For each call, mobile operators must select an encryption key (called a stream cipher) to secure the call. Normally, the stream cipher should be unique for each call. However, a
team of academics from the Ruhr University in Bochum, Germany, has discovered that not all mobile operators follow the 4G standard to the letter of the law. Researchers say that while mobile operators do, indeed, support encrypted voice calls, many calls are
encrypted with the same encryption key. In their research, academics said that the problem usually manifests at the base station (mobile cell tower) level, which, in most cases, reuse the same stream cipher, or use predictable algorithms to generate the encryption
key for voice calls.
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