“the PM is on the green scrambler”

Reasons for use

Problems

Secrecy: you don’t want people to know you’re using a code, so then they won’t (A) realize you’re up to something, and (B) start breaking the code. Another worry is being caught with encoding/decoding materials (like a one-time pad) which would be incriminating

Security: you don’t want something easy to break, and need something easy to change so that a break won’t put you out of commission for a long time.

Easy to use

No equipment
Fast encoding/decoding
Easy to learn

Hard to mix up

Flexibility

A ‘closed’ set of codes is easier to manage for one-time-pad
An open code is more flexible to encode anything, but could be harder to use, or be easier to break.

Authentication

Need to make sure that you can’t be spoofed if a member is caught and compromised, or if knowledge is intercepted

Trail

Can be useful to prove you weren’t up to anything really bad, or can be bad news if the judge doesn’t like what you were doing.

Implementations

SMS (can be recovered from the phone company)

Voice (can be overheard)

MMS (hard to intercept, but requires taking a picture of something)

Data (requires an application)

Needs:

Duress code – to use when you’re under surveillance and can’t communicate freely, and a second to say ‘disregard future communication, captured’ (has to sound innocuous)

Framework

Message in a message

Hide a message in another message, either by coded words or phrases, or with tones!
See http://www.hulu.com/watch/90211/lost-through-the-looking-glass-part-2
The code to turn off the jamming equipment is “good vibrations’ played on a keypad.

The code: DTMF signals in a voice call. Voice calls are seldom recorded, and extraneous button presses occur sometimes, and therefore would be unlikely to arouse suspicion.  And the medium can be used for both encoded and clear transmission, and it’s synchronous.

Sample DTMF codes:

1: yes

2: no/not

3: acknowledged

4: go to predetermined location

The DTMF tones would have to be set securely beforehand, therefore making this system potentially vulnerable if the system were discovered.  It has a built-in anti-spoofing mechanism in the voice of the users being distinct.  It is able to be cracked through analysis, although proper use would make this more difficult, as would changing keys.

This would be ideal for communication in a small group, for a closed set of signals.

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