Create pub/priv keypair for federation (#17071)

* add logic for creating pub/priv keypair for federation

* Apply suggestions from code review

Co-authored-by: delvh <dev.lh@web.de>

* make fmt

* Update modules/activitypub/keypair.go

Co-authored-by: delvh <dev.lh@web.de>

* add tests

* fix revert

* more tests

* Apply suggestions from code review

Co-authored-by: delvh <dev.lh@web.de>

* make fmt

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: 6543 <6543@obermui.de>

modules/activitypub/keypair.go

@@ -0,0 +1,48 @@
+// 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 (
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+)
+
+const rsaBits = 2048
+
+// GenerateKeyPair generates a public and private keypair for signing actions by users for activitypub purposes
+func GenerateKeyPair() (string, string, error) {
+	priv, _ := rsa.GenerateKey(rand.Reader, rsaBits)
+	privPem, err := pemBlockForPriv(priv)
+	if err != nil {
+		return "", "", err
+	}
+	pubPem, err := pemBlockForPub(&priv.PublicKey)
+	if err != nil {
+		return "", "", err
+	}
+	return privPem, pubPem, nil
+}
+
+func pemBlockForPriv(priv *rsa.PrivateKey) (string, error) {
+	privBytes := pem.EncodeToMemory(&pem.Block{
+		Type:  "RSA PRIVATE KEY",
+		Bytes: x509.MarshalPKCS1PrivateKey(priv),
+	})
+	return string(privBytes), nil
+}
+
+func pemBlockForPub(pub *rsa.PublicKey) (string, error) {
+	pubASN1, err := x509.MarshalPKIXPublicKey(pub)
+	if err != nil {
+		return "", err
+	}
+	pubBytes := pem.EncodeToMemory(&pem.Block{
+		Type:  "PUBLIC KEY",
+		Bytes: pubASN1,
+	})
+	return string(pubBytes), nil
+}