From 501e81a36a4ecc3a8fe7f58cecb80a212e0fc7a2 Mon Sep 17 00:00:00 2001
From: Miguel Oliveira <miguel.oliveira4224@gmail.com>
Date: Wed, 2 Mar 2022 13:51:51 -0300
Subject: [PATCH] Add masked X25519

---
 internal/fq.lua     |   2 +-
 internal/x25519.lua |   5 +-
 random.lua          |  66 ++++++++++++++++++++++++++
 x25519.lua          |   6 +--
 x25519c.lua         | 111 ++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 184 insertions(+), 6 deletions(-)
 create mode 100644 random.lua
 create mode 100644 x25519c.lua

diff --git a/internal/fq.lua b/internal/fq.lua
index bd65d3a..294044b 100644
--- a/internal/fq.lua
+++ b/internal/fq.lua
@@ -3,7 +3,7 @@
 -- @module ccryptolib.internal.fq
 --
 
-local util = require "ccryptolib.util"
+local util = require "ccryptolib.internal.util"
 
 local unpack = unpack or table.unpack
 
diff --git a/internal/x25519.lua b/internal/x25519.lua
index b4f7a20..23ba994 100644
--- a/internal/x25519.lua
+++ b/internal/x25519.lua
@@ -1,4 +1,6 @@
-local fp = require "ccryptolibinternal.fp"
+local fp = require "ccryptolib.internal.fp"
+
+local G = {9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
 
 local function step(dx, x1, z1, x2, z2)
     local a = fp.add(x1, z1)
@@ -57,6 +59,7 @@ local function bits(str)
 end
 
 return {
+    G = G,
     step = step,
     ladder = ladder,
     bits = bits,
diff --git a/random.lua b/random.lua
new file mode 100644
index 0000000..34e7c5a
--- /dev/null
+++ b/random.lua
@@ -0,0 +1,66 @@
+local blake3   = require "ccryptolib.blake3"
+local chacha20 = require "ccryptolib.chacha20"
+
+-- Initialize from local context.
+local ctx = {
+    os.epoch("utc"),
+    math.random(0, 2 ^ 24 - 1),
+    math.random(0, 2 ^ 24 - 1),
+    tostring({}),
+    tostring({}),
+}
+
+local state = blake3.digest(table.concat(ctx, "|"), 32)
+
+local function seed(data)
+    state = blake3.digestKeyed(state, data, 32)
+end
+
+local function stir(n)
+    -- Collect samples from jitter.
+    local epoch = os.epoch
+    local acc = {}
+    local byte = 0
+    for i = 1, n do
+        local t0 = epoch("utc")
+        repeat byte = byte + 1 until epoch("utc") ~= t0
+        acc[i] = byte % 256
+    end
+
+    -- Extract into the new state.
+    seed(string.char(table.unpack(acc)))
+end
+
+local function random(len)
+    local msg = ("\0"):rep(len + 32)
+    local nonce = ("\0"):rep(12)
+    local out = chacha20.crypt(state, nonce, msg, 8, 0)
+    state = out:sub(1, 32)
+    return out:sub(33)
+end
+
+local function save()
+    local file = fs.open("/.random", "wb")
+    file.write(random(32))
+    file.close()
+end
+
+-- Load.
+if fs.exists("./random") then
+    local file = fs.open("./random", "rb")
+    seed(file.read(32) or "")
+end
+
+-- Add extra entropy.
+stir(512)
+
+-- Save.
+math.randomseed(("I4"):unpack(random(4)))
+save()
+
+return {
+    seed = seed,
+    stir = stir,
+    random = random,
+    save = save,
+}
diff --git a/x25519.lua b/x25519.lua
index 89d959e..65ea4ab 100644
--- a/x25519.lua
+++ b/x25519.lua
@@ -2,15 +2,13 @@ local expect = require "cc.expect".expect
 local fp     = require "ccryptolib.internal.fp"
 local x25519 = require "ccryptolib.internal.x25519"
 
-local G = {9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
-
 local mod = {}
 
 function mod.publicKey(sk)
     expect(1, sk, "string")
     assert(#sk == 32, "secret key length must be 32")
 
-    return fp.encode(x25519.ladder(G, x25519.bits(sk)))
+    return fp.encode(x25519.ladder(x25519.G, x25519.bits(sk)))
 end
 
 function mod.exchange(sk, pk)
@@ -19,7 +17,7 @@ function mod.exchange(sk, pk)
     expect(2, pk, "string")
     assert(#pk == 32, "public key length must be 32")
 
-    return fp.encode(x25519(fp.decode(pk), x25519.bits(sk)))
+    return fp.encode(x25519.ladder(fp.decode(pk), x25519.bits(sk)))
 end
 
 return mod
diff --git a/x25519c.lua b/x25519c.lua
new file mode 100644
index 0000000..1854c70
--- /dev/null
+++ b/x25519c.lua
@@ -0,0 +1,111 @@
+local expect = require "cc.expect".expect
+local fp     = require "ccryptolib.internal.fp"
+local fq     = require "ccryptolib.internal.fq"
+local x25519 = require "ccryptolib.internal.x25519"
+local random = require "ccryptolib.random"
+
+local ORDER = 4
+
+--- The inverse of 8 modulo q (in montgomery form).
+local INV8Q = {
+    5110253,
+    3039345,
+    2503500,
+    11779568,
+    15416472,
+    16766550,
+    16777215,
+    16777215,
+    16777215,
+    16777215,
+    4095,
+}
+
+local function fqRandom()
+    return fq.decodeWide(random.random(64))
+end
+
+local function fqDecodeStd(str)
+    -- Decode.
+    local words = {("<I3I3I3I3I3I3I3I3I3I3I2"):unpack(str)} words[12] = nil
+
+    -- Clamp.
+    words[1] = bit32.band(words[1], 0xfffff8)
+    words[11] = bit32.band(words[11], 0x7fff)
+    words[11] = bit32.bor(words[11], 0x4000)
+
+    return fq.montgomery(words)
+end
+
+local mod = {}
+
+function mod.secretKeyInit(sk)
+    sk = fqDecodeStd(sk)
+
+    -- Set up the mask.
+    local sks = {}
+    local sum = fq.num(0)
+    for i = 1, ORDER - 1 do
+        sks[i] = fqRandom()
+        sum = fq.add(sum, sks[i])
+    end
+    sks[ORDER] = fq.add(sk, fq.neg(sum))
+
+    return sks
+end
+
+function mod.secretKeyEncode(sks)
+    local out = {}
+    for i = 1, ORDER do out[i] = fq.encode(sks[i]) end
+    return table.concat(out)
+end
+
+function mod.secretKeyDecode(str)
+    expect(1, str, "string")
+    assert(#str == ORDER * 32, ("secret key length must be %d"):format(ORDER * 32))
+
+    local out = {}
+    for i = 1, ORDER do out[i] = fq.decode(str:sub(i * 32 - 31, i * 32)) end
+    return out
+end
+
+function mod.secretKeyRemask(sk)
+    local sum = fq.num(0)
+    local out = {}
+
+    for i = 1, ORDER - 1 do
+        local element = fqRandom()
+        out[i] = fq.add(sk[i], element)
+        sum = fq.add(sum, element)
+    end
+
+    out[ORDER] = fq.add(sk[ORDER], fq.neg(sum))
+
+    return out
+end
+
+function mod.exchange(sk, pk, mc)
+    expect(2, pk, "string")
+    assert(#pk == 32, "public key length must be 32")
+    expect(3, mc, "string")
+    assert(#mc == 32, "multiplier length must be 32")
+
+    -- Get the multiplier in Fq.
+    mc = fqDecodeStd(mc)
+
+    -- Multiply secret key members and add them together.
+    -- This unwraps into the "true" secret key times the multiplier (mod q).
+    local skmt = fq.num(0)
+    for i = 1, #sk do skmt = fq.add(skmt, fq.mul(sk[i], mc)) end
+
+    -- Get bits.
+    -- We have our exponent modulo q. We also know that its value is 0 modulo 8.
+    -- Use the Chinese Remainder Theorem to find its value modulo 8q.
+    local bits = fq.bits(fq.mul(skmt, INV8Q))
+    local bits8 = {0, 0, 0}
+    for i = 1, 253 do bits8[i + 3] = bits[i] end
+
+    return fp.encode(x25519.ladder(fp.decode(pk), bits8))
+end
+
+return mod