this post was submitted on 03 Oct 2025
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It is potentially doable:
A short message is 140 bytes of gsm7-bit packed characters (I.e. each character is translated to "ascii" format which only take up 7-bit space, which also is packed together forming unharmonic bytes), so we can probably get away with 160 characters per SMS.
According to crypto.stackexchange, a 2048-bit private key generates a base64 encoded public key of 392 characters.
That would mean 3 SMSs per person you send your public key to. For a 4096-bit private key, this accounts to 5 SMSs.
As key exchange only has to be sent once per contact it sounds totally doable.
After you sent your public key around, you should now be able to receive encrypted short messages from your contacts.
The output length of a ciphertext depends on the key size according to crypto.stackexchange and rfc8017. This means we have 256 bytes of ciphertext for each 2048-bit key encrypted plaintext message, and 512 bytes for 4096-bit keys. Translated into short messages, it would mean 2 or 4 SMSs for each text message respectively, a 1:2, or 1:4 ratio.
Hope you have a good SMS plan ๐