SSL Encryption

Introduction

For more details see #1252.

The TCP to SSL socket upgrade can be used with SSL clients, websockets clients (html5 clients) or HTTPS connections.

This option can more easily go through some firewalls and may be required by some network policies. Client certificates can also be used for authentication.

There are a lot more options to configure and certificates to deal with. See ​https://docs.python.org/2/library/ssl.html, on which this is based.

It is only applicable to TCP sockets, not unix domain sockets. Do not assume that you can just enable SSL to make your connection secure.

When using the binary packages, a self-signed SSL certificate will be generated during the first installation. It is placed in:

• /etc/xpra/ssl-cert.pem on Posix platforms
• C:\ProgramData\Xpra\ssl-cert.pem on MS Windows
• /Library/Application Support/Xpra/ssl-cert.pem on Mac OS

Example

• use an existing certificate "cert.pem" (see below for creating one)
• start a server with TCP and SSL support:

  xpra start --start=xterm \
      --bind-tcp=0.0.0.0:10000 --ssl-cert=/path/to/ssl-cert.pem --ssl=on
  

or for SSL only:

xpra start --start=xterm \
    --bind-ssl=0.0.0.0:10000 --ssl-cert=/path/to/ssl-cert.pem

• client:

  xpra attach ssl://127.0.0.1:10001/
  

Using a self signed certificate

Generate a certificate:

openssl req -new -x509 -days 365 -nodes -out cert.pem -keyout key.pem -sha256
cat key.pem cert.pem > ssl-cert.pem

To avoid this error when the client connects:

[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)

You can:

• temporarily add --ssl-server-verify-mode=none to your client command line
• or copy the key to the client then use ssl-ca-certs to use it for validation:

  xpra attach ssl://host:10000/ --ssl-ca-certs=./cert.pem
  

---

For trusting your own certificates and testing with localhost, see ​certificates for localhost

Securing SSL with self signed CA and certificates

See ​The Most Dangerous Code in the World: Validating SSL Certificates in Non-Browser Software and ​Beware of Unverified TLS Certificates in PHP & Python. See also: ​Fallout from the Python certificate verification change.

Since the server certificate will not be signed by any recognized certificate authorities, you will need to send the verification data to the client via some other means... This will no be handled by xpra, it simply cannot be. (same as the AES key, at which point... you might as well use AES)

---

# generate your CA key and certificate:
openssl genrsa -out ca.key 4096
# (provide the 'Common Name', ie: 'Example Internal CA')
openssl req -new -x509 -days 365 -key ca.key -out ca.crt
# generate your server key:
openssl genrsa -out server.key 4096
# make a signing request from the server key:
# (you must provide the 'Common Name' here, ie: 'localhost' or 'test.internal')
openssl req -new -key server.key -out server.csr
# sign it with your CA key:
openssl x509 -req -days 365 \
    -in server.csr -out server.crt \
    -CA ca.crt -CAkey ca.key \
    -CAserial ./caserial -CAcreateserial
# verify it (it should print "OK"):
openssl verify -CAfile ca.crt ./server.crt

You can now start your xpra server using this key:

xpra start --start=xterm \
    --bind-tcp=0.0.0.0:10000 --ssl=on --ssl-cert=`pwd`/server.crt --ssl-key=`pwd`/server.key

Use openssl to verify that this xpra server uses SSL and that the certificate can be verified using the "ca.crt" authority file: (it should print Verify return code: 0 (ok)):

openssl s_client -connect 127.0.0.1:10000  -CAfile /path/to/ca.crt < /dev/null

Connect the xpra client:

xpra attach ssl:localhost:10000 --ssl-ca-cert=/path/to/ca.crt

Sending the CA data

In some cases, it may be desirable to supply the CA certificate on the command line, in a URL string or in a session file. Here's how.

Convert a CA file to a hexadecimal string:

python -c "import sys,binascii;print(binascii.hexlify(open(sys.argv[1]).read()))" ca.crt

Convert hex back to data to verify (only part of the data shown here):

python -c "import sys,binascii;print binascii.unhexlify(sys.argv[1])" \
    2d2d2d2d2d424547494e2043455254494649434154452d2d2d2d2d0a4d4949

Use it directly in the xpra command:

xpra attach ssl:localhost:10000 \
    --ssl-ca-data=2d2d2d2d2d424547494e...4452d2d2d2d2d0a

Alternatively, place all of these in a connection file you can just double click on:

echo > ssl-test.xpra <<EOF
host=localhost
autoconnect=true
port=10000
mode=ssl
ssl-ca-data=2d2d2d2d2d424547494e...4452d2d2d2d2d0a
EOF

The cadata can also be encoded using base64, which more dense:

python -c "import sys,base64;print(base64.b64encode(open(sys.argv[1]).read()))" ca.crt