An eavesdropper intercepting an encrypted signal hears only bursts of incoherent noise. The encryption transforms analog voice signals into digital signals and rearranges them as they travel over the wire or radio airwaves. This scrambling is reversed at the other end of the network. An eavesdropper must have an identical encryptation device to break into the conversation. Programmable codes called 'keys' can protect encryptations even when the eavesdropper has obtained identical equipment. Changing a 'key' is quick and easy with data encryption products. A telephone with an encryption devise looks and acts like a regular telephone. The government has set standards for encryption design. Purchasers of systems should identify their specific needs in the areas of security, packaging, and performance. Guidelines on making choices and diagrams of sample security systems.