Following on from Opabina Regalis - Token Fetch, this challenge listens on ssl-added-and-removed-here.ctfcompetition.com:20691.

This challenge also uses protocol buffers. We’re using full duplex sockets to act as both a client and a server on the same connection. When we first connect, we are sent a request to /protected/not-secret. The idea is to respond to this request with a 401 Unauthorized, giving a WWW-Authenticate: Basic header. We should get back a request with an Authorization header, containing a base64-encoded username and password. We can then use this password to make our own authenticated request to /protected/secret.

Initial request from server:

GET /protected/not-secret HTTP/1.1
User-Agent: opabina-regalis.go

Our 401 response:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Basic realm="In the realm of hackers"

The server will then respond with a request containing the base64 username and password:

GET /protected/not-secret HTTP/1.1
User-Agent: opabina-regalis.go
Authorization: Basic Z29vZ2xlLmN0ZjoxODAxMTg2MjEwLjEwMDA2NTI3NjEuOTA4OTcyODQy

Which decodes to google.ctf:1801186210.1000652761.908972842. Note that a new password is generated for each session.

If we make a request to /protected/secret, we get a 401, specifying Digest authorization:

HTTP/1.1 401 Unauthorized
Server: opabina-regalis.go
WWW-Authenticate: Digest realm="In the realm of hackers",qop="auth",nonce="f2c8bb0f1b5bb7ed",opaque="f2c8bb0f1b5bb7ed"

Since we have the password, nonce, realm, and opaque parameters, we can now compute the digest response:

def calc_response(username, realm, password, method, uri, nonce, nc, cnonce):
    ha1 = hashlib.md5(':'.join((username, realm, password))).hexdigest()
    ha2 = hashlib.md5(':'.join((method, uri))).hexdigest()
    r = hashlib.md5(':'.join((ha1, nonce, nc, cnonce, 'auth', ha2))).hexdigest()
    return r

And we can build the header and send it with our request:

GET /protected/secret HTTP/1.1
Authorization: Digest username="google.ctf",realm="In the realm of hackers",nonce="af8a6a5f1c64ade3",uri="/protected/secret",qop=auth,nc=00000001,cnonce="1115fd1d40a5b9d5",response="ec0555e97853f8e14b2b8cae23208d7b",opaque="af8a6a5f1c64ade3"

Bingo!

HTTP/1.1 200 OK
Server: opabina-regalis.go

CTF{What:is:green:and:goes:to:summer:camp...A:brussel:scout}