Voice Scrambling and Encryption
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.)
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.
Motorola markets phase inversion scrambling under the trademark "Secure
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
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
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.
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 speech, 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.
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
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
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
A discussion of various cryptographic algorithms can be viewed at http://www.counterpane.com/
Problems with scrambling and voice encryption:
Voice encryption units and speech scramblers both suffer several drawbacks.
The largest problem 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 sacrifice 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)
SourcesThe companies below retail secure
fax and telephone equipment.
For more information, please Email firstname.lastname@example.org