Presence

Lib and tool to generate short discrete presence tokens.

The token can be verified for validity in a certain time range, but otherwise appears random and is plausibly deniable. This should be good for privacy!

Anatomy

The token shall be conveyed in a upper-hex string or in binary 8 bytes array.

The first 4 bytes is a sha256 hash of the salt upper-hex + the unix timestamp decimal string.

The other 4 bytes is the random salt.

CLI

The companion CLI tool will verify or generate presence tokens. It give a non-zero exit code if no presence is matched or generated (like grep)

It can also produce presence signed with ed25519 if you specify SECRET (string to be hashed into a seed), SECRET_SEED (32 bytes as hex), or SECRET_SEED_FILE (file containing 32+ bytes in binary, only first 32 are read).

Install:

$ cargo install presence

Usage:

generate a new token or a few

$ presence new
67CE238185EA6D0B
$ presence new new new new
54C2BACC2E2DAF45
FBB0E058058CFF1E
71FE18E54F52DC76
22E8DAEBBDD7A220

check a token

$ presence 1234567890123456
1234567890123456 None
$ presence nonsense
nonsense Failed: string must be 16 bytes long
$ presence nonsenseXXXXXXXX
nonsenseXXXXXXXX Failed: string must be a hex

also works with stdin when no args are given

$ presence 1234567890123456 | presence
1234567890123456 None

and you can use stdin to also generate tokens with empty line

$ echo | presence
FFA86CA7292F9225

ouroboros, printing the matching timestamp

$ echo | presence | presence
B615BEDA94640BA7 OK 1710799816

we can create the same presence token in shell easily

presence_sh () {
    local time="${time:-$(date +%s)}"
    local salt="${salt:-$(head -c32 /dev/random | sha256sum | head -c8 | tr 'a-f' 'A-F')}"
    printf "$salt$time" | sha256sum | head -c8 | tr 'a-f' 'A-F'
    printf "$salt\n"
}
time=1710799816 salt=94640BA7 presence_sh

generate 3 x ed25519 signed presence revealing the public key

$ printf "12345678901234567890123456789012\n\n\n" |
    SECRET_SEED_FILE="/dev/stdin" REVEAL_PUBLIC_KEY=yp  presence
pubkey: 02F8C6129D816CF51C374BC7F8C3E63ED156CF78AEFB4A6550D97B87997977EE
76DF4CA121D3253EbazupoVjFItfM6gPG0bZce7adrWwFjrTiePI7KUwifVQwBc9Q8hWNOLcNpNQr6LlcMi0t5HMWxBajCFWhQ-LCQ
A1FB3851EFB04D71OUvhSXJ1WaVEKGTVJLO-0FiI9CJk1CjxC0JI2vYj_8Pyx0UHqN-gqN0z9g6J1vWSmT09pqvq7JZy7Jb3T_JqDg
772A7DBBFC75C305oZnUM5XX_TAIQ-0UAyFClaWGudOKuq5CUnvirTiHbVKBAtK7g8nfefel9Ep5dxd7gmoxDWv8r3cY-j5ejweuCA

stay present

$ while echo; do sleep 1; done | presence
C0C6792F43904B94
3418E6B820D54FE3
DBA0FD7F65FDD4D7
492F9AED9F2A904F
AC646257A1B78CCE
A09249459EA29CAA
4D0AAFB96D3970F0
...