88 lines
1.9 KiB
Lua
88 lines
1.9 KiB
Lua
local expect = require "cc.expect".expect
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local fp = require "ccryptolib.fp"
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local add = fp.add
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local sub = fp.sub
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local kmul = fp.kmul
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local mul = fp.mul
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local square = fp.square
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local invert = fp.invert
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local encode = fp.encode
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local decode = fp.decode
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local function step(dx, x1, z1, x2, z2)
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local a = add(x1, z1)
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local aa = square(a)
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local b = sub(x1, z1)
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local bb = square(b)
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local e = sub(aa, bb)
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local c = add(x2, z2)
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local d = sub(x2, z2)
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local da = mul(d, a)
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local cb = mul(c, b)
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local x4 = square(add(da, cb))
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local z4 = mul(dx, square(sub(da, cb)))
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local x3 = mul(aa, bb)
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local z3 = mul(e, add(bb, kmul(e, 121666)))
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return x3, z3, x4, z4
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end
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local function x25519(dx, bits)
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local x1 = {1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
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local z1 = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
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local x2, z2 = dx, x1
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for i = #bits, 1, -1 do
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if bits[i] == 0 then
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x1, z1, x2, z2 = step(dx, x1, z1, x2, z2)
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else
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x2, z2, x1, z1 = step(dx, x2, z2, x1, z1)
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end
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end
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return mul(x1, invert(z1))
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end
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local function bits(str)
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-- Decode.
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local bytes = {str:byte(1, 32)}
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local out = {}
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for i = 1, 32 do
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local byte = bytes[i]
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for j = -7, 0 do
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local bit = byte % 2
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out[8 * i + j] = bit
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byte = (byte - bit) / 2
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end
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end
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-- Clamp.
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out[1] = 0
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out[2] = 0
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out[3] = 0
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out[256] = 0
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out[255] = 1
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return out
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end
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local mod = {}
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function mod.publicKey(sk)
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expect(1, sk, "string")
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assert(#sk == 32, "secret key length must be 32")
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return encode(x25519({9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, bits(sk)))
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end
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function mod.exchange(sk, pk)
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expect(1, sk, "string")
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assert(#sk == 32, "secret key length must be 32")
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expect(2, pk, "string")
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assert(#pk == 32, "public key length must be 32")
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return encode(x25519(decode(pk), bits(sk)))
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end
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return mod
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