60 lines
1.5 KiB
Lua
60 lines
1.5 KiB
Lua
--- The X25519 key exchange scheme.
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--
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-- @module x25519
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--
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local expect = require "cc.expect".expect
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local fp = require "ccryptolib.internal.fp"
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local mont = require "ccryptolib.internal.curve25519"
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-- TODO This function feels out of place anywhere I try putting it on.
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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 mod = {}
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--- Computes the public key from a secret key.
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--
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-- @tparam string sk A random 32-byte secret key.
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-- @treturn string The matching public key.
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--
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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(mont.ladder8(fp.kmul, 9, bits(sk)))
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end
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--- Performs the key exchange.
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--
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-- @tparam string sk A secret key.
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-- @tparam string pk A public key, usually derived from a second secret key.
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-- @treturn string The 32-byte shared secret between both keys.
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--
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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(mont.ladder8(fp.mul, fp.decode(pk), bits(sk)))
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end
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return mod
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