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 const requiredSigHeaders = "(request-target) host date digest"
18 
19 // Sign signs the given HTTP request with the matching private key of the
20 // public key available at pubkeyURL.
21 func Sign(req *http.Request, key *rsa.PrivateKey, pubkeyURL string) error {
22 	if pubkeyURL == "" {
23 		return fmt.Errorf("no pubkey url")
24 	}
25 	date := time.Now().UTC().Format(http.TimeFormat)
26 	req.Header.Set("Date", date)
27 	hash := sha256.New()
28 	toSign := []string{"(request-target)", "host", "date"}
29 	fmt.Fprintln(hash, "(request-target):", strings.ToLower(req.Method), req.URL.Path)
30 	fmt.Fprintln(hash, "host:", req.URL.Hostname())
31 	fmt.Fprintf(hash, "date: %s", date)
32 
33 	if req.Body != nil {
34 		// we're adding one more entry to our signature, so one more line.
35 		fmt.Fprint(hash, "\n")
36 		buf := &bytes.Buffer{}
37 		io.Copy(buf, req.Body)
38 		req.Body.Close()
39 		req.Body = io.NopCloser(buf)
40 		digest := sha256.Sum256(buf.Bytes())
41 		d := "SHA-256=" + base64.StdEncoding.EncodeToString(digest[:])
42 		toSign = append(toSign, "digest")
43 		fmt.Fprintf(hash, "digest: %s", d)
44 		req.Header.Set("Digest", d)
45 	}
46 	sig, err := rsa.SignPKCS1v15(rand.Reader, key, crypto.SHA256, hash.Sum(nil))
47 	if err != nil {
48 		return err
49 	}
50 	bsig := base64.StdEncoding.EncodeToString(sig)
51 
52 	val := fmt.Sprintf("keyId=%q,algorithm=%q,headers=%q,signature=%q", pubkeyURL, "rsa-sha256", strings.Join(toSign, " "), bsig)
53 	req.Header.Set("Signature", val)
54 	return nil
55 }
56 
57 type signature struct {
58 	keyID     string
59 	algorithm string
60 	headers   string
61 	signature string
62 }
63 
64 func parseSignatureHeader(line string) (signature, error) {
65 	var sig signature
66 	for _, v := range strings.Split(line, ",") {
67 		name, val, ok := strings.Cut(v, "=")
68 		if !ok {
69 			return sig, fmt.Errorf("bad field: %s from %s", v, line)
70 		}
71 		val = strings.Trim(val, `"`)
72 		switch name {
73 		case "keyId":
74 			sig.keyID = val
75 		case "algorithm":
76 			sig.algorithm = val
77 		case "headers":
78 			sig.headers = val
79 		case "signature":
80 			sig.signature = val
81 		default:
82 			return signature{}, fmt.Errorf("bad field name %s", name)
83 		}
84 	}
85 
86 	if sig.keyID == "" {
87 		return sig, fmt.Errorf("missing signature field keyId")
88 	} else if sig.algorithm == "" {
89 		return sig, fmt.Errorf("missing signature field algorithm")
90 	} else if sig.headers == "" {
91 		return sig, fmt.Errorf("missing signature field headers")
92 	} else if sig.signature == "" {
93 		return sig, fmt.Errorf("missing signature field signature")
94 	}
95 	return sig, nil
96 }