Simplify random initialization

The entropy is now provided by the user. They are tasked with finding a
high-quality source of entropy to initialize the generator with.

ccryptolib/random.lua

@@ -1,14 +1,15 @@
 local expect   = require "cc.expect".expect
 local blake3   = require "ccryptolib.blake3"
 local chacha20 = require "ccryptolib.chacha20"
-local packing  = require "ccryptolib.internal.packing"
+local util     = require "ccryptolib.internal.util"
 
-local u1x4, fmt1x4 = packing.compileUnpack("<I4")
+local lassert = util.lassert
 
--- Initialize from local context.
+-- Extract local context.
 local ctx = {
-    "ccryptolib 2022-03-05T08:50:36Z random.lua initialization context",
+    "ccryptolib 2023-04-11T19:43Z random.lua initialization context",
     os.epoch("utc"),
+    os.epoch("ingame"),
     math.random(0, 2 ^ 24 - 1),
     math.random(0, 2 ^ 24 - 1),
     tostring({}),
@@ -16,72 +17,36 @@ local ctx = {
 }
 
 local state = blake3.digest(table.concat(ctx, "|"))
-local accumulator = {}
-local accumulatorLen = 0
-
---- Adds data to the accumulator without context.
---
--- @tparam string data The input bytes.
---
-local function reseed(data)
-    local acc = accumulator
-    local len = accumulatorLen
-
-    -- Append to the accumulator.
-    acc[#acc + 1] = data
-    len = len + #data
-
-    if len < 64 then
-        accumulatorLen = len
-        return
-    end
-
-    -- Concatenate.
-    local cat = table.concat(acc)
-
-    -- Align by 64-byte block.
-    local rlen = len % 64
-    local blen = len - rlen
-
-    -- Digest.
-    state = blake3.digestKeyed(state, cat:sub(1, blen))
-    accumulator = {cat:sub(blen + 1)}
-    accumulatorLen = rlen
-end
-
-do -- Load entropy from disk.
-    local file = fs.open("/.random", "rb")
-    if file then
-        reseed(file.read(32) or "")
-        file.close()
-    end
-end
+local initialized = false
 
 local mod = {}
 
---- Adds entropy into the generator state.
+--- Mixes entropy into the generator, and marks it as initialized.
 --
--- @tparam string data The entropy data.
+-- @tparam string seed The seed data.
 --
-function mod.reseed(data)
-    expect(1, data, "string")
-    reseed(data)
+function mod.init(seed)
+    expect(1, seed, "string")
+    state = blake3.digestKeyed(state, seed)
+    initialized = true
 end
 
---- Adds entropy from sampling system noise.
+--- Mixes extra entropy into the generator state.
 --
--- @tparam number n The number of iterations to spend extracting entropy.
+-- @tparam string seed The additional entropy to mix.
 --
-function mod.stir(n)
-    expect(1, n, "number")
-    error("TODO") -- TODO
+function mod.mix(data)
+    state = blake3.digestKeyed(state, data)
 end
 
 --- Generates random bytes.
 --
 -- @tparam number len The desired output length.
+-- @throws If the generator hasn't been initialized.
 --
 function mod.random(len)
+    expect(1, len, "number")
+    lassert(initialized, "attempt to use an uninitialized random generator", 2)
     local msg = ("\0"):rep(len + 32)
     local nonce = ("\0"):rep(12)
     local out = chacha20.crypt(state, nonce, msg, 8, 0)
@@ -89,26 +54,4 @@ function mod.random(len)
     return out:sub(33)
 end
 
-local random = mod.random
-
---- Saves the state to the filesystem.
---
--- This potentially adds security when starting the generator from boot. The
--- saved path is fixed and located at `/.random`.
---
-function mod.save()
-    local file = fs.open("/.random", "wb")
-    file.write(random(32))
-    file.close()
-end
-
--- Add extra entropy.
--- mod.stir(error("TODO")) -- TODO
-
--- Save.
-mod.save()
-
--- Regenerate the math.random seed.
-math.randomseed(u1x4(fmt1x4, random(4), 1))
-
 return mod

spec/x25519_spec.lua

@@ -6,6 +6,8 @@
 local util = require "spec.util"
 local x25519 = require "ccryptolib.x25519"
 
+require "ccryptolib.random".init("mock initialization")
+
 describe("x25519.exchange", function()
     it("passes the section 5.2 test vector #1", function()
         local x = util.hexcat {

spec/x25519c_spec.lua

@@ -6,6 +6,8 @@
 local util = require "spec.util"
 local x25519c = require "ccryptolib.x25519c"
 
+require "ccryptolib.random".init("mock initialization")
+
 local function exchange(sk, pk)
     local sk = x25519c.maskX(sk)
     sk = x25519c.remask(sk)