Outlook Express is compatible with the Secure/Multipurpose Internet Mail Extensions (S/MIME) version 2 and 3 specifications. Outlook Express supports the following encryption algorithms: RC2 (40-bit and 128-bit), DES (56-bit), and 3DES (168-bit). Outlook Express can decrypt RC2 (64-bit) encrypted e-mail, but cannot send messages using this algorithm.
Outlook Express can use only SHA-1 as the hashing algorithm when signing messages. The bit length of your private key varies, depending on the certification authority from which you obtain it and the process used in generating the key.
The private keys are stored on your computer and are only as secure as your computer. Private keys installed using Microsoft cryptographic system components will not be transmitted to the certification authority that issues the digital ID; the keys are not stored in escrow with any government agency.