internal/format: implement outer layer parsing and marshaling

internal/format/format.go

@@ -0,0 +1,141 @@
+package format
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+)
+
+type Header struct {
+	Recipients []*Recipient
+	AEAD       string
+	MAC        []byte
+}
+
+type Recipient struct {
+	Type string
+	Args []string
+	Body []byte
+}
+
+var b64 = base64.RawURLEncoding.Strict()
+
+func decodeString(s string) ([]byte, error) {
+	// CR and LF are ignored by DecodeString. LF is handled by the parser,
+	// but CR can introduce malleability.
+	if strings.Contains(s, "\r") {
+		return nil, errors.New(`invalid character: \r`)
+	}
+	return b64.DecodeString(s)
+}
+
+const intro = "This is a file encrypted with age-tool.com, version 1\n"
+
+var recipientPrefix = []byte("->")
+var footerPrefix = []byte("---")
+
+func (h *Header) Marshal(w io.Writer) error {
+	if _, err := io.WriteString(w, intro); err != nil {
+		return err
+	}
+	for _, r := range h.Recipients {
+		if _, err := w.Write(recipientPrefix); err != nil {
+			return err
+		}
+		for _, a := range append([]string{r.Type}, r.Args...) {
+			if _, err := io.WriteString(w, " "+a); err != nil {
+				return err
+			}
+		}
+		if _, err := io.WriteString(w, "\n"); err != nil {
+			return err
+		}
+		if _, err := w.Write(r.Body); err != nil {
+			return err
+		}
+	}
+	mac := b64.EncodeToString(h.MAC)
+	_, err := fmt.Fprintf(w, "%s %s %s\n", footerPrefix, h.AEAD, mac)
+	return err
+}
+
+type ParseError string
+
+func (e ParseError) Error() string {
+	return "parsing age header: " + string(e)
+}
+
+func errorf(format string, a ...interface{}) error {
+	return ParseError(fmt.Sprintf(format, a...))
+}
+
+// Parse returns the header and a Reader that begins at the start of the
+// payload.
+func Parse(input io.Reader) (*Header, io.Reader, error) {
+	h := &Header{}
+	rr := bufio.NewReader(input)
+
+	line, err := rr.ReadString('\n')
+	if err != nil {
+		return nil, nil, errorf("failed to read intro: %v", err)
+	}
+	if line != intro {
+		return nil, nil, errorf("unexpected intro: %q", line)
+	}
+
+	var r *Recipient
+	for {
+		line, err := rr.ReadBytes('\n')
+		if err != nil {
+			return nil, nil, errorf("failed to read header: %v", err)
+		}
+
+		if bytes.HasPrefix(line, footerPrefix) {
+			prefix, args := splitArgs(line)
+			if prefix != string(footerPrefix) || len(args) != 2 {
+				return nil, nil, errorf("malformed closing line: %q", line)
+			}
+			h.AEAD = args[0]
+			h.MAC, err = decodeString(args[1])
+			if err != nil {
+				return nil, nil, errorf("malformed closing line %q: %v", line, err)
+			}
+			break
+
+		} else if bytes.HasPrefix(line, recipientPrefix) {
+			r = &Recipient{}
+			prefix, args := splitArgs(line)
+			if prefix != string(recipientPrefix) || len(args) < 1 {
+				return nil, nil, errorf("malformed recipient: %q", line)
+			}
+			r.Type = args[0]
+			r.Args = args[1:]
+			h.Recipients = append(h.Recipients, r)
+
+		} else if r != nil {
+			r.Body = append(r.Body, line...)
+
+		} else {
+			return nil, nil, errorf("unexpected line: %q", line)
+		}
+	}
+
+	// Unwind the bufio overread and return the unbuffered input.
+	buf, err := rr.Peek(rr.Buffered())
+	if err != nil {
+		return nil, nil, errorf("internal error: %v", err)
+	}
+	payload := io.MultiReader(bytes.NewReader(buf), input)
+
+	return h, payload, nil
+}
+
+func splitArgs(line []byte) (string, []string) {
+	l := strings.TrimSuffix(string(line), "\n")
+	parts := strings.Split(l, " ")
+	return parts[0], parts[1:]
+}