Document Curve25519

internal/curve25519.lua

@@ -43,27 +43,14 @@ local function step(dxmul, dx, x1, z1, x2, z2)
     return x3, z3, x4, z4
 end
 
-local function bits(str)
-    -- Decode.
-    local bytes = {str:byte(1, 32)}
-    local out = {}
-    for i = 1, 32 do
-        local byte = bytes[i]
-        for j = -7, 0 do
-            local bit = byte % 2
-            out[8 * i + j] = bit
-            byte = (byte - bit) / 2
-        end
-    end
-
-    -- Clamp.
-    out[256] = 0
-    out[255] = 1
-
-    -- We remove the 3 lowest bits since the ladder already multiplies by 8.
-    return {unpack(out, 4)}
-end
-
+--- Performs a Montgomery ladder operation with multiplication by 8.
+--
+-- @tparam function(a:internal.fp.fp1, dx:any):internal.fp.fpq dxmul A function
+-- to multiply an element in Fp by dx.
+-- @tparam any dx The base point's x coordinate. Z is assumed to be equal to 1.
+-- @tparam {number...} bits The multiplier scalar divided by 8, in little-endian
+-- bits.
+--
 local function ladder8(dxmul, dx, bits)
     local x1 = fp.num(1)
     local z1 = fp.num(0)
@@ -89,7 +76,5 @@ local function ladder8(dxmul, dx, bits)
 end
 
 return {
-    double = double,
-    bits = bits,
     ladder8 = ladder8,
 }

x25519.lua

@@ -7,6 +7,28 @@ local expect = require "cc.expect".expect
 local fp     = require "ccryptolib.internal.fp"
 local mont   = require "ccryptolib.internal.curve25519"
 
+-- TODO This function feels out of place anywhere I try putting it on.
+local function bits(str)
+    -- Decode.
+    local bytes = {str:byte(1, 32)}
+    local out = {}
+    for i = 1, 32 do
+        local byte = bytes[i]
+        for j = -7, 0 do
+            local bit = byte % 2
+            out[8 * i + j] = bit
+            byte = (byte - bit) / 2
+        end
+    end
+
+    -- Clamp.
+    out[256] = 0
+    out[255] = 1
+
+    -- We remove the 3 lowest bits since the ladder already multiplies by 8.
+    return {unpack(out, 4)}
+end
+
 local mod = {}
 
 --- Computes the public key from a secret key.
@@ -17,7 +39,7 @@ local mod = {}
 function mod.publicKey(sk)
     expect(1, sk, "string")
     assert(#sk == 32, "secret key length must be 32")
-    return fp.encode(mont.ladder8(fp.kmul, 9, mont.bits(sk)))
+    return fp.encode(mont.ladder8(fp.kmul, 9, bits(sk)))
 end
 
 --- Performs the key exchange.
@@ -31,7 +53,7 @@ function mod.exchange(sk, pk)
     assert(#sk == 32, "secret key length must be 32")
     expect(2, pk, "string")
     assert(#pk == 32, "public key length must be 32")
-    return fp.encode(mont.ladder8(fp.mul, fp.decode(pk), mont.bits(sk)))
+    return fp.encode(mont.ladder8(fp.mul, fp.decode(pk), bits(sk)))
 end
 
 return mod