commit b6b3c35e24b99b2538475f27cff76d0556993160
parent 0b9fe954c950237b4d9fd3df076ca08282442572
Author: Jared Tobin <jared@jtobin.io>
Date: Sat, 10 Oct 2026 15:01:06 +0400
Merge impl/fixed-scalars: ppad-fixed scalar arithmetic
Replace bolt3's hand-rolled mod-n arithmetic in private key derivation
with ppad-fixed's constant-time Montgomery scalars, add ppad-fixed to
the flake (with ppad-secp256k1 following it), and bump the
ppad-secp256k1 lower bound to 0.5.8.
Diffstat:
4 files changed, 43 insertions(+), 133 deletions(-)
diff --git a/flake.lock b/flake.lock
@@ -273,28 +273,25 @@
"ppad-fixed": {
"inputs": {
"flake-utils": [
- "ppad-secp256k1",
"ppad-fixed",
"ppad-nixpkgs",
"flake-utils"
],
"nixpkgs": [
- "ppad-secp256k1",
"ppad-fixed",
"ppad-nixpkgs",
"nixpkgs"
],
"ppad-nixpkgs": [
- "ppad-secp256k1",
"ppad-nixpkgs"
]
},
"locked": {
- "lastModified": 1780868194,
- "narHash": "sha256-Yd3J3gCNLE2vZVMMg1fD7aVpPySIbFqxtwskU6nD/mc=",
+ "lastModified": 1785603497,
+ "narHash": "sha256-rF8KdVJ+VwUT2bEfCLZMPc05rUclr0wN2uOTYvd3z7g=",
"ref": "master",
- "rev": "74ebe44f818353129c5ec973a55c8bc487e2fd7b",
- "revCount": 297,
+ "rev": "e303bda7578385f488c2f23baa2e1a6a2b0b2aaf",
+ "revCount": 305,
"type": "git",
"url": "git://git.ppad.tech/fixed.git"
},
@@ -414,7 +411,9 @@
"nixpkgs"
],
"ppad-base16": "ppad-base16_3",
- "ppad-fixed": "ppad-fixed",
+ "ppad-fixed": [
+ "ppad-fixed"
+ ],
"ppad-hmac-drbg": "ppad-hmac-drbg",
"ppad-nixpkgs": [
"ppad-nixpkgs"
@@ -425,11 +424,11 @@
"ppad-sha512": "ppad-sha512"
},
"locked": {
- "lastModified": 1783709454,
- "narHash": "sha256-6BknM38fe2vFwWWRsBeebTt+hKwaZL0ib89XqPO6Y6U=",
+ "lastModified": 1785605654,
+ "narHash": "sha256-VDNnKXmJIZZ/3u7Tf1rK93GR4x7OiAadqykhqCwkHb4=",
"ref": "master",
- "rev": "bc0d3792bcea0475ed2e4879747dbd0e0e6db3e7",
- "revCount": 266,
+ "rev": "fdb80d54b86c61bd1a15e2e8db351091d02c89ee",
+ "revCount": 280,
"type": "git",
"url": "git://git.ppad.tech/secp256k1.git"
},
@@ -556,6 +555,7 @@
],
"ppad-bolt1": "ppad-bolt1",
"ppad-bolt9": "ppad-bolt9",
+ "ppad-fixed": "ppad-fixed",
"ppad-nixpkgs": "ppad-nixpkgs",
"ppad-ripemd160": "ppad-ripemd160",
"ppad-secp256k1": "ppad-secp256k1",
diff --git a/flake.nix b/flake.nix
@@ -26,12 +26,19 @@
ref = "master";
inputs.ppad-nixpkgs.follows = "ppad-nixpkgs";
};
+ ppad-fixed = {
+ type = "git";
+ url = "git://git.ppad.tech/fixed.git";
+ ref = "master";
+ inputs.ppad-nixpkgs.follows = "ppad-nixpkgs";
+ };
ppad-secp256k1 = {
type = "git";
url = "git://git.ppad.tech/secp256k1.git";
ref = "master";
inputs.ppad-nixpkgs.follows = "ppad-nixpkgs";
inputs.ppad-sha256.follows = "ppad-sha256";
+ inputs.ppad-fixed.follows = "ppad-fixed";
};
ppad-ripemd160 = {
type = "git";
@@ -51,8 +58,8 @@
};
outputs = { self, nixpkgs, flake-utils, ppad-nixpkgs
- , ppad-bolt1, ppad-bolt9, ppad-sha256, ppad-secp256k1
- , ppad-ripemd160, ppad-tx }:
+ , ppad-bolt1, ppad-bolt9, ppad-sha256, ppad-fixed
+ , ppad-secp256k1, ppad-ripemd160, ppad-tx }:
flake-utils.lib.eachDefaultSystem (system:
let
lib = "ppad-bolt3";
@@ -68,6 +75,12 @@
(hlib.enableCabalFlag sha256 "llvm")
[ llvm clang ];
+ fixed = ppad-fixed.packages.${system}.default;
+ fixed-llvm =
+ hlib.addBuildTools
+ (hlib.enableCabalFlag fixed "llvm")
+ [ llvm clang ];
+
secp256k1 = ppad-secp256k1.packages.${system}.default;
secp256k1-llvm =
hlib.addBuildTools
@@ -93,6 +106,7 @@
ppad-bolt1 = bolt1;
ppad-bolt9 = bolt9;
ppad-sha256 = sha256-llvm;
+ ppad-fixed = fixed-llvm;
ppad-secp256k1 = secp256k1-llvm;
ppad-ripemd160 = ripemd160-llvm;
ppad-tx = tx-llvm;
diff --git a/lib/Lightning/Protocol/BOLT3/Keys.hs b/lib/Lightning/Protocol/BOLT3/Keys.hs
@@ -43,17 +43,16 @@ module Lightning.Protocol.BOLT3.Keys (
import Control.DeepSeq (NFData(..))
import qualified Crypto.Curve.Secp256k1 as S
import qualified Crypto.Hash.SHA256 as SHA256
+import qualified Data.Choice as C
import Data.Bits ((.&.), (.|.), complement, complementBit, shiftL, shiftR,
testBit, xor)
import qualified Data.ByteString as BS
-import qualified Data.ByteString.Builder as BSB
-import qualified Data.ByteString.Lazy as BSL
-import qualified Data.ByteString.Unsafe as BU
import Data.Word (Word64)
import GHC.Generics (Generic)
import Lightning.Protocol.BOLT1 (Point, PerCommitmentSecret)
import qualified Lightning.Protocol.BOLT1 as BOLT1
import Lightning.Protocol.BOLT3.Types
+import qualified Numeric.Montgomery.Secp256k1.Scalar as SC
-- points ---------------------------------------------------------------------
@@ -277,8 +276,7 @@ derive_privkey (Seckey sk_bs) (PerCommitmentPoint pcp) = do
sk <- S.parse_int256 sk_bs
bp <- to_point =<< S.derive_pub sk
let !h = SHA256.hash (BOLT1.un_point pcp <> BOLT1.un_point bp)
- !r = add_mod_n (w4 sk_bs) (reduce_n (w4 h))
- to_seckey r
+ to_seckey (SC.to sk + SC.to (S.unsafe_roll32 h))
-- | Derive a commitment's @revocationprivkey@ from the revocation
-- basepoint secret and the commitment's (revealed) per-commitment
@@ -315,119 +313,16 @@ derive_revocationprivkey (Seckey rbs_bs) pcs = do
pcp <- to_point =<< S.derive_pub ps
let !rbp_bs = BOLT1.un_point rbp
!pcp_bs = BOLT1.un_point pcp
- !h1 = reduce_n (w4 (SHA256.hash (rbp_bs <> pcp_bs)))
- !h2 = reduce_n (w4 (SHA256.hash (pcp_bs <> rbp_bs)))
- !r = add_mod_n (mul_mod_n (w4 rbs_bs) h1) (mul_mod_n (w4 pcs_bs) h2)
- to_seckey r
-
--- constant-time arithmetic modulo the secp256k1 group order ------------------
-
--- A 256-bit integer as four 64-bit limbs, least significant first.
-data W4 = W4
- {-# UNPACK #-} !Word64
- {-# UNPACK #-} !Word64
- {-# UNPACK #-} !Word64
- {-# UNPACK #-} !Word64
-
--- The secp256k1 group order n.
-_N :: W4
-_N = W4 0xBFD25E8CD0364141 0xBAAEDCE6AF48A03B
- 0xFFFFFFFFFFFFFFFE 0xFFFFFFFFFFFFFFFF
-
--- Read 32 big-endian bytes (the length is checked by the caller).
-w4 :: BS.ByteString -> W4
-w4 bs = W4 (limb 24) (limb 16) (limb 8) (limb 0) where
- limb :: Int -> Word64
- limb o = go 0 0 where
- go !acc !i
- | i == 8 = acc
- | otherwise =
- go ((acc `shiftL` 8) .|. fromIntegral (BU.unsafeIndex bs (o + i)))
- (i + 1)
-
--- Write 32 big-endian bytes.
-unw4 :: W4 -> BS.ByteString
-unw4 (W4 a b c d) = BSL.toStrict . BSB.toLazyByteString $
- BSB.word64BE d <> BSB.word64BE c <> BSB.word64BE b <> BSB.word64BE a
-
--- A valid secret key from a result in [0, n), failing on zero.
-to_seckey :: W4 -> Maybe Seckey
-to_seckey r@(W4 a b c d)
- | (a .|. b .|. c .|. d) == 0 = Nothing
- | otherwise = Just (Seckey (unw4 r))
-
--- a + b + c, for a carry c in {0, 1}; returns the sum and carry out.
-adc :: Word64 -> Word64 -> Word64 -> (Word64, Word64)
-adc a b c =
- let !s = a + b + c
- !o = ((a .&. b) .|. ((a .|. b) .&. complement s)) `shiftR` 63
- in (s, o)
-{-# INLINE adc #-}
-
--- a - b - c, for a borrow c in {0, 1}; returns the difference and
--- borrow out.
-sbb :: Word64 -> Word64 -> Word64 -> (Word64, Word64)
-sbb a b c =
- let !s = a - b - c
- !o = ((complement a .&. b) .|. (complement (a `xor` b) .&. s))
- `shiftR` 63
- in (s, o)
-{-# INLINE sbb #-}
-
-add4 :: W4 -> W4 -> (W4, Word64)
-add4 (W4 a0 a1 a2 a3) (W4 b0 b1 b2 b3) =
- let !(s0, c0) = adc a0 b0 0
- !(s1, c1) = adc a1 b1 c0
- !(s2, c2) = adc a2 b2 c1
- !(s3, c3) = adc a3 b3 c2
- in (W4 s0 s1 s2 s3, c3)
-{-# INLINE add4 #-}
-
-sub4 :: W4 -> W4 -> (W4, Word64)
-sub4 (W4 a0 a1 a2 a3) (W4 b0 b1 b2 b3) =
- let !(s0, c0) = sbb a0 b0 0
- !(s1, c1) = sbb a1 b1 c0
- !(s2, c2) = sbb a2 b2 c1
- !(s3, c3) = sbb a3 b3 c2
- in (W4 s0 s1 s2 s3, c3)
-{-# INLINE sub4 #-}
-
--- x if the bit is 1, y if it is 0.
-select :: Word64 -> W4 -> W4 -> W4
-select bit (W4 x0 x1 x2 x3) (W4 y0 y1 y2 y3) =
- let !m = negate bit
- f x y = (x .&. m) .|. (y .&. complement m)
- in W4 (f x0 y0) (f x1 y1) (f x2 y2) (f x3 y3)
-{-# INLINE select #-}
-
--- x mod n, for any 256-bit x (one subtraction suffices, as 2^256 < 2n).
-reduce_n :: W4 -> W4
-reduce_n x =
- let !(t, borrow) = sub4 x _N
- in select borrow x t
-
--- a + b mod n, for a, b < n.
-add_mod_n :: W4 -> W4 -> W4
-add_mod_n a b =
- let !(s, carry) = add4 a b
- !(t, borrow) = sub4 s _N
- in select (carry .|. (borrow `xor` 1)) t s
-
--- a * b mod n, for a, b < n, by double-and-add over all 256 bits of b.
-mul_mod_n :: W4 -> W4 -> W4
-mul_mod_n a (W4 b0 b1 b2 b3) = go (255 :: Int) (W4 0 0 0 0) where
- go !i !acc
- | i < 0 = acc
- | otherwise =
- let !d = add_mod_n acc acc
- !bit = (limb (i `shiftR` 6) `shiftR` (i .&. 63)) .&. 1
- in go (i - 1) (select bit (add_mod_n d a) d)
- limb :: Int -> Word64
- limb j = case j of
- 0 -> b0
- 1 -> b1
- 2 -> b2
- _ -> b3
+ !h1 = SC.to (S.unsafe_roll32 (SHA256.hash (rbp_bs <> pcp_bs)))
+ !h2 = SC.to (S.unsafe_roll32 (SHA256.hash (pcp_bs <> rbp_bs)))
+ to_seckey (SC.to rbs * h1 + SC.to ps * h2)
+
+-- The secret key for a nonzero scalar. Reduction, arithmetic and the
+-- zero check are constant-time (ppad-fixed).
+to_seckey :: SC.Montgomery -> Maybe Seckey
+to_seckey r
+ | C.decide (SC.eq r 0) = Nothing
+ | otherwise = Just (Seckey (S.unroll32 (SC.retr r)))
-- per-commitment secrets -----------------------------------------------------
diff --git a/ppad-bolt3.cabal b/ppad-bolt3.cabal
@@ -35,8 +35,9 @@ library
, bytestring >= 0.9 && < 0.13
, deepseq >= 1.4 && < 1.6
, ppad-bolt1 >= 0.1 && < 0.2
+ , ppad-fixed >= 0.2 && < 0.3
, ppad-ripemd160 >= 0.1.4 && < 0.2
- , ppad-secp256k1 >= 0.5.7 && < 0.6
+ , ppad-secp256k1 >= 0.5.8 && < 0.6
, ppad-sha256 >= 0.3.2 && < 0.4
, ppad-tx >= 0.2 && < 0.3
