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Secure Real-Time Transport Protocol (SRTP) is one of the most widely used protocols in VoIP applications to provide both real-time transport and security features. However, applying security mechanisms such as encryption and authentication require additional processing time and hence may require significant processing overhead. The paper analyzes this additional processing time and CPU cycles consumed by SRTP when it is used to secure VoIP on handheld devices in order to investigate the energy consumption of applying SRTP on these limited resource devices. Experiments were conducted on HP iPAQ handheld devices running PJSIP softphone installed on Windows Mobile operating system. Two types of codecs have been used in the measurements; GSM and G.711. A comparison of processing time required by SRTP encryption and AES block cipher is also performed in this paper.