Use the symmetric passphrase to encrypt data.

This command will encode the XML data with your new symmetric passphrase. The performance is greatly improved over using Asymmetric keys for subsequent transmissions. You should only use these symmetric passphrase for a limited time, perhaps only for a set number of transactions or only for a single day. The particular design considerations are left to the implementer.

$ openssl enc -e –a -aes128 -kfile passphrase-$$  -in myPriv.xml -out myPriv.xml.aes128
        

In this case I used an encryption method called AES because its ubiquitous on most systems, others are available and your partner and you should agree on the encoding method your going to use. To see a list of available ciphers that your system supports use the following command.

$ openssl ciphers -v