mirror of
https://github.com/saymrwulf/swisspost-evoting-go-poc.git
synced 2026-09-04 20:23:55 +00:00
saymrwulf
ec4be74e17
Correctness/security review of the whole PoC, with fixes and regression tests.
Cryptographic soundness:
- mixnet: enforce the multi-exponentiation c_{B_m}=commit(0;0) check that was
stubbed out with an empty if — without it a malicious mixer can prove a
non-permutation shuffle.
- zkp: derive all four Fiat-Shamir challenges via RecursiveHashToZq instead of
a biased `hash mod q` (which also capped the challenge space at 256 bits for
production-sized groups).
Verification honesty:
- protocol: VerifyTally now actually calls zkp.VerifySchnorrProof and returns
the true aggregate result instead of an unconditional true.
- protocol: persist the padded mix input (event.MixInput) so the verifier checks
shuffle 0 against the same padding the tally used (fixes false INVALID for N<2).
Other correctness:
- kdf: length-prefix BuildKDFInfo parts so the info encoding is injective.
- math: GqElementFromSquareRoot accepts the valid root q (off-by-one that could
panic in HashAndSquare); RandomGqElement samples the full canonical range.
- cmd: validate demo --voters/--options instead of panicking on degenerate values.
- protocol: use crypto/rand in the demo driver (drop the last math/rand import).
Transport security (new): pkg/transportsec exposes Ed25519 signatures and X25519
ECDH — implemented in Rust (rust/transportsec: ed25519-dalek, x25519-dalek),
linked into Go via cgo. No RSA. Cross-language conformance test proves the Rust
Ed25519 signatures interoperate with Go's crypto/ed25519. Makefile builds the
Rust static lib before the Go binary.
Tests: added unit/round-trip/tamper coverage for math, hash, elgamal, zkp,
mixnet, kdf, returncodes, protocol (end-to-end), and the Rust FFI bridge.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
103 lines
2.7 KiB
Go
103 lines
2.7 KiB
Go
package mixnet
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import (
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"math/big"
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"github.com/user/evote/pkg/elgamal"
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emath "github.com/user/evote/pkg/math"
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)
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// ProductArgument proves that the product of all elements in a committed matrix equals b.
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type ProductArgument struct {
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CB *emath.GqElement // Commitment to Hadamard product (nil if m=1)
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Hadamard *HadamardArgument // nil if m=1
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SVP SingleValueProductArgument // Always present
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}
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// GenProductArgument generates a product argument.
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func GenProductArgument(
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cA *emath.GqVector, // Commitments to A columns (size m)
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b emath.ZqElement, // Product b = Π A[i,j]
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A *emath.ZqMatrix, // n×m matrix
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r *emath.ZqVector, // Randomness for A columns
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pk elgamal.PublicKey, // Public key (needed for sub-argument hashes)
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ck CommitmentKey,
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group *emath.GqGroup,
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) ProductArgument {
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n := A.NumRows()
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m := A.NumCols()
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if m == 1 {
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// Single column: just use SVP directly
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svp := GenSingleValueProductArgument(cA.Get(0), b, A.GetColumn(0), r.Get(0), pk, ck, group)
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return ProductArgument{SVP: svp}
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}
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// m > 1: Hadamard + SVP
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zqGroup := emath.ZqGroupFromGqGroup(group)
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// Compute b_vector = row-wise products (Hadamard product of all columns)
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bVector := make([]emath.ZqElement, n)
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for i := 0; i < n; i++ {
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prod := A.Get(i, 0)
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for j := 1; j < m; j++ {
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prod = prod.Multiply(A.Get(i, j))
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}
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bVector[i] = prod
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}
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bVec := emath.ZqVectorOf(bVector...)
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// Commit to Hadamard product
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s := emath.RandomZqElement(zqGroup)
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cb := ck.Commit(bVec, s)
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// Generate Hadamard argument (now with pk)
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hadamardArg := GenHadamardArgument(cA, cb, A, bVec, r, s, pk, ck, group)
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// Generate SVP argument (now with pk)
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svpArg := GenSingleValueProductArgument(cb, b, bVec, s, pk, ck, group)
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return ProductArgument{
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CB: &cb,
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Hadamard: &hadamardArg,
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SVP: svpArg,
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}
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}
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// VerifyProductArgument verifies a product argument.
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func VerifyProductArgument(
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arg ProductArgument,
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cA *emath.GqVector,
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b emath.ZqElement,
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pk elgamal.PublicKey,
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ck CommitmentKey,
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group *emath.GqGroup,
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) bool {
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m := cA.Size()
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if m == 1 {
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return VerifySingleValueProductArgument(arg.SVP, cA.Get(0), b, pk, ck, group)
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}
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// Verify Hadamard
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if arg.CB == nil || arg.Hadamard == nil {
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return false
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}
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if !VerifyHadamardArgument(*arg.Hadamard, cA, *arg.CB, pk, ck, group) {
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return false
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}
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// Verify SVP
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return VerifySingleValueProductArgument(arg.SVP, *arg.CB, b, pk, ck, group)
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}
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func computeProduct(matrix *emath.ZqMatrix) emath.ZqElement {
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one, _ := emath.NewZqElement(big.NewInt(1), matrix.Group())
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result := one
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for i := 0; i < matrix.NumRows(); i++ {
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for j := 0; j < matrix.NumCols(); j++ {
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result = result.Multiply(matrix.Get(i, j))
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}
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}
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return result
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}
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