69 lines
1.6 KiB
Lua
69 lines
1.6 KiB
Lua
local expect = require "cc.expect".expect
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local fp = require "ccryptolib.internal.fp"
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local x25519 = require "ccryptolib.internal.x25519"
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local unpack = unpack or table.unpack
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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[256] = 0
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out[255] = 1
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-- We remove the 3 lowest bits since the ladder already multiplies by 8.
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return {unpack(out, 4)}
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end
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local function ladder8(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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-- Standard ladder.
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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 = x25519.step(dx, x1, z1, x2, z2)
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else
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x2, z2, x1, z1 = x25519.step(dx, x2, z2, x1, z1)
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end
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end
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-- Multiply by 8 (double 3 times).
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for _ = 1, 3 do
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x1, z1 = x25519.double(x1, z1)
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end
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return fp.mul(x1, fp.invert(z1))
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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 fp.encode(ladder8(x25519.G, 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 fp.encode(ladder8(fp.decode(pk), bits(sk)))
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end
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return mod
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