all: remove AEAD marker

As Thomas convinced me, we can always add it back by bumping the
version, but the fewest knobs and joints we start with, the better.

internal/age/age.go

@@ -32,8 +32,6 @@ func Encrypt(dst io.Writer, recipients ...Recipient) (io.WriteCloser, error) {
 	}
 
 	hdr := &format.Header{}
-	// TODO: remove the AEAD marker from v1.
-	hdr.AEAD = "ChaChaPoly"
 	for i, r := range recipients {
 		if r.Type() == "scrypt" && len(recipients) != 1 {
 			return nil, errors.New("an scrypt recipient must be the only one")
@@ -74,9 +72,6 @@ func Decrypt(src io.Reader, identities ...Identity) (io.Reader, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to read header: %v", err)
 	}
-	if hdr.AEAD != "ChaChaPoly" {
-		return nil, fmt.Errorf("unsupported AEAD: %v", hdr.AEAD)
-	}
 	if len(hdr.Recipients) > 20 {
 		return nil, errors.New("too many recipients")
 	}

internal/format/format.go

@@ -12,7 +12,6 @@ import (
 
 type Header struct {
 	Recipients []*Recipient
-	AEAD       string
 	MAC        []byte
 }
 
@@ -61,7 +60,7 @@ func (h *Header) MarshalWithoutMAC(w io.Writer) error {
 			return err
 		}
 	}
-	_, err := fmt.Fprintf(w, "%s %s", footerPrefix, h.AEAD)
+	_, err := fmt.Fprintf(w, "%s", footerPrefix)
 	return err
 }
 
@@ -107,11 +106,10 @@ func Parse(input io.Reader) (*Header, io.Reader, error) {
 
 		if bytes.HasPrefix(line, footerPrefix) {
 			prefix, args := splitArgs(line)
-			if prefix != string(footerPrefix) || len(args) != 2 {
+			if prefix != string(footerPrefix) || len(args) != 1 {
 				return nil, nil, errorf("malformed closing line: %q", line)
 			}
-			h.AEAD = args[0]
-			h.MAC, err = DecodeString(args[1])
+			h.MAC, err = DecodeString(args[0])
 			if err != nil {
 				return nil, nil, errorf("malformed closing line %q: %v", line, err)
 			}