USER node
CMD ["node", "index.js"]

Solution

A programming error lies in the fact that crypto.randomUUID (the function) is used as the secret, instead of calling the function.
const crypto = require('crypto');
class Database {
constructor() {
this.notes = [];
this.secret = `secret-${crypto.randomUUID}`;
console.log(this.secret);
}
Therefore, the secret is actually:
secret-function randomUUID(options) {
if (options !== undefined)
validateObject(options, 'options');
const {
disableEntropyCache = false,
} = options || {};
validateBoolean(disableEntropyCache, 'options.disableEntropyCache');
return disableEntropyCache ? getUnbufferedUUID() : getBufferedUUID();
}
Therefore, we just have to generate the token for id=0, which is the same every time.
console.log(db.generateToken(0));
The flag is dice{1_d00r_y0u_d00r_w3_a11_d00r_f0r_1_d00r}