<p>SslStream is supposed to negotiate the cipher type, key length, hash algorithm, etc. with its peer SSL stack. When using it in my code, I find that the negotiation always defaults to RC4 &amp; MD5. I would like to use 3DES or AES for some added security.</p>

<p>Looking around the web I find only a few references to this problem and no solutions; one poster is claiming this actually makes sense, since the lowest common denominator between the two stacks is secure while has the added benefit of being faster/using less CPU resources. While this may be technically correct, my particular trade-off between complexity and cost lies elsewhere (I prefer to use AES with a long key).</p>

<p>If anyone can help I'd appreciate it.</p>

It should be using the most secure set of algorithms that were in both lists. I find it hard to believe that it isn't, because SslStream is wrapping the SChannel SSPI, and if that were broken then Internet Explorer, IIS and everything else on Windows would be broken too.
It could be that you have an outdated version of SChannel.dll/Secur32.dll. What OS and Internet Explorer version do you have installed?
It is possible to
disable protocols in SCHANNEL
. Could you check that this hasn't been done?