No. SLIP works with SLIP. PPP works with PPP.
Some vendors may offer products which work both as SLIP and PPP. However, they must be configured to run in one mode or the other. There is no present method to determine, based upon the protocol passed at the time of a connection, which combination of SLIP protocols or PPP is being requested.
IT DEPENDS UPON MANY FACTORS.
The people who post this type of question have usually not read the Net-2-HOWTO document.
A good technical discussion is available at Morning Star\s www server, www.morningstar.com.
If you have the choice, use CHAP. Failing that, PAP is better than nothing.
CHAP is a Cryptographic Handshake Authentication Protocol. It means that it takes some form of a key and will encrypt the response using a one-way encryption algorithm. The algorithm is negotiated at the time that the CHAP protocol is requested. The most common is called MD5. It has an encryption code of 05 in the CHAP request.
Microsoft uses a DES algorithm which, until recently, was incompatible with the pppd process. If you wish to connect to a Windows NT server, there are a set of patches which are included with the pppd source code to support the DES style used by Microsoft.
Contrary to what some un-informed people believe at Stanford University believe, Microsoft did not just go against the recommendations of the IETF working group. The code values were properly requested and the implementation has been fully documented.