diff --git a/modules/activitypub/client.go b/modules/activitypub/client.go
new file mode 100644
index 0000000000..c3c1d9e950
--- /dev/null
+++ b/modules/activitypub/client.go
@@ -0,0 +1,117 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package activitypub
+
+import (
+	"bytes"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"net/http"
+
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/setting"
+	"github.com/go-fed/activity/pub"
+	"github.com/go-fed/httpsig"
+)
+
+const (
+	activityStreamsContentType = "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\""
+)
+
+func containsRequiredHttpHeaders(method string, headers []string) error {
+	var hasRequestTarget, hasDate, hasDigest bool
+	for _, header := range headers {
+		hasRequestTarget = hasRequestTarget || header == httpsig.RequestTarget
+		hasDate = hasDate || header == "Date"
+		hasDigest = method == "GET" || hasDigest || header == "Digest"
+	}
+	if !hasRequestTarget {
+		return fmt.Errorf("missing http header for %s: %s", method, httpsig.RequestTarget)
+	} else if !hasDate {
+		return fmt.Errorf("missing http header for %s: Date", method)
+	} else if !hasDigest {
+		return fmt.Errorf("missing http header for %s: Digest", method)
+	}
+	return nil
+}
+
+type Client struct {
+	clock       pub.Clock
+	client      *http.Client
+	algs        []httpsig.Algorithm
+	digestAlg   httpsig.DigestAlgorithm
+	getHeaders  []string
+	postHeaders []string
+	priv        *rsa.PrivateKey
+	pubId       string
+}
+
+func NewClient(user *user_model.User, pubId string) (c *Client, err error) {
+	if err = containsRequiredHttpHeaders(http.MethodGet, setting.Federation.GetHeaders); err != nil {
+		return
+	} else if err = containsRequiredHttpHeaders(http.MethodPost, setting.Federation.PostHeaders); err != nil {
+		return
+	} else if !httpsig.IsSupportedDigestAlgorithm(setting.Federation.DigestAlgorithm) {
+		err = fmt.Errorf("unsupported digest algorithm: %s", setting.Federation.DigestAlgorithm)
+		return
+	}
+	algos := make([]httpsig.Algorithm, len(setting.Federation.Algorithms))
+	for i, algo := range setting.Federation.Algorithms {
+		algos[i] = httpsig.Algorithm(algo)
+	}
+	clock, err := NewClock()
+	if err != nil {
+		return
+	}
+
+	priv, err := GetPrivateKey(user)
+	if err != nil {
+		return
+	}
+	privPem, _ := pem.Decode([]byte(priv))
+	privParsed, err := x509.ParsePKCS1PrivateKey(privPem.Bytes)
+	if err != nil {
+		return
+	}
+
+	c = &Client{
+		clock:       clock,
+		client:      &http.Client{},
+		algs:        algos,
+		digestAlg:   httpsig.DigestAlgorithm(setting.Federation.DigestAlgorithm),
+		getHeaders:  setting.Federation.GetHeaders,
+		postHeaders: setting.Federation.PostHeaders,
+		priv:        privParsed,
+		pubId:       pubId,
+	}
+	return
+}
+
+func (c *Client) Post(b []byte, to string) (resp *http.Response, err error) {
+	byteCopy := make([]byte, len(b))
+	copy(byteCopy, b)
+	buf := bytes.NewBuffer(byteCopy)
+	var req *http.Request
+	req, err = http.NewRequest(http.MethodPost, to, buf)
+	if err != nil {
+		return
+	}
+	req.Header.Add("Content-Type", activityStreamsContentType)
+	req.Header.Add("Accept-Charset", "utf-8")
+	req.Header.Add("Date", fmt.Sprintf("%s GMT", c.clock.Now().UTC().Format("Mon, 02 Jan 2006 15:04:05")))
+
+	signer, _, err := httpsig.NewSigner(c.algs, c.digestAlg, c.postHeaders, httpsig.Signature, 60)
+	if err != nil {
+		return
+	}
+	err = signer.SignRequest(c.priv, c.pubId, req, b)
+	if err != nil {
+		return
+	}
+	resp, err = c.client.Do(req)
+	return
+}
diff --git a/modules/activitypub/client_test.go b/modules/activitypub/client_test.go
new file mode 100644
index 0000000000..e29117ea13
--- /dev/null
+++ b/modules/activitypub/client_test.go
@@ -0,0 +1,49 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package activitypub
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"regexp"
+	"testing"
+
+	_ "code.gitea.io/gitea/models" // https://discourse.gitea.io/t/testfixtures-could-not-clean-table-access-no-such-table-access/4137/4
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestActivityPubSignedPost(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1}).(*user_model.User)
+	pubId := "https://example.com/pubId"
+	c, err := NewClient(user, pubId)
+	assert.NoError(t, err)
+
+	expected := "BODY"
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Regexp(t, regexp.MustCompile("^"+setting.Federation.DigestAlgorithm), r.Header.Get("Digest"))
+		assert.Contains(t, r.Header.Get("Signature"), pubId)
+		assert.Equal(t, r.Header.Get("Content-Type"), activityStreamsContentType)
+		body, err := ioutil.ReadAll(r.Body)
+		assert.NoError(t, err)
+		assert.Equal(t, expected, string(body))
+		fmt.Fprintf(w, expected)
+	}))
+	defer srv.Close()
+
+	r, err := c.Post([]byte(expected), srv.URL)
+	assert.NoError(t, err)
+	defer r.Body.Close()
+	body, err := io.ReadAll(r.Body)
+	assert.NoError(t, err)
+	assert.Equal(t, expected, string(body))
+}
diff --git a/modules/activitypub/main_test.go b/modules/activitypub/main_test.go
new file mode 100644
index 0000000000..c2be6f661e
--- /dev/null
+++ b/modules/activitypub/main_test.go
@@ -0,0 +1,16 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package activitypub
+
+import (
+	"path/filepath"
+	"testing"
+
+	"code.gitea.io/gitea/models/unittest"
+)
+
+func TestMain(m *testing.M) {
+	unittest.MainTest(m, filepath.Join("..", ".."))
+}
diff --git a/modules/setting/federation.go b/modules/setting/federation.go
index c300060789..12330c6cf3 100644
--- a/modules/setting/federation.go
+++ b/modules/setting/federation.go
@@ -9,9 +9,17 @@ import "code.gitea.io/gitea/modules/log"
 // Federation settings
 var (
 	Federation = struct {
-		Enabled bool
+		Enabled         bool
+		Algorithms      []string
+		DigestAlgorithm string
+		GetHeaders      []string
+		PostHeaders     []string
 	}{
-		Enabled: true,
+		Enabled:         true,
+		Algorithms:      []string{"rsa-sha256", "rsa-sha512"},
+		DigestAlgorithm: "SHA-256",
+		GetHeaders:      []string{"(request-target)", "Date"},
+		PostHeaders:     []string{"(request-target)", "Date", "Digest"},
 	}
 )