s2s

package main

import (
	"crypto/x509"
	"encoding/pem"
	"fmt"
	"net/http"
	"net/http/httptest"
	"strings"
	"time"

	"github.com/go-ap/activitypub"
)

var (
	prvPem = `-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----`

	block, _  = pem.Decode([]byte(prvPem))
	prv, _    = x509.ParsePKCS1PrivateKey(block.Bytes)
	millenium = time.Date(2001, time.January, 1, 0, 0, 0, 0, time.UTC)

	srv = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		fmt.Printf("%s", r.Header.Get("Signature"))
		w.WriteHeader(http.StatusOK)
	}))
)

func main() {
	actor := new(activitypub.Actor)
	actor.ID = "https://example.com/~johndoe"

	pub := prv.Public()
	pubEnc, _ := x509.MarshalPKIXPublicKey(pub)
	pubEncoded := pem.EncodeToMemory(&pem.Block{
		Type:  "PUBLIC KEY",
		Bytes: pubEnc,
	})

	actor.PublicKey = activitypub.PublicKey{
		ID:           activitypub.IRI(fmt.Sprintf("%s#main", actor.ID)),
		Owner:        actor.ID,
		PublicKeyPem: string(pubEncoded),
	}

	tr := New(WithTransport(http.DefaultTransport), WithActor(actor, prv))

	// The below functionality would be equivalent to this, more idiomatic, usage:
	//http.DefaultClient.Transport = tr
	//res, err := http.Get(srv.URL)

	req, _ := http.NewRequest(http.MethodGet, srv.URL, nil)
	host := strings.TrimPrefix(srv.URL, "http://")
	host = host[:strings.Index(host, ":")]
	req.Header.Set("Host", host)
	req.Header.Set("Date", millenium.Format(http.TimeFormat))
	_, _ = tr.RoundTrip(req)

}

Output:

keyId="https://example.com/~johndoe#main",algorithm="hs2019",headers="(request-target) host date",signature="lotUyRDWnYs/AxAy+oOMAcgXPaUXsCjn8yjhPsJ7o/Ek2Q66e61V57qTALgBU+zHftTj9u/dHZehp/1M/JKhkygiEw2Av16MzpNFmsC6lxMTP4Pvs9wxXxLPXQKWzlYVaHKlEzRMoFI3AJyRlf/eT2mNoJVh1f89ETV704Jl8eE="