age: move the scrypt lone recipient check out of Decrypt

The important one is the decryption side one, because when a user types
a password they expect it to both decrypt and authenticate the file.
Moved that one out of Decrypt and into ScryptIdentity, now that
Identities get all the stanzas. special_cases--

This also opens the door to other Identity implementations that do allow
multiple scrypt recipients, if someone really wants that. The CLI will
never allow it, but an explicit choice by an API consumer feels like
something we shouldn't interfere with.

Moreover, this also allows alternative Identity implementations that use
different recipient types to replicate the behavior if they have the
same authentication semantics.

The encryption side one is only a courtesy, to stop API users from
making files that won't decrypt. Unfortunately, that one needs to stay
as a special case in Encrypt, as the Recipient can't see around itself.
However, changed it to a type assertion, so custom recipients can
generate multiple scrypt recipient stanzas, if they really want.

README.md

@@ -105,9 +105,7 @@ $ age-keygen | age -p > key.age
 Public key: age1yhm4gctwfmrpz87tdslm550wrx6m79y9f2hdzt0lndjnehwj0ukqrjpyx5
 Enter passphrase (leave empty to autogenerate a secure one):
 Using the autogenerated passphrase "hip-roast-boring-snake-mention-east-wasp-honey-input-actress".
-
 $ age -r age1yhm4gctwfmrpz87tdslm550wrx6m79y9f2hdzt0lndjnehwj0ukqrjpyx5 secrets.txt > secrets.txt.age
-
 $ age -d -i key.age secrets.txt.age > secrets.txt
 Enter passphrase for identity file "key.age":
 ```

age.go

@@ -103,6 +103,16 @@ func Encrypt(dst io.Writer, recipients ...Recipient) (io.WriteCloser, error) {
 		return nil, errors.New("no recipients specified")
 	}
 
+	// As a best effort, prevent an API user from generating a file that the
+	// ScryptIdentity will refuse to decrypt. This check can't unfortunately be
+	// implemented as part of the Recipient interface, so it lives as a special
+	// case in Encrypt.
+	for _, r := range recipients {
+		if _, ok := r.(*ScryptRecipient); ok && len(recipients) != 1 {
+			return nil, errors.New("an ScryptRecipient must be the only one for the file")
+		}
+	}
+
 	fileKey := make([]byte, fileKeySize)
 	if _, err := rand.Read(fileKey); err != nil {
 		return nil, err
@@ -118,11 +128,6 @@ func Encrypt(dst io.Writer, recipients ...Recipient) (io.WriteCloser, error) {
 			hdr.Recipients = append(hdr.Recipients, (*format.Stanza)(s))
 		}
 	}
-	for _, s := range hdr.Recipients {
-		if s.Type == "scrypt" && len(hdr.Recipients) != 1 {
-			return nil, errors.New("an scrypt recipient must be the only one")
-		}
-	}
 	if mac, err := headerMAC(fileKey, hdr); err != nil {
 		return nil, fmt.Errorf("failed to compute header MAC: %v", err)
 	} else {
@@ -169,12 +174,6 @@ func Decrypt(src io.Reader, identities ...Identity) (io.Reader, error) {
 		return nil, fmt.Errorf("failed to read header: %v", err)
 	}
 
-	for _, r := range hdr.Recipients {
-		if r.Type == "scrypt" && len(hdr.Recipients) != 1 {
-			return nil, errors.New("an scrypt recipient must be the only one")
-		}
-	}
-
 	stanzas := make([]*Stanza, 0, len(hdr.Recipients))
 	for _, s := range hdr.Recipients {
 		stanzas = append(stanzas, (*Stanza)(s))

cmd/age/age_test.go

@@ -18,24 +18,30 @@ import (
 )
 
 func TestVectors(t *testing.T) {
-	defaultIDs, err := parseIdentitiesFile("testdata/default_key.txt")
+	var defaultIDs []age.Identity
+
+	password, err := ioutil.ReadFile("testdata/default_password.txt")
 	if err != nil {
 		t.Fatal(err)
 	}
-	password, err := ioutil.ReadFile("testdata/default_password.txt")
-	if err == nil {
-		p := strings.TrimSpace(string(password))
-		i, err := age.NewScryptIdentity(p)
-		if err != nil {
-			t.Fatal(err)
-		}
-		defaultIDs = append(defaultIDs, i)
+	p := strings.TrimSpace(string(password))
+	i, err := age.NewScryptIdentity(p)
+	if err != nil {
+		t.Fatal(err)
 	}
+	defaultIDs = append(defaultIDs, i)
+
+	ids, err := parseIdentitiesFile("testdata/default_key.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defaultIDs = append(defaultIDs, ids...)
 
 	files, _ := filepath.Glob("testdata/*.age")
 	for _, f := range files {
 		_, name := filepath.Split(f)
 		name = strings.TrimSuffix(name, ".age")
+		expectPass := strings.HasPrefix(name, "good_")
 		expectFailure := strings.HasPrefix(name, "fail_")
 		expectNoMatch := strings.HasPrefix(name, "nomatch_")
 		t.Run(name, func(t *testing.T) {
@@ -73,7 +79,7 @@ func TestVectors(t *testing.T) {
 				if e := new(age.NoIdentityMatchError); !errors.As(err, &e) {
 					t.Errorf("expected ErrIncorrectIdentity, got %v", err)
 				}
-			} else {
+			} else if expectPass {
 				if err != nil {
 					t.Fatal(err)
 				}
@@ -82,6 +88,8 @@ func TestVectors(t *testing.T) {
 					t.Fatal(err)
 				}
 				t.Logf("%s", out)
+			} else {
+				t.Fatal("invalid test vector")
 			}
 		})
 	}

cmd/age/encrypted_keys.go

@@ -24,6 +24,11 @@ type LazyScryptIdentity struct {
 var _ age.Identity = &LazyScryptIdentity{}
 
 func (i *LazyScryptIdentity) Unwrap(stanzas []*age.Stanza) (fileKey []byte, err error) {
+	for _, s := range stanzas {
+		if s.Type == "scrypt" && len(stanzas) != 1 {
+			return nil, errors.New("an scrypt recipient must be the only one")
+		}
+	}
 	if len(stanzas) != 1 || stanzas[0].Type != "scrypt" {
 		return nil, age.ErrIncorrectIdentity
 	}

scrypt.go

@@ -122,6 +122,11 @@ func (i *ScryptIdentity) SetMaxWorkFactor(logN int) {
 }
 
 func (i *ScryptIdentity) Unwrap(stanzas []*Stanza) ([]byte, error) {
+	for _, s := range stanzas {
+		if s.Type == "scrypt" && len(stanzas) != 1 {
+			return nil, errors.New("an scrypt recipient must be the only one")
+		}
+	}
 	return multiUnwrap(i.unwrap, stanzas)
 }