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 government and military use, 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.
Older types of secure telephones generally use a technique called
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.