Compression techniques used in Voice over IP (VoIP) may leave phone conversations open to interception.

According to researchers at John Hopkins University in the US, variable bitrate compression, which is already in use in some VoIP applications, compresses different sounds in different ways. This uses different sized packets for different sounds made during conversations.

The researchers identified that by analysing the size of the packets, they could decipher the words and phrases used during a conversation. in this way criminals could evesdrop without having to decrypt the data packets first.

Rather than decrypting entire conversations, the team searched conversations for words and phrases and then encoded with variable bitrate compression to highlight how it appears in a packet.

See also: Hackers target VoIP accounts

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