feat: Minstrel can put and list objects in a path-style S3 bucket, signed with SigV4 and no SDK (#6771)

Bucket.Put stores a body under a key, signed over its SHA-256; Bucket.List
pages through ListObjectsV2. A backup only adds and lists, so an
append-only key is enough. Keys are encoded once, exactly as signed, so
music paths with spaces, plus signs and accents reach the bucket intact.
A refusal names S3's code in words ("the secret key is wrong").

The signer is pinned to AWS's published GET Object, PUT Object and
ListObjects examples.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/backup/sigv4.go

@@ -0,0 +1,119 @@
+package backup
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
+	"net/http"
+	"net/url"
+	"slices"
+	"strings"
+	"time"
+)
+
+// emptySHA256 is the payload hash of a request with no body.
+const emptySHA256 = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
+
+// sign adds AWS Signature Version 4 headers for S3 to req: X-Amz-Date,
+// X-Amz-Content-Sha256 and Authorization. Every header already on the
+// request is signed, along with the host.
+func sign(req *http.Request, payloadSHA256, accessKey, secretKey, region string, at time.Time) {
+	at = at.UTC()
+	stamp := at.Format("20060102T150405Z")
+	date := at.Format("20060102")
+	req.Header.Set("X-Amz-Date", stamp)
+	req.Header.Set("X-Amz-Content-Sha256", payloadSHA256)
+
+	host := req.Host
+	if host == "" {
+		host = req.URL.Host
+	}
+	headers := map[string]string{"host": host}
+	for name, values := range req.Header {
+		headers[strings.ToLower(name)] = strings.TrimSpace(strings.Join(values, ","))
+	}
+	names := make([]string, 0, len(headers))
+	for name := range headers {
+		names = append(names, name)
+	}
+	slices.Sort(names)
+	var canonHeaders strings.Builder
+	for _, name := range names {
+		canonHeaders.WriteString(name + ":" + headers[name] + "\n")
+	}
+	signed := strings.Join(names, ";")
+
+	canonical := strings.Join([]string{
+		req.Method,
+		canonicalPath(req.URL),
+		canonicalQuery(req.URL.Query()),
+		canonHeaders.String(),
+		signed,
+		payloadSHA256,
+	}, "\n")
+	scope := date + "/" + region + "/s3/aws4_request"
+	toSign := "AWS4-HMAC-SHA256\n" + stamp + "\n" + scope + "\n" + hexSHA256([]byte(canonical))
+
+	key := hmacSHA256([]byte("AWS4"+secretKey), date)
+	key = hmacSHA256(key, region)
+	key = hmacSHA256(key, "s3")
+	key = hmacSHA256(key, "aws4_request")
+	signature := hex.EncodeToString(hmacSHA256(key, toSign))
+
+	req.Header.Set("Authorization", "AWS4-HMAC-SHA256 Credential="+accessKey+"/"+scope+
+		", SignedHeaders="+signed+", Signature="+signature)
+}
+
+// canonicalPath is the request path with each segment URI-encoded once.
+func canonicalPath(u *url.URL) string {
+	p := u.EscapedPath()
+	if p == "" {
+		return "/"
+	}
+	if unescaped, err := url.PathUnescape(p); err == nil {
+		p = unescaped
+	}
+	segments := strings.Split(p, "/")
+	for i, s := range segments {
+		segments[i] = uriEncode(s)
+	}
+	return strings.Join(segments, "/")
+}
+
+// canonicalQuery is the query sorted by name, names and values URI-encoded.
+func canonicalQuery(q url.Values) string {
+	pairs := make([]string, 0, len(q))
+	for name, values := range q {
+		for _, v := range values {
+			pairs = append(pairs, uriEncode(name)+"="+uriEncode(v))
+		}
+	}
+	slices.Sort(pairs)
+	return strings.Join(pairs, "&")
+}
+
+// uriEncode percent-encodes everything but the unreserved characters, as
+// SigV4 requires; url.QueryEscape differs on space and '~'.
+func uriEncode(s string) string {
+	var b strings.Builder
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z' || c >= '0' && c <= '9' || c == '-' || c == '_' || c == '.' || c == '~' {
+			b.WriteByte(c)
+			continue
+		}
+		b.WriteString("%" + strings.ToUpper(hex.EncodeToString([]byte{c})))
+	}
+	return b.String()
+}
+
+func hmacSHA256(key []byte, msg string) []byte {
+	m := hmac.New(sha256.New, key)
+	m.Write([]byte(msg))
+	return m.Sum(nil)
+}
+
+func hexSHA256(b []byte) string {
+	sum := sha256.Sum256(b)
+	return hex.EncodeToString(sum[:])
+}