Your Second Life IMs history is being stored at SpeedLight server. This allows you switching devices easily, but you may worry that some database stores your messages text.
You can conceal your IMs history completely by encrypting them.
The IMs history encryption can be enabled in "Avatar settings":
What happens when you enter the IMs history password:
Your browser generates two keys: one to encrypt messages, and one to decrypt
The key to encrypt goes to the SpeedLight server
The key to decrypt stays on your PC
Now all your further messages will be encrypted on server but visible in your browser
If you open SpeedLight on another device, you won't see your messages' text because is is encrypted:
Click the link and enter your IMs history password to see IMs on this device.
You can disable the IM encryption at any time if you know the password. Just untick the "Encrypt" checkbox and confirm the decryption.
All your existing IMs will be decrypted back.
If you don't remember the password, you can turn the encryption off too. But the text contents of the IMs will be lost.
To change the IM history password, disable the encryption first. Then enable the encryption back and specify the new encryption password.