I'm currently working on a port of an application from iPhone onto WP7 and the local content has to be decrypted on the device. The content was original encrypted for a different platform and the cipher & padding used are not supported in the Silverlight version for WP7. The content was encrypted using the AES symmetric algorithm with an ECB cipher and PCKS5 padding. As stated this combination is not supported by the AESManaged class available in the System.Security.Cryptography namespace. The documentation states only the following is supported: 'The cipher mode is always CBC, and the padding mode is always PKCS7' I'm not sure why this is, but I'm aware that ECB is no longer viewed a secure cipher - the choice to use this was out of our hands. The next stage was to find OSS library for WP7. After several recommendations we decided to use Bouncy Castle . BC (Bouncy Castle) is a well established cryptography library from the Java world with a port to C#...