diff --git a/.github/workflows/codeql.yml b/.github/workflows/codeql.yml
new file mode 100644
index 0000000..6b56df0
--- /dev/null
+++ b/.github/workflows/codeql.yml
@@ -0,0 +1,39 @@
+name: "CodeQL"
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+  schedule:
+    - cron: '0 19 * * 0'
+
+jobs:
+  analyze:
+    name: Analyze
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
+
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [ 'go' ]
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      # Initializes the CodeQL tools for scanning.
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v2
+        with:
+          languages: ${{ matrix.language }}
+
+      - name: Autobuild
+        uses: github/codeql-action/autobuild@v2
+
+      - name: Perform CodeQL Analysis
+        uses: github/codeql-action/analyze@v2
diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
new file mode 100644
index 0000000..e77120c
--- /dev/null
+++ b/.github/workflows/lint.yml
@@ -0,0 +1,42 @@
+name: Lint
+
+on:
+  push:
+
+jobs:
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go: [1.16, 1.17, 1.18]
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 2
+
+      - name: Setup go
+        id: go
+        uses: actions/setup-go@v2
+        with:
+          go-version: ${{ matrix.go }}
+
+      - name: Cache go
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.cache/go-build
+            ~/go/pkg/mod
+          key: ${{ runner.os }}-go-v${{ matrix.go }}-${{ hashFiles('**/go.sum') }}
+          restore-keys: |
+            ${{ runner.os }}-go-
+
+      - name: Golangci lint
+        uses: golangci/golangci-lint-action@v3
+        with:
+          version: v1.45.2
+          skip-pkg-cache: true
+          skip-build-cache: true
+          only-new-issues: true
+          args: -v ./...
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
new file mode 100644
index 0000000..af4f778
--- /dev/null
+++ b/.github/workflows/tests.yml
@@ -0,0 +1,51 @@
+name: Tests
+
+on:
+  push:
+  schedule:
+    - cron: '0 19 * * 0'
+
+jobs:
+  test:
+    name: Tests
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go: [1.16, 1.17, 1.18]
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 2
+
+      - name: Setup go
+        id: go
+        uses: actions/setup-go@v2
+        with:
+          go-version: ${{ matrix.go }}
+
+      - name: Cache go
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.cache/go-build
+            ~/go/pkg/mod
+          key: ${{ runner.os }}-go-v${{ matrix.go }}-${{ hashFiles('**/go.sum') }}
+          restore-keys: |
+            ${{ runner.os }}-go-
+
+      - name: Run tests
+        id: test
+        run: |
+          make test
+          echo "::set-output name=coverage::$(make coverage)"
+
+      - name: Create Coverage Badge
+        uses: schneegans/dynamic-badges-action@v1.2.0
+        with:
+          auth: ${{ secrets.GIST_SECRET }}
+          gistID: cce912d48b587ba656b45a0cba34510b
+          filename: pow-hc-test-coverage.json
+          label: Coverage
+          message: ${{ steps.test.outputs.coverage }}
+          color: green
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..7b8c71a
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,4 @@
+.idea
+.DS_Store
+
+coverage.txt
\ No newline at end of file
diff --git a/.golangci.yml b/.golangci.yml
new file mode 100644
index 0000000..c031dc4
--- /dev/null
+++ b/.golangci.yml
@@ -0,0 +1,46 @@
+# This file contains all available configuration options
+# with their default values.
+
+# options for analysis running
+run:
+  # timeout for analysis, e.g. 30s, 5m, default is 1m
+  timeout: 5m
+
+  # include test files or not, default is true
+  tests: false
+
+linters:
+  disable:
+    - scopelint
+  enable:
+    - errcheck
+    - goimports
+    - gofmt
+    - revive
+    - exportloopref
+    - prealloc
+    - lll
+    - whitespace
+    - unconvert
+    - goconst
+    - staticcheck
+    - govet
+    - gocritic
+  presets:
+    - bugs
+    - unused
+
+linters-settings:
+  lll:
+    line-length: 180
+  revive:
+    ignore-generated-header: true
+    rules:
+      - name: unexported-return
+        disabled: true
+
+issues:
+  exclude-rules:
+    - linters:
+        - lll
+      source: "^//go:generate "
\ No newline at end of file
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..dd3d9c0
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2022 Kirill Rodin
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
\ No newline at end of file
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..118a87e
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,22 @@
+.PHONY: install
+install:
+	@go mod tidy
+	@go install \
+	  github.com/abice/go-enum \
+	  github.com/golang/mock/mockgen
+
+.PHONY: gen
+gen: install
+	@go generate ./...
+
+.PHONY: test
+test: gen
+	@go test $$(go list ./... | grep -v /mock) -coverprofile=coverage.txt -covermode=atomic -timeout 60s
+
+.PHONY: coverage
+coverage:
+	@make test | tr -d '\n' | sed -e 's/.*coverage:\(.*\)of statements.*/\1/' | tr -d '\n '
+
+.PHONY: lint
+lint:
+	@golangci-lint --timeout 10m0s run
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..b3c23d0
--- /dev/null
+++ b/README.md
@@ -0,0 +1,82 @@
+# Hashcash
+
+[![GoDoc](https://img.shields.io/badge/reference-007d9c.svg?logo=go&logoColor=white&label=doc)](https://pkg.go.dev/github.com/PoW-HC/hashcash)
+[![Go Report Card](https://goreportcard.com/badge/github.com/PoW-HC/hashcash)](https://goreportcard.com/report/github.com/PoW-HC/hashcash)
+[![Lint](https://github.com/PoW-HC/hashcash/actions/workflows/lint.yml/badge.svg)](https://github.com/PoW-HC/hashcash/actions/workflows/lint.yml)
+[![Tests](https://github.com/PoW-HC/hashcash/actions/workflows/tests.yml/badge.svg)](https://github.com/PoW-HC/hashcash/actions/workflows/tests.yml)
+![Coverage](https://img.shields.io/endpoint?url=https://gist.githubusercontent.com/halfi/cce912d48b587ba656b45a0cba34510b/raw/pow-hc-test-coverage.json)
+[![Code quality](https://github.com/PoW-HC/hashcash/actions/workflows/codeql.yml/badge.svg)](https://github.com/PoW-HC/hashcash/actions/workflows/codeql.yml)
+[![license](https://img.shields.io/github/license/PoW-HC/hashcash)](https://github.com/PoW-HC/hashcash/blob/main/LICENSE)
+
+---
+
+Hashcash is a Go library which implements the hashcash [proof-of-work](https://en.wikipedia.org/wiki/Proof_of_work)
+algorithm. Hashcash has been used as a denial-of-service counter measure technique in a number of systems. To learn more
+about hashcash visit [official page](http://hashcash.org/).
+
+## Usage
+
+Computing a valid hashcash:
+
+```go
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/PoW-HC/hashcash/pkg/hash"
+	"github.com/PoW-HC/hashcash/pkg/pow"
+)
+
+const maxIterations = 1 << 30
+
+func main() {
+	hasher, err := hash.NewHasher("sha256")
+	if err != nil {
+		// handle error
+	}
+
+	p := pow.New(hasher)
+
+	hashcash, err := pow.InitHashcash(5, "127.0.0.1", pow.SignExt("secret", hasher))
+	if err != nil {
+		// handle error
+	}
+
+	solution, err := p.Compute(context.Background(), hashcash, maxIterations)
+	if err != nil {
+		// handle error
+	}
+	fmt.Println(solution)
+}
+
+```
+
+Outputs:
+
+```
+1:5:1649257375:127.0.0.1:41965c8500f67f79b35672d7fb7e19fe5af0d51da582cbfcdbedb0f5944198bb:MgjmhCBiRV4=:MTk3YzA0
+```
+
+```shell
+echo -n "1:5:1649257375:127.0.0.1:41965c8500f67f79b35672d7fb7e19fe5af0d51da582cbfcdbedb0f5944198bb:MgjmhCBiRV4=:MTk3YzA0" | shasum -a 256
+0000067c716fa612cee5eb31eab6163e804c002841cfe96cc81f4f8034fd6006  -
+```
+
+Verification token:
+
+```go
+	err := p.Verify(solution, "127.0.0.1")
+	if err != nil {
+		// hashcash token failed verification.
+	}
+```
+
+## Documentation
+
+https://pkg.go.dev/github.com/PoW-HC/hashcash
+
+## License
+
+See the [LICENSE](LICENSE.md) file for license rights and limitations (MIT).
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..39c509f
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,21 @@
+module github.com/PoW-HC/hashcash
+
+go 1.16
+
+require (
+	github.com/abice/go-enum v0.4.0
+	github.com/golang/mock v1.6.0
+	github.com/minio/sha256-simd v1.0.0
+	github.com/stretchr/testify v1.7.1
+)
+
+require (
+	github.com/google/uuid v1.3.0 // indirect
+	github.com/klauspost/cpuid/v2 v2.0.12 // indirect
+	github.com/kr/pretty v0.3.0 // indirect
+	github.com/mattn/go-colorable v0.1.12 // indirect
+	github.com/mitchellh/copystructure v1.2.0 // indirect
+	golang.org/x/crypto v0.0.0-20220331220935-ae2d96664a29 // indirect
+	golang.org/x/mod v0.6.0-dev.0.20220330205332-605edab4323b // indirect
+	golang.org/x/sys v0.0.0-20220405210540-1e041c57c461 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..252ffde
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,131 @@
+github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
+github.com/Masterminds/goutils v1.1.1 h1:5nUrii3FMTL5diU80unEVvNevw1nH4+ZV4DSLVJLSYI=
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+github.com/Masterminds/semver v1.5.0 h1:H65muMkzWKEuNDnfl9d70GUjFniHKHRbFPGBuZ3QEww=
+github.com/Masterminds/semver v1.5.0/go.mod h1:MB6lktGJrhw8PrUyiEoblNEGEQ+RzHPF078ddwwvV3Y=
+github.com/Masterminds/sprig v2.22.0+incompatible h1:z4yfnGrZ7netVz+0EDJ0Wi+5VZCSYp4Z0m2dk6cEM60=
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diff --git a/internal/tools/tools.go b/internal/tools/tools.go
new file mode 100644
index 0000000..8cf0604
--- /dev/null
+++ b/internal/tools/tools.go
@@ -0,0 +1,9 @@
+//go:build tools
+// +build tools
+
+package tools
+
+import (
+	_ "github.com/abice/go-enum"
+	_ "github.com/golang/mock/mockgen"
+)
diff --git a/pkg/hash/factory.go b/pkg/hash/factory.go
new file mode 100644
index 0000000..b0ae9b2
--- /dev/null
+++ b/pkg/hash/factory.go
@@ -0,0 +1,26 @@
+package hash
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/PoW-HC/hashcash/pkg/hash/hasher"
+)
+
+func NewHasher(hasherName string) (Hasher, error) {
+	h, err := hasher.ParseHasher(strings.ToUpper(hasherName))
+	if err != nil {
+		return nil, fmt.Errorf("parse hasher by name error: %w", err)
+	}
+
+	switch h {
+	case hasher.HasherSHA1:
+		return NewSHA1(), nil
+	case hasher.HasherSHA256:
+		return NewSHA256(), nil
+	case hasher.HasherSHA512:
+		return NewSHA512(), nil
+	default:
+		return NewSHA256(), nil
+	}
+}
diff --git a/pkg/hash/hasher.go b/pkg/hash/hasher.go
new file mode 100644
index 0000000..8b9f63a
--- /dev/null
+++ b/pkg/hash/hasher.go
@@ -0,0 +1,8 @@
+package hash
+
+//go:generate mockgen -package=mock -destination=./mock/hasher.go github.com/PoW-HC/hashcash/pkg/hash Hasher
+
+// Hasher interface to hash function
+type Hasher interface {
+	Hash(str string) (string, error)
+}
diff --git a/pkg/hash/hasher/const.go b/pkg/hash/hasher/const.go
new file mode 100644
index 0000000..f6a1208
--- /dev/null
+++ b/pkg/hash/hasher/const.go
@@ -0,0 +1,12 @@
+package hasher
+
+//go:generate go-enum -f=$GOFILE --marshal
+
+// Hasher is an enumeration of hash types that are allowed.
+/* ENUM(
+	SHA1,
+	SHA256,
+	SHA512,
+)
+*/
+type Hasher int
diff --git a/pkg/hash/hasher/const_enum.go b/pkg/hash/hasher/const_enum.go
new file mode 100644
index 0000000..6fee3f5
--- /dev/null
+++ b/pkg/hash/hasher/const_enum.go
@@ -0,0 +1,66 @@
+// Code generated by go-enum DO NOT EDIT.
+// Version:
+// Revision:
+// Build Date:
+// Built By:
+
+package hasher
+
+import (
+	"fmt"
+)
+
+const (
+	// HasherSHA1 is a Hasher of type SHA1.
+	HasherSHA1 Hasher = iota
+	// HasherSHA256 is a Hasher of type SHA256.
+	HasherSHA256
+	// HasherSHA512 is a Hasher of type SHA512.
+	HasherSHA512
+)
+
+const _HasherName = "SHA1SHA256SHA512"
+
+var _HasherMap = map[Hasher]string{
+	HasherSHA1:   _HasherName[0:4],
+	HasherSHA256: _HasherName[4:10],
+	HasherSHA512: _HasherName[10:16],
+}
+
+// String implements the Stringer interface.
+func (x Hasher) String() string {
+	if str, ok := _HasherMap[x]; ok {
+		return str
+	}
+	return fmt.Sprintf("Hasher(%d)", x)
+}
+
+var _HasherValue = map[string]Hasher{
+	_HasherName[0:4]:   HasherSHA1,
+	_HasherName[4:10]:  HasherSHA256,
+	_HasherName[10:16]: HasherSHA512,
+}
+
+// ParseHasher attempts to convert a string to a Hasher.
+func ParseHasher(name string) (Hasher, error) {
+	if x, ok := _HasherValue[name]; ok {
+		return x, nil
+	}
+	return Hasher(0), fmt.Errorf("%s is not a valid Hasher", name)
+}
+
+// MarshalText implements the text marshaller method.
+func (x Hasher) MarshalText() ([]byte, error) {
+	return []byte(x.String()), nil
+}
+
+// UnmarshalText implements the text unmarshaller method.
+func (x *Hasher) UnmarshalText(text []byte) error {
+	name := string(text)
+	tmp, err := ParseHasher(name)
+	if err != nil {
+		return err
+	}
+	*x = tmp
+	return nil
+}
diff --git a/pkg/hash/mock/hasher.go b/pkg/hash/mock/hasher.go
new file mode 100644
index 0000000..0d5f8f9
--- /dev/null
+++ b/pkg/hash/mock/hasher.go
@@ -0,0 +1,49 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: github.com/PoW-HC/hashcash/pkg/hash (interfaces: Hasher)
+
+// Package mock is a generated GoMock package.
+package mock
+
+import (
+	reflect "reflect"
+
+	gomock "github.com/golang/mock/gomock"
+)
+
+// MockHasher is a mock of Hasher interface.
+type MockHasher struct {
+	ctrl     *gomock.Controller
+	recorder *MockHasherMockRecorder
+}
+
+// MockHasherMockRecorder is the mock recorder for MockHasher.
+type MockHasherMockRecorder struct {
+	mock *MockHasher
+}
+
+// NewMockHasher creates a new mock instance.
+func NewMockHasher(ctrl *gomock.Controller) *MockHasher {
+	mock := &MockHasher{ctrl: ctrl}
+	mock.recorder = &MockHasherMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use.
+func (m *MockHasher) EXPECT() *MockHasherMockRecorder {
+	return m.recorder
+}
+
+// Hash mocks base method.
+func (m *MockHasher) Hash(arg0 string) (string, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Hash", arg0)
+	ret0, _ := ret[0].(string)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// Hash indicates an expected call of Hash.
+func (mr *MockHasherMockRecorder) Hash(arg0 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Hash", reflect.TypeOf((*MockHasher)(nil).Hash), arg0)
+}
diff --git a/pkg/hash/mock/hasher_params.go b/pkg/hash/mock/hasher_params.go
new file mode 100644
index 0000000..fc64444
--- /dev/null
+++ b/pkg/hash/mock/hasher_params.go
@@ -0,0 +1,29 @@
+package mock
+
+import (
+	"github.com/golang/mock/gomock"
+)
+
+type HasherMockParams struct {
+	HashTimes    int
+	HashAnyTimes bool
+	HashReq      gomock.Matcher
+	HashRes      string
+	HashResErr   error
+}
+
+func (p HasherMockParams) NewHasher(ctrl *gomock.Controller) *MockHasher {
+	mock := NewMockHasher(ctrl)
+
+	callTimes(mock.EXPECT().Hash(p.HashReq), p.HashTimes, p.HashAnyTimes).Return(p.HashRes, p.HashResErr)
+
+	return mock
+}
+
+func callTimes(c *gomock.Call, times int, anyTimes bool) *gomock.Call {
+	if anyTimes {
+		return c.AnyTimes()
+	}
+
+	return c.Times(times)
+}
diff --git a/pkg/hash/sha1.go b/pkg/hash/sha1.go
new file mode 100644
index 0000000..80980c9
--- /dev/null
+++ b/pkg/hash/sha1.go
@@ -0,0 +1,27 @@
+package hash
+
+import (
+	"crypto/sha1" //nolint:gosec
+	"encoding/hex"
+	"fmt"
+)
+
+type SHA1 struct {
+}
+
+// NewSHA1 sha1 hash function
+func NewSHA1() *SHA1 {
+	return new(SHA1)
+}
+
+func (s *SHA1) Hash(str string) (string, error) {
+	// Weak cryptographic primitive. I know, I know..
+	sha := sha1.New() //nolint:gosec
+
+	_, err := sha.Write([]byte(str))
+	if err != nil {
+		return "", fmt.Errorf("sha256 hash error: %w", err)
+	}
+
+	return hex.EncodeToString(sha.Sum(nil)), err
+}
diff --git a/pkg/hash/sha256.go b/pkg/hash/sha256.go
new file mode 100644
index 0000000..f94fa50
--- /dev/null
+++ b/pkg/hash/sha256.go
@@ -0,0 +1,27 @@
+package hash
+
+import (
+	"encoding/hex"
+	"fmt"
+
+	"github.com/minio/sha256-simd"
+)
+
+type SHA256 struct {
+}
+
+// NewSHA256 sha256 hash function
+func NewSHA256() *SHA256 {
+	return new(SHA256)
+}
+
+func (s *SHA256) Hash(str string) (string, error) {
+	sha := sha256.New()
+
+	_, err := sha.Write([]byte(str))
+	if err != nil {
+		return "", fmt.Errorf("sha256 hash error: %w", err)
+	}
+
+	return hex.EncodeToString(sha.Sum(nil)), err
+}
diff --git a/pkg/hash/sha512.go b/pkg/hash/sha512.go
new file mode 100644
index 0000000..8d3b5d7
--- /dev/null
+++ b/pkg/hash/sha512.go
@@ -0,0 +1,26 @@
+package hash
+
+import (
+	"crypto/sha512"
+	"encoding/hex"
+	"fmt"
+)
+
+type SHA512 struct {
+}
+
+// NewSHA512 sha512 hash function
+func NewSHA512() *SHA512 {
+	return new(SHA512)
+}
+
+func (s *SHA512) Hash(str string) (string, error) {
+	sha := sha512.New()
+
+	_, err := sha.Write([]byte(str))
+	if err != nil {
+		return "", fmt.Errorf("sha512 hash error: %w", err)
+	}
+
+	return hex.EncodeToString(sha.Sum(nil)), err
+}
diff --git a/pkg/pow/hashcach.go b/pkg/pow/hashcach.go
new file mode 100644
index 0000000..ebcf2d5
--- /dev/null
+++ b/pkg/pow/hashcach.go
@@ -0,0 +1,108 @@
+package pow
+
+import (
+	"bytes"
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"math"
+	"math/big"
+	mrand "math/rand"
+	"strconv"
+	"time"
+)
+
+const (
+	versionV1 = 1
+)
+
+var (
+	ErrExtInvalid    = fmt.Errorf("extension sum invalid")
+	ErrHashcashEmpty = fmt.Errorf("hashcash empty")
+)
+
+// Hashcach struct to marshal and unmarshal hashcach to string or proto buf
+type Hashcach struct {
+	Version  int32
+	Bits     int32
+	Date     time.Time
+	Resource string
+	Ext      string
+	Rand     []byte
+	Counter  int64
+}
+
+func NewHashcach(
+	version int32,
+	bits int32,
+	date time.Time,
+	resource string,
+	ext string,
+	rand []byte,
+	counter int64,
+) *Hashcach {
+	return &Hashcach{
+		Version:  version,
+		Bits:     bits,
+		Date:     date,
+		Resource: resource,
+		Ext:      ext,
+		Rand:     rand,
+		Counter:  counter,
+	}
+}
+
+// InitHashcash initiate new hashcash
+func InitHashcash(bits int32, resource string, extGenerator ExtGeneratorFunc) (*Hashcach, error) {
+	t := time.Now()
+	randBytes := randomBytes()
+
+	hc := NewHashcach(
+		versionV1,
+		bits,
+		t,
+		resource,
+		"",
+		randBytes,
+		0,
+	)
+
+	if extGenerator != nil {
+		ext, err := extGenerator(hc)
+		if err != nil {
+			return nil, fmt.Errorf("ext generator error: %w", err)
+		}
+		hc.Ext = ext
+	}
+
+	return hc, nil
+}
+
+// String implements fmt.Stringer interface to get string hashcash
+func (h *Hashcach) String() string {
+	var buf bytes.Buffer
+	buf.WriteString(strconv.Itoa(int(h.Version)))
+	buf.WriteString(":")
+	buf.WriteString(strconv.Itoa(int(h.Bits)))
+	buf.WriteString(":")
+	buf.WriteString(strconv.Itoa(int(h.Date.Unix())))
+	buf.WriteString(":")
+	buf.WriteString(h.Resource)
+	buf.WriteString(":")
+	buf.WriteString(h.Ext)
+	buf.WriteString(":")
+	buf.WriteString(base64.StdEncoding.EncodeToString(h.Rand))
+	buf.WriteString(":")
+	buf.WriteString(base64.StdEncoding.EncodeToString([]byte(strconv.FormatInt(h.Counter, 16))))
+	return buf.String()
+}
+
+func randomBytes() []byte {
+	b, err := rand.Int(rand.Reader, big.NewInt(math.MaxInt64))
+	if err != nil {
+		// this is fallback in case of unweak rand function fall with error (exceptional case)
+		b = big.NewInt(mrand.Int63n(math.MaxInt64)) //nolint:gosec
+	}
+
+	return b.Bytes()
+}
diff --git a/pkg/pow/hashcash_test.go b/pkg/pow/hashcash_test.go
new file mode 100644
index 0000000..d0c62ff
--- /dev/null
+++ b/pkg/pow/hashcash_test.go
@@ -0,0 +1,22 @@
+package pow
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRandomBytes(t *testing.T) {
+	assert.Greater(t, len(randomBytes()), 0)
+}
+
+func TestInitHashcash(t *testing.T) {
+	var bits int32 = 3
+	var resource = "127.0.0.1"
+	a := assert.New(t)
+
+	actual, err := InitHashcash(bits, resource, nil)
+	a.Nil(err)
+	a.Equal(bits, actual.Bits)
+	a.Equal(resource, actual.Resource)
+}
diff --git a/pkg/pow/options.go b/pkg/pow/options.go
new file mode 100644
index 0000000..010edd0
--- /dev/null
+++ b/pkg/pow/options.go
@@ -0,0 +1,29 @@
+package pow
+
+import (
+	"time"
+)
+
+type options struct {
+	validateExtFunc      ExtValidatorFunc
+	challengeExpDuration time.Duration
+}
+
+type Options func(*options)
+
+func WithValidateExtFunc(callback ExtValidatorFunc) Options {
+	return func(pow *options) {
+		pow.validateExtFunc = callback
+	}
+}
+func WithChallengeExpDuration(callback time.Duration) Options {
+	return func(pow *options) {
+		pow.challengeExpDuration = callback
+	}
+}
+
+func setDefaultOptions(o *options) {
+	if o.challengeExpDuration == 0 {
+		o.challengeExpDuration = defaultChallengeDuration
+	}
+}
diff --git a/pkg/pow/pow.go b/pkg/pow/pow.go
new file mode 100644
index 0000000..2ec7adf
--- /dev/null
+++ b/pkg/pow/pow.go
@@ -0,0 +1,112 @@
+package pow
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/PoW-HC/hashcash/pkg/hash"
+)
+
+const (
+	zero rune = 48 // ASCII code for number zero
+
+	defaultChallengeDuration = 120 * time.Second
+	maxHashSize              = 1 << 7 // max length of sha512
+)
+
+var (
+	ErrMaxIterationsExceeded = fmt.Errorf("max iterations exceeded")
+	ErrWrongResource         = fmt.Errorf("wrong resource")
+	ErrChallengeExpired      = fmt.Errorf("challenge expired")
+	ErrWrongChallenge        = fmt.Errorf("wrong challenge")
+)
+
+// POW proof of work class
+type POW struct {
+	s     hash.Hasher
+	zeros []rune
+
+	options options
+}
+
+// New constructor
+func New(s hash.Hasher, opts ...Options) *POW {
+	p := &POW{
+		s:     s,
+		zeros: make([]rune, maxHashSize),
+	}
+
+	for i := range p.zeros {
+		p.zeros[i] = zero
+	}
+
+	for i := range opts {
+		opts[i](&p.options)
+	}
+
+	setDefaultOptions(&p.options)
+
+	return p
+}
+
+// Compute time waster. Do all useless load.
+func (p *POW) Compute(ctx context.Context, h *Hashcach, max int64) (*Hashcach, error) {
+	if max > 0 {
+		for h.Counter <= max {
+			if err := ctx.Err(); err != nil {
+				break
+			}
+
+			hashString, err := p.s.Hash(h.String())
+			if err != nil {
+				return nil, fmt.Errorf("calculate pow hash sum error: %w", err)
+			}
+
+			if isHashCorrect(hashString, p.zeros, int(h.Bits)) {
+				return h, nil
+			}
+
+			h.Counter++
+		}
+	}
+
+	return nil, ErrMaxIterationsExceeded
+}
+
+// Verify that hashcash correct and provided by server
+func (p *POW) Verify(h *Hashcach, resource string) error {
+	if h == nil || h.Resource != resource {
+		return ErrWrongResource
+	}
+
+	if h.Date.Add(p.options.challengeExpDuration).Before(time.Now()) {
+		return ErrChallengeExpired
+	}
+
+	hashString, err := p.s.Hash(h.String())
+	if err != nil {
+		return fmt.Errorf("calculate pow hash sum error: %w", err)
+	}
+
+	if !isHashCorrect(hashString, p.zeros, int(h.Bits)) {
+		return ErrWrongChallenge
+	}
+
+	if p.options.validateExtFunc != nil {
+		err = p.options.validateExtFunc(h)
+		if err != nil {
+			return fmt.Errorf("validation extension error: %w", err)
+		}
+	}
+
+	return nil
+}
+
+func isHashCorrect(hash string, zeroHash []rune, zerosCount int) bool {
+	if zerosCount > len(hash) || zerosCount > len(zeroHash) {
+		return false
+	}
+
+	return hash[:zerosCount] == string(zeroHash[:zerosCount])
+}
diff --git a/pkg/pow/pow_test.go b/pkg/pow/pow_test.go
new file mode 100644
index 0000000..4d132b2
--- /dev/null
+++ b/pkg/pow/pow_test.go
@@ -0,0 +1,368 @@
+package pow
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/assert"
+
+	"github.com/PoW-HC/hashcash/pkg/hash/mock"
+)
+
+var zeros = make([]rune, maxHashSize)
+
+func TestMain(m *testing.M) {
+	for i := range zeros {
+		zeros[i] = zero
+	}
+	os.Exit(m.Run())
+}
+
+func TestIsHashCorrect(t *testing.T) {
+	for _, tCase := range []struct {
+		name     string
+		hash     string
+		zeros    int
+		expected bool
+	}{
+		{
+			name:     "positive",
+			hash:     "00000e89df98a05e524fdcd29d8040d64d0259e2d5109ca1998e567a3c1c1c68",
+			zeros:    5,
+			expected: true,
+		},
+		{
+			name:     "wrong 5 zeros",
+			hash:     "00000e89df98a05e524fdcd29d8040d64d0259e2d5109ca1998e567a3c1c1c68",
+			zeros:    6,
+			expected: false,
+		},
+		{
+			name:     "wrong 0",
+			hash:     "d59d15c9a1842bc4563897803799e94f1f242d7e7e8c618f047e068211543998",
+			zeros:    5,
+			expected: false,
+		},
+		{
+			name:     "too short",
+			hash:     "0000",
+			zeros:    6,
+			expected: false,
+		},
+	} {
+		t.Run(tCase.name, func(t *testing.T) {
+			actual := isHashCorrect(tCase.hash, zeros, tCase.zeros)
+			assert.Equal(t, tCase.expected, actual)
+		})
+	}
+}
+
+func TestPowCompute(t *testing.T) {
+	hasherErr := fmt.Errorf("expected error")
+	ctrl := gomock.NewController(t)
+	ctx, cancel := context.WithTimeout(context.Background(), time.Minute)
+	defer cancel()
+	deadCTX, deadCancel := context.WithCancel(context.Background())
+	deadCancel()
+
+	for _, tCase := range []struct {
+		name        string
+		ctx         context.Context
+		hashcash    *Hashcach
+		max         int64
+		hasherMock  mock.HasherMockParams
+		expected    *Hashcach
+		expectedErr error
+	}{
+		{
+			name: "positive",
+			ctx:  ctx,
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret1648762844",
+			},
+			max: 1,
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Eq("0:5:1648762844:resource:resource\nsecret1648762844:Cg==:MA=="),
+				HashRes:    "00000e89df98a05e524fdcd29d8040d64d0259e2d5109ca1998e567a3c1c1c68",
+				HashResErr: nil,
+			},
+			expected: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret1648762844",
+				Counter:  0,
+			},
+			expectedErr: nil,
+		},
+		{
+			name: "hasher error",
+			ctx:  ctx,
+			hashcash: &Hashcach{
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret1648762844",
+			},
+			max: 1,
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Eq("0:0:1648762844:resource:resource\nsecret1648762844:Cg==:MA=="),
+				HashRes:    "",
+				HashResErr: hasherErr,
+			},
+			expected:    nil,
+			expectedErr: hasherErr,
+		},
+		{
+			name: "deadline exceeded",
+			ctx:  ctx,
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret1648762844",
+			},
+			max: 1,
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  2,
+				HashReq:    gomock.Any(),
+				HashRes:    "d59d15c9a1842bc4563897803799e94f1f242d7e7e8c618f047e068211543998",
+				HashResErr: nil,
+			},
+			expected:    nil,
+			expectedErr: ErrMaxIterationsExceeded,
+		},
+		{
+			name: "dead ctx",
+			ctx:  deadCTX,
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret1648762844",
+			},
+			max:         1,
+			expected:    nil,
+			expectedErr: ErrMaxIterationsExceeded,
+		},
+	} {
+		t.Run(tCase.name, func(t *testing.T) {
+			var (
+				a      = assert.New(t)
+				hasher = tCase.hasherMock.NewHasher(ctrl)
+				pow    = New(hasher)
+			)
+
+			actual, err := pow.Compute(tCase.ctx, tCase.hashcash, tCase.max)
+			a.Equal(tCase.expected, actual)
+			a.ErrorIs(err, tCase.expectedErr)
+		})
+	}
+}
+
+func TestPowVerify(t *testing.T) {
+	hasherErr := fmt.Errorf("expected error")
+	ctrl := gomock.NewController(t)
+	now := time.Now()
+
+	for _, tCase := range []struct {
+		name        string
+		powOptions  []Options
+		hashcash    *Hashcach
+		resource    string
+		hasherMock  mock.HasherMockParams
+		expectedErr error
+	}{
+		{
+			name: "positive",
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     now,
+				Ext:      "resource\nsecret1648762844",
+			},
+			resource: "resource",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Any(),
+				HashRes:    "00000e89df98a05e524fdcd29d8040d64d0259e2d5109ca1998e567a3c1c1c68",
+				HashResErr: nil,
+			},
+			expectedErr: nil,
+		},
+		{
+			name: "positive validate ext",
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     now,
+				Ext:      "resource\nsecret1648762844",
+			},
+			powOptions: []Options{
+				WithValidateExtFunc(func(h *Hashcach) error {
+					assert.Equal(
+						t,
+						&Hashcach{
+							Bits:     5,
+							Resource: "resource",
+							Rand:     []byte{10},
+							Date:     now,
+							Ext:      "resource\nsecret1648762844",
+						},
+						h,
+					)
+					return nil
+				}),
+			},
+			resource: "resource",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Any(),
+				HashRes:    "00000e89df98a05e524fdcd29d8040d64d0259e2d5109ca1998e567a3c1c1c68",
+				HashResErr: nil,
+			},
+			expectedErr: nil,
+		},
+		{
+			name: "positive duration",
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     now.Add(50 * time.Second),
+				Ext:      "resource\nsecret1648762844",
+			},
+			powOptions: []Options{
+				WithChallengeExpDuration(time.Minute),
+			},
+			resource: "resource",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Any(),
+				HashRes:    "00000e89df98a05e524fdcd29d8040d64d0259e2d5109ca1998e567a3c1c1c68",
+				HashResErr: nil,
+			},
+			expectedErr: nil,
+		},
+		{
+			name: "wrong resource",
+			hashcash: &Hashcach{
+				Resource: "resource",
+			},
+			resource:    "resource2",
+			expectedErr: ErrWrongResource,
+		},
+		{
+			name: "challenge expired",
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret1648762844",
+			},
+			resource:    "resource",
+			expectedErr: ErrChallengeExpired,
+		},
+		{
+			name: "hasher error",
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Now(),
+				Ext:      "resource\nsecret1648762844",
+			},
+			resource: "resource",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Any(),
+				HashRes:    "",
+				HashResErr: hasherErr,
+			},
+			expectedErr: hasherErr,
+		},
+		{
+			name: "wrong hash",
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Now(),
+				Ext:      "resource\nsecret1648762844",
+			},
+			resource: "resource",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Any(),
+				HashRes:    "d59d15c9a1842bc4563897803799e94f1f242d7e7e8c618f047e068211543998",
+				HashResErr: nil,
+			},
+			expectedErr: ErrWrongChallenge,
+		},
+		{
+			name: "validate ext error",
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Now(),
+				Ext:      "resource\nsecret1648762844",
+			},
+			powOptions: []Options{
+				WithValidateExtFunc(func(h *Hashcach) error {
+					return hasherErr
+				}),
+			},
+			resource: "resource",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Any(),
+				HashRes:    "00000e89df98a05e524fdcd29d8040d64d0259e2d5109ca1998e567a3c1c1c68",
+				HashResErr: nil,
+			},
+			expectedErr: hasherErr,
+		},
+		{
+			name: "error duration",
+			hashcash: &Hashcach{
+				Bits:     5,
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     now.Add(-2 * time.Minute),
+				Ext:      "resource\nsecret1648762844",
+			},
+			powOptions: []Options{
+				WithChallengeExpDuration(time.Minute),
+			},
+			resource:    "resource",
+			expectedErr: ErrChallengeExpired,
+		},
+	} {
+		t.Run(tCase.name, func(t *testing.T) {
+			var (
+				a      = assert.New(t)
+				hasher = tCase.hasherMock.NewHasher(ctrl)
+				pow    = New(hasher, tCase.powOptions...)
+			)
+
+			err := pow.Verify(tCase.hashcash, tCase.resource)
+			a.ErrorIs(err, tCase.expectedErr)
+		})
+	}
+}
diff --git a/pkg/pow/sign.go b/pkg/pow/sign.go
new file mode 100644
index 0000000..2e35c01
--- /dev/null
+++ b/pkg/pow/sign.go
@@ -0,0 +1,64 @@
+package pow
+
+import (
+	"bytes"
+	"fmt"
+	"strconv"
+	"time"
+
+	"github.com/PoW-HC/hashcash/pkg/hash"
+)
+
+type ExtGeneratorFunc func(*Hashcach) (string, error)
+type ExtValidatorFunc func(*Hashcach) error
+
+// SignExt creates signed extension
+// See extSum description for hash generating details
+func SignExt(secret string, hasher hash.Hasher) ExtGeneratorFunc {
+	return func(h *Hashcach) (string, error) {
+		ext, err := extSum(h.Resource, secret, h.Bits, h.Rand, h.Date, hasher)
+		if err != nil {
+			return "", err
+		}
+
+		return ext, nil
+	}
+}
+
+// VerifyExt verify extension from hashcash to validate hashcash was provided by server.
+// See extSum description for hash generating details
+func VerifyExt(secret string, hasher hash.Hasher) ExtValidatorFunc {
+	return func(h *Hashcach) error {
+		extSum, err := extSum(h.Resource, secret, h.Bits, h.Rand, h.Date, hasher)
+		if err != nil {
+			return fmt.Errorf("verify ext sum error: %w", err)
+		}
+
+		if h.Ext != extSum {
+			return ErrExtInvalid
+		}
+
+		return nil
+	}
+}
+
+// extSum generates hash sum with hasher interface from fields:
+//    - resource  - ip address
+//    - randBytes - random number
+//    - secret    - secret known only on server
+//    - time      - timestamp
+func extSum(resource, secret string, bits int32, randBytes []byte, t time.Time, hasher hash.Hasher) (string, error) {
+	var ext bytes.Buffer
+	ext.WriteString(resource)
+	ext.Write(randBytes)
+	ext.WriteString(secret)
+	ext.WriteString(strconv.Itoa(int(t.Unix())))
+	ext.WriteString(strconv.Itoa(int(bits)))
+
+	extSum, err := hasher.Hash(ext.String())
+	if err != nil {
+		return "", fmt.Errorf("calculate hashcash ext hash sum error: %w", err)
+	}
+
+	return extSum, nil
+}
diff --git a/pkg/pow/sign_test.go b/pkg/pow/sign_test.go
new file mode 100644
index 0000000..4c8888f
--- /dev/null
+++ b/pkg/pow/sign_test.go
@@ -0,0 +1,226 @@
+package pow
+
+import (
+	"fmt"
+	"testing"
+	"time"
+
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/assert"
+
+	"github.com/PoW-HC/hashcash/pkg/hash/mock"
+)
+
+func TestExtSum(t *testing.T) {
+	var (
+		resource       = "resource1"
+		secret         = "secret"
+		rand           = []byte{10}
+		bits     int32 = 5
+		date           = time.Unix(1648762844, 0)
+		expected       = fmt.Sprintf("%s%s%s%d%d", resource, rand, secret, date.Unix(), bits)
+		a              = assert.New(t)
+		hasher         = (mock.HasherMockParams{
+			HashTimes:  1,
+			HashReq:    gomock.Eq(expected),
+			HashRes:    expected,
+			HashResErr: nil,
+		}).NewHasher(gomock.NewController(t))
+	)
+
+	actual, err := extSum(resource, secret, bits, rand, date, hasher)
+	a.Nil(err)
+	a.Equal(expected, actual)
+}
+
+func TestExtSumErr(t *testing.T) {
+	var (
+		resource          = "resource"
+		secret            = "secret"
+		rand              = []byte{10}
+		bits        int32 = 5
+		date              = time.Unix(1648762844, 0)
+		expected          = fmt.Sprintf("%s%s%s%d%d", resource, rand, secret, date.Unix(), bits)
+		expectedErr       = fmt.Errorf("expected error")
+		a                 = assert.New(t)
+		hasher            = (mock.HasherMockParams{
+			HashTimes:  1,
+			HashReq:    gomock.Eq(expected),
+			HashRes:    "",
+			HashResErr: expectedErr,
+		}).NewHasher(gomock.NewController(t))
+	)
+
+	actual, err := extSum(resource, secret, bits, rand, date, hasher)
+	a.Empty(actual)
+	a.ErrorIs(err, expectedErr)
+}
+
+func TestVerifyExt(t *testing.T) {
+	hasherErr := fmt.Errorf("expected error")
+	ctrl := gomock.NewController(t)
+
+	for _, tCase := range []struct {
+		name        string
+		hashcash    *Hashcach
+		secret      string
+		hasherMock  mock.HasherMockParams
+		expectedErr error
+	}{
+		{
+			name: "positive",
+			hashcash: &Hashcach{
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret16487628440",
+			},
+			secret: "secret",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Eq("resource\nsecret16487628440"),
+				HashRes:    "resource\nsecret16487628440",
+				HashResErr: nil,
+			},
+			expectedErr: nil,
+		},
+		{
+			name: "wrong ext",
+			hashcash: &Hashcach{
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "wrong",
+			},
+			secret: "secret",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Eq("resource\nsecret16487628440"),
+				HashRes:    "resource\nsecret16487628440",
+				HashResErr: nil,
+			},
+			expectedErr: ErrExtInvalid,
+		},
+		{
+			name: "wrong hasher response",
+			hashcash: &Hashcach{
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret16487628440",
+			},
+			secret: "secret",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Eq("resource\nsecret16487628440"),
+				HashRes:    "wrong",
+				HashResErr: nil,
+			},
+			expectedErr: ErrExtInvalid,
+		},
+		{
+			name: "wrong hasher response",
+			hashcash: &Hashcach{
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret16487628440",
+			},
+			secret: "secret",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Eq("resource\nsecret16487628440"),
+				HashRes:    "wrong",
+				HashResErr: nil,
+			},
+			expectedErr: ErrExtInvalid,
+		},
+		{
+			name: "hasher error",
+			hashcash: &Hashcach{
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret16487628440",
+			},
+			secret: "secret",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Eq("resource\nsecret16487628440"),
+				HashRes:    "",
+				HashResErr: hasherErr,
+			},
+			expectedErr: hasherErr,
+		},
+	} {
+		t.Run(tCase.name, func(t *testing.T) {
+			var (
+				a      = assert.New(t)
+				hasher = tCase.hasherMock.NewHasher(ctrl)
+			)
+
+			err := VerifyExt(tCase.secret, hasher)(tCase.hashcash)
+			a.ErrorIs(err, tCase.expectedErr)
+		})
+	}
+}
+
+func TestSignExt(t *testing.T) {
+	hasherErr := fmt.Errorf("expected error")
+	_ = hasherErr
+	ctrl := gomock.NewController(t)
+
+	for _, tCase := range []struct {
+		name        string
+		hashcash    *Hashcach
+		secret      string
+		hasherMock  mock.HasherMockParams
+		expected    string
+		expectedErr error
+	}{
+		{
+			name: "positive",
+			hashcash: &Hashcach{
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+			},
+			secret: "secret",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Eq("resource\nsecret16487628440"),
+				HashRes:    "resource\nsecret16487628440",
+				HashResErr: nil,
+			},
+			expected: "resource\nsecret16487628440",
+		},
+		{
+			name: "hasher error",
+			hashcash: &Hashcach{
+				Resource: "resource",
+				Rand:     []byte{10},
+				Date:     time.Unix(1648762844, 0),
+				Ext:      "resource\nsecret16487628440",
+			},
+			secret: "secret",
+			hasherMock: mock.HasherMockParams{
+				HashTimes:  1,
+				HashReq:    gomock.Eq("resource\nsecret16487628440"),
+				HashRes:    "",
+				HashResErr: hasherErr,
+			},
+			expectedErr: hasherErr,
+		},
+	} {
+		t.Run(tCase.name, func(t *testing.T) {
+			var (
+				a      = assert.New(t)
+				hasher = tCase.hasherMock.NewHasher(ctrl)
+			)
+
+			ext, err := SignExt(tCase.secret, hasher)(tCase.hashcash)
+			a.ErrorIs(err, tCase.expectedErr)
+			a.Equal(tCase.expected, ext)
+		})
+	}
+}