BURP - BackUp and Restore Program


SSL - Certificate Authority and certificates

As of burp-1.3.2, the process of generating SSL certificates has been
automated with the use of burp_ca.

Effectively, the automation means that you can set up a new client on the
server (in /etc/burp/clientconfdir/<client>), and the first client to connect
that matches that client name and password will get its certificate signed
and given back. If a different, second client, happens to connect with the
same client name and password, it will be rejected because it doesn't have
a matching certificate.

The clientconfdir passwords can now be thought of as being 'first contact'
passwords, as after that, SSL will provide assurance that the peers are who
they claim to be (although the server will still check the passwords too).

There is the option of setting 'password_check = 0' on the server, which
will disable the checking of passwords completely (but won't disable the
checking of SSL certificates).

Of course, you should take into account the environment that your server and
clients are running in before making the choice to either disable the
passwords, leave them all the same, or make them all different.


Server CA options:
ca_conf = /etc/burp/CA.cnf
ca_name = burpCA
ca_server_name = burpserver
ca_burp_ca = /usr/sbin/burp_ca

Server SSL options:
ssl_dhfile = /etc/burp/dhfile.pem
ssl_cert_ca = /etc/burp/ssl_cert_ca.pem
ssl_cert = /etc/burp/ssl_cert-server.pem
ssl_key = /etc/burp/ssl_cert-server.key
ssl_key_password = password (only needed if you have an encrypted certificate)

If you do not want the server to automate the CA stuff, do not set ca_conf.

When the server starts, the ca_conf file will be read, and the CA_DIR value
from that file will be read.
If the CA_DIR directory already exists, the automatic CA setup finishes here.

Next, if ssl_dhfile does not exist, an appropriate dhfile will be generated.
The dhfile is used in encypting the SSL connection with the Diffie-Hellman

Next, the CA directory is initialised using ca_burp_ca, which should point to
the burp_ca script.
The name of the CA will be the value of ca_name. 

A key for the burp server is then generated, along with a certificate signing
request to give to the CA. The CA then signs the request. The ssl_cert_ca,
ssl_cert and ssl_key paths are then overwritten with symlinks to the new keys
and certificates.

This concludes the CA setup on the server.


Client CA options:

Client SSL options:
ssl_cert_ca = /etc/burp/ssl_cert_ca.pem
ssl_cert = /etc/burp/ssl_cert-client.pem
ssl_key = /etc/burp/ssl_cert-client.key
ssl_key_password = password (only needed if you have an encrypted certificate)
ssl_peer_cn = burpserver

If you do not want the client to automate the CA stuff, do not set ca_burp_ca.
Clients as of 1.3.2 will not include the old default certificates, so if you
want to install new clients connecting to a server that is using the old
certificates, you will also have to copy the old certificates onto the client.

When the client runs (for example, with 'burp -a b'), it will try to connect
to the server using the burp client name (cname) and password. If you haven't
got the client name and password right, everything stops here. 

If the path to ssl_key exists, the client will not try to generate a key and
certificate signing request - it assumes that this has already been done.
Otherwise, a client key is generated at the path of ssl_key, and a certificate
request is generated in the ca_csr_dir directory, named after the client name.

This is copied to CA_DIR on the server, unless the server already has a file
named the same, or a signed certificate with the same name. If either already
exists, the server will reject the request.

If the server accepts the request, it will generate a signed certificate
and send it back to the client, along with the CA certificate.
The ssl_cert_ca and ssl_cert paths will be overwritten on the client, and
burp.conf will be rewritten with ssl_peer_cn set correctly.

The connection between server and client is dropped and the client will
reconnect using the new certificates to start the real work (for example, a
backup). In previous versions of burp, you needed to set ssl_peer_cn in the
client's clientconfdir file on the server. You don't need to set this any
more, because the server will default to using the client name instead.

This concludes the certificate setup on the client.


The following is the (slightly tweaked) original documentation for burp_ca. It
might still be of use in order to help understand the process, or if it is
necessary to do it by hand. Note that there is a burp_ca.bat script if you are
generating certificate signing requests on Windows.

Setup the server:

1) generate Diffie-Hellman Parameters

 burp_ca --dhfile /etc/burp/dhfile.pem

2) Initialise CA

 burp_ca --init --ca myCA

 This creates /etc/burp/CA, generate private key for CA and self signed

3) Generate server key and cert signing request

 burp_ca --key --request --name myServer

4) Sign request

 burp_ca --sign --ca myCA --name myServer --batch

5) Link or copy cert and key to /etc/burp

 ln -s CA/CA_myCA.crt /etc/burp/ssl_cert_ca.pem
 ln -s CA/myServer.crt /etc/burp/ssl_cert-server.pem
 ln -s CA/myServer.key /etc/burp/ssl_cert-server.key

 (use the ssl_cert and ssl_key options in burp-server.conf !)

Setup the client (the easy way - insecure)

1) Generate client key and cert __on the server__

  burp_ca --name myClient --ca myCA --key --request --sign --batch

2) copy key and certs from server to client

  server:/etc/burp/ssl_cert_ca.pem -> client:/etc/burp/ssl_cert_ca.pem
  server:/etc/burp/CA/myClient.crt -> client:/etc/burp/ssl_cert-client.pem
  server:/etc/burp/CA/myClient.key -> client:/etc/burp/ssl_cert-client.key
  chmod 600 /etc/burp/ssl_cert-client.key

  (use the ssl_cert and ssl_key options in burp.conf !)

Setup the client (the usual way)

1) on the client you dont need a CA, so just

  mkdir /etc/burp/CA
  chmod 700 /etc/burp/CA

2) Generate client key and cert signing request

  burp_ca --key --request --name myClient
(on Windows: burp_ca.bat --key --keypath <path> \
	--request --requestpath <path> --name myClient)

3) copy the request from client to the server

  client:/etc/burp/CA/myClient.csr -> server:/etc/burp/CA/myClient.csr

4) sign the request __on the server__
  burp_ca --name myClient --ca myCA --sign --batch

5) copy the certs back to the client

  server:/etc/burp/CA/CA_myCA.crt -> client:/etc/burp/CA/CA_myCA.crt
  server:/etc/burp/CA/myClient.crt -> client:/etc/burp/CA/myClient.crt

6) link or copy the files

 ln -s CA/CA_myCA.crt /etc/burp/ssl_cert_ca.pem
 ln -s CA/myClient.crt /etc/burp/ssl_cert-client.pem
 ln -s CA/myClient.key /etc/burp/ssl_cert-client.key
 (use the ssl_cert and ssl_key options in burp.conf !)

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Burp, don't suck. Last updated: March 2017
By Graham Keeling
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