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Crypto phones
Secure telephones

Secure telephones are telephones that provide some level of voice security, in order to protect the calling parties from eavesdropping. Crypto phones are a special type of secure telephones that use advanced digital techniques for the encryption of voice data. Depending on the type and level of encryption used, crypto phones may be restricted or classified items.

Computer algorithms for secure voice transmission are generally controlled by the National Security Agency (NSA) in the US. They are divided into several security levels, of which Type 1 represents the highest level of security. Such devices can safely be used for top secret messages.
 
Secure telephones on this website:
British WWII scrambler phone Secraphone Secure Telehone Unit STU-I (KY-70) STU-I Secure Telehone Unit (ITT, Northern Telecom) STU-II Motorola STU-II/B SECTEL (NATO-version) STU-II/B Secure Telehone Unit (Motorola, AT&T, RCA, etc.) STU-III Secure Terminal Equipment STE Motorola SECTEL range of secure telephones SECTEL AT&T/Lucent 1100 STU-III secure phone (later sold by General Dynamics) 1100
CVAS III Secure Telephone CVAS III KY-68 Digital Secure Telephone KY-68 Philips Spendex-40 secure telephone for voice, fax and computer SP-40 Philips Spendex-50, military secure crypto phone (a.k.a. DBT) SP-50 Philips PNVX secure crypto telephone PNVX Racal MA-44741 Secure Phone Adapter MA-4471 AT&T TSD-3600 Telephone Encryptor TSD-3600 AT&T/Lucent 4100 crypto phone (later sold by General Dynamics) 4100
The Siemens DSM Voice telephone encryptor DSM Voice Siemens Crypset 100 crypto phone CS 100 Hagelin (Crypto AG) HC-3300 crypto phone HC-3300 Hagelin HC-4220 Fax Encryptor HC-4220 Hagelin HC-2203 PSTN Phone Encryptor HC-2203 Mieco 25A telephone scrambler Mieco 25A Telsy TDS-2003 portable voice encryptor TDS2003 Telsy TDS-2004 telephone voice encryptor TDS2004
Telsy Cryptophone 7000 CP-7000 Terma ET-10, Digital Tactical Terminal with secure voice and data capability ET-10 Siemens CTE-020 Remote Digital Engineering Order-Wire Terminal CTE-020 Tele Security Timmann, TST-7700 Voice and data encryption system TST 7700 Gretacoder 101, speech scrambler GC-101 Elcrovox 1-4D narrow band voice and data terminal (STU-II compatible) Elcrovox 1-4 Rohde & Schwarz Elcrodat 6-2 Elcrodat 6-2 Not truely a crypto phone, but usable on crypto-enabled telephone exchanges KWF

 
Voice scramblers
Older types of secure telephones generally use a technique called frequency inversion. They are sometimes called scramblers and are inherently insecure. An example of such a scrambler phone is the Mieco 25A (shown above). Even voice scramblers that use time-division speech scrambling should be considered insecure by today's standards, even when the two techniques are combined.
 
Further information

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