Sending/receiving encrypted Text Messages (SMS) without a smartphone

[Stefan Claas]

Since politicians have the possibility to use crypto
phones, I thought about how you can also use encrypted
SMS as a normal small citizen without a smartphone.

For example, you can use a small GPD MicroPC [1],
either with a SurfStick and modem-manager-gui (Linux),
without Internet or completely offline with an old
dumbphone.

SMS messages can then be generated like this, for example:

$ echo -n ‘Hi Bob, coming round around 6pm.' | mp -p 5 \
| xchacha20 key iv | az | ug -g
Data successfully padded.
Original size: 37 bytes
Padded size: 40 bytes
UOOTW GJMMO IXKKO XQANR ZBBZW EJPLO EUWLF LTJHY RLPKO AAYSN
AKOAA WWOAV NDHVX CBNMF CPXFN PMKMZ

This then allows messages to be typed into the old dumbphone,
since only capital letters are used, which are grouped into
groups of five for good readability.

If you use modem-manager-gui, SMS messages can also be longer
than 160 characters, as they are divided and sent as several
SMS messages.

XChaCha20 uses a 256 bit key and a 192 bit nonce and is in my
opinion well suited for SMS.

[1] https://www.gpd.hk/gpdmicropc

P.S. The old PGPfone, for Windows, for encrypted phone calls,
also works under Linux/wine.

[yeti]

<https://www.smssolutions.net/tutorials/gsm/sendsmsat/>

Someone may have a link to a better overview of AT commands or even a
complete GSM/UMTS/...-modem manual?

[Stefan Claas]

Post by yeti
<https://www.smssolutions.net/tutorials/gsm/sendsmsat/>
Someone may have a link to a better overview of AT commands or even a
complete GSM/UMTS/...-modem manual?

I first thought also about using AT commands, but must admit
modem-manager-gui, for Linux, is a really nice app, for SMS usage.