Cryptographic Message Syntax (CMS) is a standard for protecting messages cryptographically. Using CMS, messages can be protected in different content types such as signed-data, enveloped-data, digested-data and authenticated-data. CMS is architected around certificate based key management and symmetric cryptography. In this thesis, a lightweight CMS envelope is proposed for the mobile phones which have limited memory and processing power, in order to provide the privacy of the data either stored on them or exchanged by means of multimedia messaging (MMS) and e-mail. A sample prototype is implemented on mobile phone which makes use of the proposed lightweight CMS. The prototype is compared with standard CMS in terms of application size and performance. The application size decreases approximately by 35% and the envelope preparation duration is much shorter with lightweight CMS in comparison to standard CMS.