Simple libsodium crypto_secretbox_easy bindings for Lua. Meant for use with discord.

crypto_secretbox.lua

local ffi = require("ffi")

-- load libsodium so we can invoke it
local sodium = ffi.load("sodium")

-- define the headers so we can wrap them
ffi.cdef([[
int crypto_secretbox_easy(unsigned char *c, const unsigned char *m,
                          unsigned long long mlen, const unsigned char *n,
                          const unsigned char *k);

int crypto_secretbox_open_easy(unsigned char *m, const unsigned char *c,
                               unsigned long long clen, const unsigned char *n,
                               const unsigned char *k);
]])

-- encrypts a message with the given nonce and key.
-- throws an error when encryption is unsuccessful
local function crypto_secretbox_easy(message, nonce, key)
    assert(#key == 32, "key does not meet size requirement")
    assert(#nonce == 24, "nonce does not meet size requirement")
    
    local mlen = #message
    local clen = mlen + 16 -- crypto_secretbox_MACBYTES + mlen
    
    local c = ffi.new("unsigned char[?]", clen)
    
    local s = sodium.crypto_secretbox_easy(c, message, mlen, nonce, key)
    
    assert(s == 0, "message encrypted unsuccessfully")
    
    return ffi.string(c, clen)
end

-- decrypts a message with the given nonce and key.
-- throws an error when decryption is unsuccessful
local function crypto_secretbox_open_easy(ciphertext, nonce, key)
    assert(#key == 32, "key does not meet size requirement")
    assert(#nonce == 24, "nonce does not meet size requirement")
    assert(#ciphertext > 16, "ciphertext does not meet size requirement")
    
    local mlen = #ciphertext - 16
    local clen = #ciphertext
    
    local m = ffi.new("unsigned char[?]", mlen)
    
    local s = sodium.crypto_secretbox_open_easy(m, ciphertext, clen, nonce, key)
    
    assert(s == 0, "message decrypted unsuccessfully")
    
    return ffi.string(m, mlen)
end

return {
    encrypt = crypto_secretbox_easy,
    decrypt = crypto_secretbox_open_easy
}