1 package apub
2 
3 import (
4 	"bytes"
5 	"crypto"
6 	"crypto/rand"
7 	"crypto/rsa"
8 	"crypto/sha256"
9 	"encoding/base64"
10 	"fmt"
11 	"io"
12 	"net/http"
13 	"strings"
14 	"time"
15 )
16 
17 // Sign signs the given HTTP request with the matching private key of the
18 // public key available at pubkeyURL.
19 func Sign(req *http.Request, key *rsa.PrivateKey, pubkeyURL string) error {
20 	if pubkeyURL == "" {
21 		return fmt.Errorf("no pubkey url")
22 	}
23 	date := time.Now().UTC().Format(http.TimeFormat)
24 	req.Header.Set("Date", date)
25 	hash := sha256.New()
26 	toSign := []string{"(request-target)", "host", "date"}
27 	fmt.Fprintln(hash, "(request-target):", strings.ToLower(req.Method), req.URL.Path)
28 	fmt.Fprintln(hash, "host:", req.URL.Hostname())
29 	fmt.Fprintf(hash, "date: %s", date)
30 
31 	if req.Body != nil {
32 		// we're adding one more entry to our signature, so one more line.
33 		fmt.Fprint(hash, "\n")
34 		buf := &bytes.Buffer{}
35 		io.Copy(buf, req.Body)
36 		req.Body.Close()
37 		req.Body = io.NopCloser(buf)
38 		digest := sha256.Sum256(buf.Bytes())
39 		d := "SHA-256=" + base64.StdEncoding.EncodeToString(digest[:])
40 		toSign = append(toSign, "digest")
41 		fmt.Fprintf(hash, "digest: %s", d)
42 		req.Header.Set("Digest", d)
43 	}
44 	sig, err := rsa.SignPKCS1v15(rand.Reader, key, crypto.SHA256, hash.Sum(nil))
45 	if err != nil {
46 		return err
47 	}
48 	bsig := base64.StdEncoding.EncodeToString(sig)
49 
50 	val := fmt.Sprintf("keyId=%q,algorithm=%q,headers=%q,signature=%q", pubkeyURL, "rsa-sha256", strings.Join(toSign, " "), bsig)
51 	req.Header.Set("Signature", val)
52 	return nil
53 }
54 
55 type signature struct {
56 	keyID     string
57 	algorithm string
58 	headers   string
59 	signature string
60 }
61 
62 func parseSignatureHeader(line string) (signature, error) {
63 	var sig signature
64 	for _, v := range strings.Split(line, ",") {
65 		name, val, ok := strings.Cut(v, "=")
66 		if !ok {
67 			return sig, fmt.Errorf("bad field: %s from %s", v, line)
68 		}
69 		val = strings.Trim(val, `"`)
70 		switch name {
71 		case "keyId":
72 			sig.keyID = val
73 		case "algorithm":
74 			sig.algorithm = val
75 		case "headers":
76 			sig.headers = val
77 		case "signature":
78 			sig.signature = val
79 		default:
80 			return signature{}, fmt.Errorf("bad field name %s", name)
81 		}
82 	}
83 
84 	if sig.keyID == "" {
85 		return sig, fmt.Errorf("missing signature field keyId")
86 	} else if sig.algorithm == "" {
87 		return sig, fmt.Errorf("missing signature field algorithm")
88 	} else if sig.headers == "" {
89 		return sig, fmt.Errorf("missing signature field headers")
90 	} else if sig.signature == "" {
91 		return sig, fmt.Errorf("missing signature field signature")
92 	}
93 	return sig, nil
94 }