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Voice Scrambling and Encryption

  • Scrambling
  • Encryption
  • Sources

  • Speech Scrambling:

    Speech inversion is a very common method of speech scrambling, probably because its the cheapest. Speech inversion works be taking a signal and turning it 'inside out', reversing the signal around a pre-set frequency. Speech inversion can be broken down into three types, base-band inversion (also called 'phase inversion'), variable-band inversion (or 'rolling phase inversion') and split band inversion. Images will be used to help clarify what different inversion systems do. The non-scrambled wave is represented at right. (Note: Box waves were used for clarity, though telephones output a sine wave.)

    Box Wave

    Base band inversion inverts the signal around a pre-set frequency that never changes. Because of this, base-band inversion is useless!! Because the inverting frequency never changes, running the frequency through another inverter set on the same frequency unscrambles it. Motorolla markets phase inversion scrambling under the trademark "Secure Clear".

    Descrambling baseband inversion is simple. Take the scrambled input and re-invert it around the same inversion point used to scramble it. Manufacturers often have a stock inversion point.

    To hear what base-band scrambling sounds like, click here.

    Baseband Inverted Box Wave

    Variable-band inversion inverts the signal around a constantly varying frequency, making decryption possible, but not bloody likely. Variable band inversion can be identified by the burst of modem noise at the beginning of the transmission (its a 1200 bps carrier) and the repeated clicking sounds as the inverting frequency changes.

    Descrambling variable band inversion would be a chore for the amateur eavesdropper, as the inversion point changes every fraction of a second. Professionals however would likely have little trouble extracting clear speech.

    To hear what variable band inversion scrambling sounds like, click here.

    Split-band inversion is another method for making inversion more secure. Split band inversion divides the signal into two frequencies and inverts them (sually baseband) seperately. Some split band inversion systems provide enhanced security by randomly changing the frequency where the signal is split at given intervals. It far from perfect, but better than vanilla baseband.

    Split-band inverted Box wave

    Speech Encryption

    Encryption is a much stronger method of protecting speech communications than any form of scrambling. Voice encryptors work by digitizing the conversation at the telephone and applying a cryptographic technique to the resulting bit-stream. In order to decrypt the speach, the correct encryption method and key must be used.

    Hardware Based Encryption Systems:

    Hard encryption systems are voice encryption schemes that utilize hardware to encrypt conversations. Hard encryption devices are useful because they don't need a computer to work (allowing them to be built into things like radios and cellular phones), are usually more secure, and are simpler to use. On the downside, hardware encryption systems are very expensive and can be hard to acquire.

    Software Based Encryption Systems:

    Soft encryption systems are exactly what they sound like, software based encryption. While the inconvenience of having to use a computer is the primary drawback to soft voice encryption, most of the available programs use good crypto and are free.

    Digital Voice Protection:

    Digital Voice Protection (DVP) is a proprietary speech encryption technique used by Motorola for their higher-end secure communications products. DVP is considered to be very secure. DVP sounds like data being passed.


    PGPfone is another offering from Pretty Good Privacy Inc., a secure voice program for the PC. The interface is pleasantly intuitive, and there are options for different encoders and decoders (for either cellphone or landline use). PGPfone offers a selection of encryption schemes: 128 bit CAST key (a DES-like crypto system), 168 bit Triple-DES key (estimated key strength is 112 bits) or 192 bit Blowfish key (unknown estimated key strength). .

    STU III:

    STU III (Secure Telephone Unit, Generation III) is the U.S. Government's standard for voice encryption. STU IIIs utilize the NSA Skipjack encryption algorythim, which is considered secure against most eavesdropping but is supposedly backdoored by the NSA. STU IIIs are considered incredibly secure, hence are restricted to government and military users, along with their civillian contractors. STU IIIs are manufactured by Lucent, L-3 Communication Systems, General Dynamics, Lockheed Martin, AT&T and Motorola. STU III compatible fax equipment is manufactured by Ilex

    The DOD STU III handbook may be viewed here


    Nautilus is a free secure communications program. Its lacks many of the features of other communications programs, and its interface is best described as user-hateful. Unlike most other voice encryption programs, Nautilus uses a proprietary algorithym with a key negotiated by the Diffie-Hellman Key Exchange.


    The NSA has finally realized that high-bandwidth telecom applications (like ISDN and DSL) are all the rage, and decided to update their crypto systems accordingly with the development of STEs (Secure Terminal Equipment). STEs provide a platform for secure 'high-speed' (128k) data allowing for video-conferencing, fax and voice applications. Unlike STU IIIs STEs have their crypto engine on a PCMCIA card, so they can be distributed more freely. STEs are manufactured exclusively by L-3 Communication Systems.

    Speak Freely

    Speak Freely is a versitile, simple voice encryption system. Speak Freely offers a selection of voice encryption techniques (IDEA or DES). Speak Freely also permits conferencing, and contains several other useful functions. Unlike most voice encryption platforms, Speak Freely includes options that it to connect to other encrypting and non-encrypting internet telephones.


    A discussion of various cryptographic algorithms can be viewed at

    Problems with scrambling and voice encryption:

    Voice encryption units and speech scramblers both suffer several drawbacks. The largest proplem with any communications security device is that a unit is required at each end of the conversation for the system to work; so unless you're prepared to sacrafice some of your privacy by making unencrypted calls you'll swiftly end up broke. Any method of securing against eavesdropping is expensive (scramblers start at around $100 and encrypting telephones can range into the thousands)


    The companies below retail secure fax and telephone equipment.












    Voice Telephone