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To provide these services, S/MIME uses the X.509v3 format for digital certificates, along with various encryption algorithms. Non-repudiation actions, for example, require a public-key algorithm, while privacy and data security need a fast and efficient symmetric encryption algorithm.
RFC 3370 identifies the algorithms that all S/MIME version 3 software must support. These are Secure Hash Algorithm 1 (SHA-1) and Message Digest-5 (MD5) for hashing, Digital Signature Algorithm (DSA) and RSA for signatures, and RC2 and triple Data Encryption Standard (3DES) for message encryption. The requirements ensure a base level of interoperability among all S/MIME implementations. Email clients, however, can add additional algorithms, provided they correctly identify which algorithms a particular message uses.
The United States government no longer restricts encryption strength, and the default encryption algorithm in Outlook, Outlook Web Access and most email clients is 3DES. Although it is slower than the original DES, the triple Data Encryption Standard is more secure. When Microsoft Outlook runs on a 40-bit operating system that does not have 128-bit encryption capabilities, it uses the RC2 algorithm by default.
Depending on the email client you use, it can be tricky to change this default setting. The figure below shows the security properties that you can set in Outlook Express 6, which include the encryption algorithm. In Outlook 2003's online help feature, there is no mention of how to change the algorithm. However, there is no particular reason to need to change this setting.
Figure 1: Outlook Express 6 Security Settings
This was first published in September 2007