The creators of banking trojan programs in Brazil are using block ciphers to encrypt their malware, making detection by anti-virus products more difficult. The discovery, by Kaspersky Lab researcher Dmitry Bestuzhev, is the first known report of block cipher encryption being used in connection with banking malware in that region. When used to encrypt the contents of malware executables, the encryption can cause malware detection and analysis systems not to work properly. Block-cipher encrypted malicious links, for example, can be downloaded and analyzed, but not detected as malicious.
Source: https://threatpost.com/malware-writers-use-block-cipher-latin-america-111011/75880/