feature: mimc bls12 377 (#31)

add mimc hash implementation over bls12_377 ecc from gnark

.github/workflows/circomtest.yml

@@ -8,13 +8,13 @@ jobs:
         node-version: [16.x]
         go-version: [1.23.x]
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Use Node.js ${{ matrix.node-version }}
-      uses: actions/setup-node@v1
+      uses: actions/setup-node@v4
       with:
         node-version: ${{ matrix.node-version }}
     - name: Install Go
-      uses: actions/setup-go@v1
+      uses: actions/setup-go@v5
       with:
         go-version: ${{ matrix.go-version }}
     - name: run circom tests

.github/workflows/lint.yml

@@ -2,15 +2,24 @@ name: Lint
 on: [ push, pull_request ]
 jobs:
   lint:
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go-version: [1.23.x]
+        platform: [ubuntu-latest]
+    runs-on: ${{ matrix.platform }}
     steps:
-      - name: Install Go
-        uses: actions/setup-go@v1
-        with:
-          go-version: 1.23.x
       - name: Checkout code
-        uses: actions/checkout@v2
-      - name: Lint
-        run: |
-          curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(go env GOPATH)/bin v1.30.0
-          $(go env GOPATH)/bin/golangci-lint run --timeout=5m -c .golangci.yml
+        uses: actions/checkout@v4
+      - name: Install Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ matrix.go-version }}
+          cache: true
+      - name: Run golangci-lint
+        uses: golangci/golangci-lint-action@v3
+        with:
+          version: latest
+          skip-cache: false
+          skip-pkg-cache: false
+          skip-build-cache: false
+          only-new-issues: true

.github/workflows/test.yml

@@ -9,11 +9,12 @@ jobs:
         platform: [ubuntu-latest]
     runs-on: ${{ matrix.platform }}
     steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
     - name: Install Go
-      uses: actions/setup-go@v1
+      uses: actions/setup-go@v5
       with:
         go-version: ${{ matrix.go-version }}
-    - name: Checkout code
-      uses: actions/checkout@v2
+        cache: true
     - name: Run tests
       run: go test ./...

.golangci.yml

@@ -1,18 +0,0 @@
-issues:
-        max-same-issues: 0
-        exclude-use-default: false
-linters:
-        enable:
-        - whitespace
-        - gosec
-        - gci
-        - misspell
-        - gomnd
-        - gofmt
-        - goimports
-        - lll
-        - golint
-        - gocyclo
-linters-settings:
-        lll:
-                line-length: 100

dbg.go

@@ -28,7 +28,7 @@ func (d *dbgStats) incHash() {
 	d.hash++
 }
 
-//nolint:unused
+//nolint:all
 func (d *dbgStats) incDbGet() {
 	if d == nil {
 		return
@@ -36,7 +36,7 @@ func (d *dbgStats) incDbGet() {
 	d.dbGet++
 }
 
-//nolint:unused
+//nolint:all
 func (d *dbgStats) incDbPut() {
 	if d == nil {
 		return

ff.go

@@ -0,0 +1,22 @@
+package arbo
+
+import "math/big"
+
+var (
+	// BN254BaseField is the base field for the BN254 curve.
+	BN254BaseField, _ = new(big.Int).SetString("21888242871839275222246405745257275088548364400416034343698204186575808495617", 10)
+	// BLS12377BaseField is the base field for the BLS12377 curve.
+	BLS12377BaseField, _ = new(big.Int).SetString("25825498262808887005865186224201665565126143020923472090132963926938185026661", 10)
+)
+
+// BigToFF function returns the finite field representation of the big.Int
+// provided. It uses the curve scalar field to represent the provided number.
+func BigToFF(baseField, iv *big.Int) *big.Int {
+	z := big.NewInt(0)
+	if c := iv.Cmp(baseField); c == 0 {
+		return z
+	} else if c != 1 && iv.Cmp(z) != -1 {
+		return iv
+	}
+	return z.Mod(iv, baseField)
+}

ff_test.go

@@ -0,0 +1,28 @@
+package arbo
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestBigToFF(t *testing.T) {
+	baseField := BN254BaseField
+	iv := new(big.Int).Sub(baseField, big.NewInt(1))
+	// test with iv < baseField (the result should be iv)
+	z := BigToFF(baseField, iv)
+	if z.Cmp(iv) != 0 {
+		t.Fatalf("BigToFF failed: %v != %v", z, iv)
+	}
+	// test with iv > baseField (the result should be iv % baseField)
+	iv = new(big.Int).Add(baseField, big.NewInt(1))
+	z = BigToFF(baseField, iv)
+	if z.Cmp(big.NewInt(1)) != 0 {
+		t.Fatalf("BigToFF failed: %v != 0", z)
+	}
+	// test with iv == baseField (the result should be 0)
+	iv = baseField
+	z = BigToFF(baseField, iv)
+	if z.Cmp(big.NewInt(0)) != 0 {
+		t.Fatalf("BigToFF failed: %v != 0", z)
+	}
+}

go.mod

@@ -14,6 +14,7 @@ require (
 require (
 	github.com/DataDog/zstd v1.5.2 // indirect
 	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/bits-and-blooms/bitset v1.14.2 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/cockroachdb/errors v1.11.3 // indirect
 	github.com/cockroachdb/fifo v0.0.0-20240606204812-0bbfbd93a7ce // indirect
@@ -21,6 +22,8 @@ require (
 	github.com/cockroachdb/pebble v1.1.2 // indirect
 	github.com/cockroachdb/redact v1.1.5 // indirect
 	github.com/cockroachdb/tokenbucket v0.0.0-20230807174530-cc333fc44b06 // indirect
+	github.com/consensys/bavard v0.1.13 // indirect
+	github.com/consensys/gnark-crypto v0.14.0 // indirect
 	github.com/getsentry/sentry-go v0.27.0 // indirect
 	github.com/gogo/protobuf v1.3.2 // indirect
 	github.com/golang/snappy v0.0.5-0.20220116011046-fa5810519dcb // indirect
@@ -28,6 +31,7 @@ require (
 	github.com/klauspost/compress v1.17.9 // indirect
 	github.com/kr/pretty v0.3.1 // indirect
 	github.com/kr/text v0.2.0 // indirect
+	github.com/mmcloughlin/addchain v0.4.0 // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/prometheus/client_golang v1.19.1 // indirect
@@ -39,4 +43,5 @@ require (
 	golang.org/x/sys v0.26.0 // indirect
 	golang.org/x/text v0.19.0 // indirect
 	google.golang.org/protobuf v1.34.2 // indirect
+	rsc.io/tmplfunc v0.0.3 // indirect
 )

go.sum

@@ -2,6 +2,8 @@ github.com/DataDog/zstd v1.5.2 h1:vUG4lAyuPCXO0TLbXvPv7EB7cNK1QV/luu55UHLrrn8=
 github.com/DataDog/zstd v1.5.2/go.mod h1:g4AWEaM3yOg3HYfnJ3YIawPnVdXJh9QME85blwSAmyw=
 github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
 github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
+github.com/bits-and-blooms/bitset v1.14.2 h1:YXVoyPndbdvcEVcseEovVfp0qjJp7S+i5+xgp/Nfbdc=
+github.com/bits-and-blooms/bitset v1.14.2/go.mod h1:7hO7Gc7Pp1vODcmWvKMRA9BNmbv6a/7QIWpPxHddWR8=
 github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
 github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/cockroachdb/datadriven v1.0.3-0.20230413201302-be42291fc80f h1:otljaYPt5hWxV3MUfO5dFPFiOXg9CyG5/kCfayTqsJ4=
@@ -18,6 +20,10 @@ github.com/cockroachdb/redact v1.1.5 h1:u1PMllDkdFfPWaNGMyLD1+so+aq3uUItthCFqzwP
 github.com/cockroachdb/redact v1.1.5/go.mod h1:BVNblN9mBWFyMyqK1k3AAiSxhvhfK2oOZZ2lK+dpvRg=
 github.com/cockroachdb/tokenbucket v0.0.0-20230807174530-cc333fc44b06 h1:zuQyyAKVxetITBuuhv3BI9cMrmStnpT18zmgmTxunpo=
 github.com/cockroachdb/tokenbucket v0.0.0-20230807174530-cc333fc44b06/go.mod h1:7nc4anLGjupUW/PeY5qiNYsdNXj7zopG+eqsS7To5IQ=
+github.com/consensys/bavard v0.1.13 h1:oLhMLOFGTLdlda/kma4VOJazblc7IM5y5QPd2A/YjhQ=
+github.com/consensys/bavard v0.1.13/go.mod h1:9ItSMtA/dXMAiL7BG6bqW2m3NdSEObYWoH223nGHukI=
+github.com/consensys/gnark-crypto v0.14.0 h1:DDBdl4HaBtdQsq/wfMwJvZNE80sHidrK3Nfrefatm0E=
+github.com/consensys/gnark-crypto v0.14.0/go.mod h1:CU4UijNPsHawiVGNxe9co07FkzCeWHHrb1li/n1XoU0=
 github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc h1:U9qPSI2PIWSS1VwoXQT9A3Wy9MM3WgvqSxFWenqJduM=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -34,6 +40,7 @@ github.com/golang/snappy v0.0.5-0.20220116011046-fa5810519dcb/go.mod h1:/XxbfmMg
 github.com/google/go-cmp v0.5.9/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
 github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+github.com/google/subcommands v1.2.0/go.mod h1:ZjhPrFU+Olkh9WazFPsl27BQ4UPiG37m3yTrtFlrHVk=
 github.com/iden3/go-iden3-crypto v0.0.17 h1:NdkceRLJo/pI4UpcjVah4lN/a3yzxRUGXqxbWcYh9mY=
 github.com/iden3/go-iden3-crypto v0.0.17/go.mod h1:dLpM4vEPJ3nDHzhWFXDjzkn1qHoBeOT/3UEhXsEsP3E=
 github.com/kisielk/errcheck v1.5.0/go.mod h1:pFxgyoBC7bSaBwPgfKdkLd5X25qrDl4LWUI2bnpBCr8=
@@ -46,6 +53,9 @@ github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
 github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
 github.com/leanovate/gopter v0.2.9 h1:fQjYxZaynp97ozCzfOyOuAGOU4aU/z37zf/tOujFk7c=
 github.com/leanovate/gopter v0.2.9/go.mod h1:U2L/78B+KVFIx2VmW6onHJQzXtFb+p5y3y2Sh+Jxxv8=
+github.com/mmcloughlin/addchain v0.4.0 h1:SobOdjm2xLj1KkXN5/n0xTIWyZA2+s99UCY1iPfkHRY=
+github.com/mmcloughlin/addchain v0.4.0/go.mod h1:A86O+tHqZLMNO4w6ZZ4FlVQEadcoqkyU72HC5wJ4RlU=
+github.com/mmcloughlin/profile v0.1.1/go.mod h1:IhHD7q1ooxgwTgjxQYkACGA77oFTDdFVejUS1/tS/qU=
 github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 h1:C3w9PqII01/Oq1c1nUAm88MOHcQC9l5mIlSMApZMrHA=
 github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822/go.mod h1:+n7T8mK8HuQTcFwEeznm/DIxMOiR9yIdICNftLE1DvQ=
 github.com/pingcap/errors v0.11.4 h1:lFuQV/oaUMGcD2tqt+01ROSmJs75VG1ToEOkZIZ4nE4=
@@ -111,3 +121,5 @@ google.golang.org/protobuf v1.34.2 h1:6xV6lTsCfpGD21XK49h7MhtcApnLqkfYgPcdHftf6h
 google.golang.org/protobuf v1.34.2/go.mod h1:qYOHts0dSfpeUzUFpOMr/WGzszTmLH+DiWniOlNbLDw=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+rsc.io/tmplfunc v0.0.3 h1:53XFQh69AfOa8Tw0Jm7t+GV7KZhOi6jzsCzTtKbMvzU=
+rsc.io/tmplfunc v0.0.3/go.mod h1:AG3sTPzElb1Io3Yg4voV9AGZJuleGAwaVRxL9M49PhA=

hash.go

@@ -4,6 +4,7 @@ import (
 	"crypto/sha256"
 	"math/big"
 
+	"github.com/consensys/gnark-crypto/ecc/bls12-377/fr/mimc"
 	"github.com/iden3/go-iden3-crypto/poseidon"
 	"golang.org/x/crypto/blake2b"
 )
@@ -16,6 +17,9 @@ var (
 	TypeHashPoseidon = []byte("poseidon")
 	// TypeHashBlake2b represents the label for the HashFunction of Blake2b
 	TypeHashBlake2b = []byte("blake2b")
+	// TypeHashMiMC_BLS12_377 represents the label for the HashFunction of MiMC
+	// over BLS12-377 curve
+	TypeHashMiMC_BLS12_377 = []byte("mimc_bls12_377")
 
 	// HashFunctionSha256 contains the HashSha256 struct which implements
 	// the HashFunction interface
@@ -26,6 +30,9 @@ var (
 	// HashFunctionBlake2b contains the HashBlake2b struct which implements
 	// the HashFunction interface
 	HashFunctionBlake2b HashBlake2b
+	// HashFunctionMiMC_BLS12_377 contains the HashMiMC_BLS12_377 struct which
+	// implements the HashFunction interface
+	HashFunctionMiMC_BLS12_377 HashMiMC_BLS12_377
 )
 
 // Once Generics are at Go, this will be updated (August 2021
@@ -37,8 +44,6 @@ type HashFunction interface {
 	Type() []byte
 	Len() int
 	Hash(...[]byte) ([]byte, error)
-	// CheckInput checks if the input is valid without computing the hash
-	// CheckInput(...[]byte) error
 }
 
 // HashSha256 implements the HashFunction interface for the Sha256 hash
@@ -120,3 +125,28 @@ func (f HashBlake2b) Hash(b ...[]byte) ([]byte, error) {
 	}
 	return hasher.Sum(nil), nil
 }
+
+// HashMiMC_BLS12_377 implements the HashFunction interface for the MiMC hash
+// over the BLS12-377 curve
+type HashMiMC_BLS12_377 struct{}
+
+// Type returns the type of HashFunction for the HashMiMC_BLS12_377
+func (f HashMiMC_BLS12_377) Type() []byte {
+	return TypeHashMiMC_BLS12_377
+}
+
+// Len returns the length of the Hash output for the HashMiMC_BLS12_377
+func (f HashMiMC_BLS12_377) Len() int {
+	return mimc.BlockSize
+}
+
+// Hash implements the hash method for the HashFunction HashMiMC_BLS12_377
+func (f HashMiMC_BLS12_377) Hash(b ...[]byte) ([]byte, error) {
+	h := mimc.NewMiMC()
+	for i := 0; i < len(b); i++ {
+		if _, err := h.Write(SwapEndianness(b[i])); err != nil {
+			return nil, err
+		}
+	}
+	return SwapEndianness(h.Sum(nil)), nil
+}

hash_test.go

@@ -53,3 +53,17 @@ func TestHashBlake2b(t *testing.T) {
 		qt.Equals,
 		"928b20366943e2afd11ebc0eae2e53a93bf177a4fcf35bcc64d503704e65e202")
 }
+
+func TestHashMiMC(t *testing.T) {
+	// MiMC hash
+	HashFunction := &HashMiMC_BLS12_377{}
+	b := []byte("test")
+	h, err := HashFunction.Hash(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	c := qt.New(t)
+	c.Assert(hex.EncodeToString(h),
+		qt.Equals,
+		"f881f34991492d823e02565c778b824bac5eacef6340b70ee90a8966a2e63900")
+}

helpers_test.go

@@ -3,7 +3,6 @@ package arbo
 import (
 	"bytes"
 	"io"
-	"io/ioutil"
 	"os"
 	"testing"
 	"time"
@@ -41,13 +40,13 @@ func checkRoots(c *qt.C, tree1, tree2 *Tree) {
 func storeTree(c *qt.C, tree *Tree, path string) {
 	dump, err := tree.Dump(nil)
 	c.Assert(err, qt.IsNil)
-	err = ioutil.WriteFile(path+"-"+time.Now().String()+".debug", dump, 0600)
+	err = os.WriteFile(path+"-"+time.Now().String()+".debug", dump, 0600)
 	c.Assert(err, qt.IsNil)
 }
 
 // nolint:unused
 func readTree(c *qt.C, tree *Tree, path string) {
-	b, err := ioutil.ReadFile(path) //nolint:gosec
+	b, err := os.ReadFile(path) //nolint:gosec
 	c.Assert(err, qt.IsNil)
 	err = tree.ImportDump(b)
 	c.Assert(err, qt.IsNil)
@@ -102,7 +101,7 @@ func TestReadTreeDBG(t *testing.T) {
 
 	// tree1 is generated by a loop of .Add
 	path := "err-dump/tree1-2021-06-03 16:45:54.104449306 +0200 CEST m=+0.073874545.debug"
-	b, err := ioutil.ReadFile(path)
+	b, err := os.ReadFile(path)
 	c.Assert(err, qt.IsNil)
 	err = importDumpLoopAdd(tree1, b)
 	c.Assert(err, qt.IsNil)

testvectors/circom/go-data-generator/generator_test.go

@@ -2,8 +2,8 @@ package main
 
 import (
 	"encoding/json"
-	"io/ioutil"
 	"math/big"
+	"os"
 	"testing"
 
 	qt "github.com/frankban/quicktest"
@@ -42,7 +42,7 @@ func TestGenerator(t *testing.T) {
 	jCvp, err := json.Marshal(cvp)
 	c.Assert(err, qt.IsNil)
 	// store the data into a file that will be used at the circom test
-	err = ioutil.WriteFile("go-smt-verifier-inputs.json", jCvp, 0600)
+	err = os.WriteFile("go-smt-verifier-inputs.json", jCvp, 0600)
 	c.Assert(err, qt.IsNil)
 
 	// proof of non-existence
@@ -52,6 +52,6 @@ func TestGenerator(t *testing.T) {
 	jCvp, err = json.Marshal(cvp)
 	c.Assert(err, qt.IsNil)
 	// store the data into a file that will be used at the circom test
-	err = ioutil.WriteFile("go-smt-verifier-non-existence-inputs.json", jCvp, 0600)
+	err = os.WriteFile("go-smt-verifier-non-existence-inputs.json", jCvp, 0600)
 	c.Assert(err, qt.IsNil)
 }

tree.go

@@ -303,7 +303,7 @@ func (t *Tree) addBatchInDisk(wTx db.WriteTx, keys, values [][]byte) ([]Invalid,
 	}
 
 	if len(subRoots) != nCPU {
-		return nil, fmt.Errorf("This error should not be reached."+
+		return nil, fmt.Errorf("this error should not be reached."+
 			" len(subRoots) != nCPU, len(subRoots)=%d, nCPU=%d."+
 			" Please report it in a new issue:"+
 			" https://github.com/vocdoni/arbo/issues/new", len(subRoots), nCPU)
@@ -1463,9 +1463,11 @@ func (t *Tree) Graphviz(w io.Writer, fromRoot []byte) error {
 // generate a string Graphviz representation of the first NLevels of the tree
 // and writes it to w
 func (t *Tree) GraphvizFirstNLevels(w io.Writer, fromRoot []byte, untilLvl int) error {
-	fmt.Fprintf(w, `digraph hierarchy {
+	if _, err := fmt.Fprintf(w, `digraph hierarchy {
 node [fontname=Monospace,fontsize=10,shape=box]
-`)
+`); err != nil {
+		return err
+	}
 	if fromRoot == nil {
 		var err error
 		fromRoot, err = t.RootWithTx(t.db)
@@ -1482,14 +1484,20 @@ node [fontname=Monospace,fontsize=10,shape=box]
 		switch v[0] {
 		case PrefixValueEmpty:
 		case PrefixValueLeaf:
-			fmt.Fprintf(w, "\"%v\" [style=filled];\n", hex.EncodeToString(k[:nChars]))
+			if _, err := fmt.Fprintf(w, "\"%v\" [style=filled];\n", hex.EncodeToString(k[:nChars])); err != nil {
+				return false
+			}
 			// key & value from the leaf
 			kB, vB := ReadLeafValue(v)
-			fmt.Fprintf(w, "\"%v\" -> {\"k:%v\\nv:%v\"}\n",
+			if _, err := fmt.Fprintf(w, "\"%v\" -> {\"k:%v\\nv:%v\"}\n",
 				hex.EncodeToString(k[:nChars]), hex.EncodeToString(kB[:nChars]),
-				hex.EncodeToString(vB[:nChars]))
-			fmt.Fprintf(w, "\"k:%v\\nv:%v\" [style=dashed]\n",
-				hex.EncodeToString(kB[:nChars]), hex.EncodeToString(vB[:nChars]))
+				hex.EncodeToString(vB[:nChars])); err != nil {
+				return false
+			}
+			if _, err := fmt.Fprintf(w, "\"k:%v\\nv:%v\" [style=dashed]\n",
+				hex.EncodeToString(kB[:nChars]), hex.EncodeToString(vB[:nChars])); err != nil {
+				return false
+			}
 		case PrefixValueIntermediate:
 			l, r := ReadIntermediateChilds(v)
 			lStr := hex.EncodeToString(l[:nChars])
@@ -1507,14 +1515,20 @@ node [fontname=Monospace,fontsize=10,shape=box]
 					rStr)
 				nEmpties++
 			}
-			fmt.Fprintf(w, "\"%v\" -> {\"%v\" \"%v\"}\n", hex.EncodeToString(k[:nChars]),
-				lStr, rStr)
-			fmt.Fprint(w, eStr)
+			if _, err := fmt.Fprintf(w, "\"%v\" -> {\"%v\" \"%v\"}\n", hex.EncodeToString(k[:nChars]),
+				lStr, rStr); err != nil {
+				return false
+			}
+			if _, err := fmt.Fprint(w, eStr); err != nil {
+				return false
+			}
 		default:
 		}
 		return false
 	})
-	fmt.Fprintf(w, "}\n")
+	if _, err := fmt.Fprintf(w, "}\n"); err != nil {
+		return err
+	}
 	return err
 }
 
@@ -1540,15 +1554,19 @@ func (t *Tree) PrintGraphvizFirstNLevels(fromRoot []byte, untilLvl int) error {
 		}
 	}
 	w := bytes.NewBufferString("")
-	fmt.Fprintf(w,
-		"--------\nGraphviz of the Tree with Root "+hex.EncodeToString(fromRoot)+":\n")
+	if _, err := fmt.Fprintf(w,
+		"--------\nGraphviz of the Tree with Root %s:\n", hex.EncodeToString(fromRoot)); err != nil {
+		return err
+	}
 	err := t.GraphvizFirstNLevels(w, fromRoot, untilLvl)
 	if err != nil {
 		fmt.Println(w)
 		return err
 	}
-	fmt.Fprintf(w,
-		"End of Graphviz of the Tree with Root "+hex.EncodeToString(fromRoot)+"\n--------\n")
+	if _, err := fmt.Fprintf(w,
+		"End of Graphviz of the Tree with Root %s\n--------\n", hex.EncodeToString(fromRoot)); err != nil {
+		return err
+	}
 
 	fmt.Println(w)
 	return nil

vt.go

@@ -171,7 +171,7 @@ func (t *vt) addBatch(ks, vs [][]byte) ([]Invalid, error) {
 		}
 	}
 	if len(nodesAtL) != nCPU {
-		return nil, fmt.Errorf("This error should not be reached."+
+		return nil, fmt.Errorf("this error should not be reached."+
 			" len(nodesAtL) != nCPU, len(nodesAtL)=%d, nCPU=%d."+
 			" Please report it in a new issue:"+
 			" https://github.com/vocdoni/arbo/issues/new", len(nodesAtL), nCPU)
@@ -239,7 +239,7 @@ func (n *node) getNodesAtLevel(currLvl, l int) ([]*node, error) {
 		return []*node{n}, nil
 	}
 	if currLvl >= l {
-		return nil, fmt.Errorf("This error should not be reached."+
+		return nil, fmt.Errorf("this error should not be reached."+
 			" currLvl >= l, currLvl=%d, l=%d."+
 			" Please report it in a new issue:"+
 			" https://github.com/vocdoni/arbo/issues/new", currLvl, l)
@@ -627,13 +627,17 @@ func (n *node) computeHashes(currLvl, maxLvl int, p *params, pairs [][2][]byte)
 
 //nolint:unused
 func (t *vt) graphviz(w io.Writer) error {
-	fmt.Fprintf(w, `digraph hierarchy {
+	if _, err := fmt.Fprintf(w, `digraph hierarchy {
 node [fontname=Monospace,fontsize=10,shape=box]
-`)
+`); err != nil {
+		return err
+	}
 	if _, err := t.root.graphviz(w, t.params, 0); err != nil {
 		return err
 	}
-	fmt.Fprintf(w, "}\n")
+	if _, err := fmt.Fprintf(w, "}\n"); err != nil {
+		return err
+	}
 	return nil
 }
 
@@ -650,7 +654,9 @@ func (n *node) graphviz(w io.Writer, p *params, nEmpties int) (int, error) {
 		if err != nil {
 			return nEmpties, err
 		}
-		fmt.Fprintf(w, "\"%p\" [style=filled,label=\"%v\"];\n", n, hex.EncodeToString(leafKey[:nChars]))
+		if _, err := fmt.Fprintf(w, "\"%p\" [style=filled,label=\"%v\"];\n", n, hex.EncodeToString(leafKey[:nChars])); err != nil {
+			return nEmpties, err
+		}
 
 		k := n.k
 		v := n.v
@@ -661,14 +667,20 @@ func (n *node) graphviz(w io.Writer, p *params, nEmpties int) (int, error) {
 			v = n.v[:nChars]
 		}
 
-		fmt.Fprintf(w, "\"%p\" -> {\"k:%v\\nv:%v\"}\n", n,
+		if _, err := fmt.Fprintf(w, "\"%p\" -> {\"k:%v\\nv:%v\"}\n", n,
 			hex.EncodeToString(k),
-			hex.EncodeToString(v))
-		fmt.Fprintf(w, "\"k:%v\\nv:%v\" [style=dashed]\n",
+			hex.EncodeToString(v)); err != nil {
+			return nEmpties, err
+		}
+		if _, err := fmt.Fprintf(w, "\"k:%v\\nv:%v\" [style=dashed]\n",
 			hex.EncodeToString(k),
-			hex.EncodeToString(v))
+			hex.EncodeToString(v)); err != nil {
+			return nEmpties, err
+		}
 	case vtMid:
-		fmt.Fprintf(w, "\"%p\" [label=\"\"];\n", n)
+		if _, err := fmt.Fprintf(w, "\"%p\" [label=\"\"];\n", n); err != nil {
+			return nEmpties, err
+		}
 
 		lStr := fmt.Sprintf("%p", n.l)
 		rStr := fmt.Sprintf("%p", n.r)
@@ -685,8 +697,12 @@ func (n *node) graphviz(w io.Writer, p *params, nEmpties int) (int, error) {
 				rStr)
 			nEmpties++
 		}
-		fmt.Fprintf(w, "\"%p\" -> {\"%v\" \"%v\"}\n", n, lStr, rStr)
-		fmt.Fprint(w, eStr)
+		if _, err := fmt.Fprintf(w, "\"%p\" -> {\"%v\" \"%v\"}\n", n, lStr, rStr); err != nil {
+			return nEmpties, err
+		}
+		if _, err := fmt.Fprint(w, eStr); err != nil {
+			return nEmpties, err
+		}
 		nEmpties, err := n.l.graphviz(w, p, nEmpties)
 		if err != nil {
 			return nEmpties, err
@@ -707,15 +723,19 @@ func (n *node) graphviz(w io.Writer, p *params, nEmpties int) (int, error) {
 //nolint:unused
 func (t *vt) printGraphviz() error {
 	w := bytes.NewBufferString("")
-	fmt.Fprintf(w,
-		"--------\nGraphviz:\n")
+	if _, err := fmt.Fprintf(w,
+		"--------\nGraphviz:\n"); err != nil {
+		return err
+	}
 	err := t.graphviz(w)
 	if err != nil {
 		fmt.Println(w)
 		return err
 	}
-	fmt.Fprintf(w,
-		"End of Graphviz --------\n")
+	if _, err := fmt.Fprintf(w,
+		"End of Graphviz --------\n"); err != nil {
+		return err
+	}
 	fmt.Println(w)
 	return nil
 }