Smartphones, whose market share has increased by 54% between 2009 and 2010, is one of the favored platform for "Convergence Computing". Convergence Computing is a technology in which a single device can provide various services without any restrictions from external devices or networks. Today, smartphones as convergent single device have diverse functions and features such as calling, Internet surfing, game playing, banking, storage of personal and professional data, etc. Some of these use encryption algorithms such as AES (Advanced Encryption Standard). For example, this algorithm is used to authenticate server protocols or to encrypt confidential information. This paper shows that an Electromagnetic Analysis (EMA) on AES is possible on a Java mobile phone to extract secret keys. The latter can then be used for forensic purposes or to recover encrypted data stored in the device. Experiments involving two successful approaches are described and compared : Spectral Density based Approach (SDA) and Template based Resynchronisation Approach (TRA).