[mike@mwxm4]

creds.go

@@ -11,19 +11,30 @@
 // nothing. A run without a terminal — the certbot hooks, cron — never asks: it
 // says what is missing and stops.
 //
+// ~/.dnsrc is encrypted too, under FILEKEY, which the program carries and
+// nobody is asked for. It is the same AES-256-GCM, and the file opens on every
+// machine dns runs on, so cron and the hooks notice nothing. What it buys is
+// that the password no longer stands in the clear in a backup, in a synced home
+// directory or on a screen someone else is looking at. A file from before this,
+// plain JSON, is still read, and written back encrypted on the next run.
+//
 // Rotating the infoblox password means 'dns --seal', pasting the line it
 // prints into this file, rebuilding, and removing the stale ~/.dnsrc wherever
 // one exists.
 //
 // What this is not: whoever knows the passphrase has the login, and so has
-// whoever can read ~/.dnsrc. It keeps the credentials out of the repository and
-// out of the binary. It is not a vault.
+// whoever holds ~/.dnsrc together with a copy of dns — FILEKEY is in every one
+// of them, and prising it out is an afternoon's work, not a cluster's. That is
+// why the file stays 0600. It keeps the credentials out of the repository and
+// out of plain sight on disk. It is not a vault.
 package main
 
 import (
 	"crypto/aes"
 	"crypto/cipher"
+	"crypto/hkdf"
 	"crypto/rand"
+	"crypto/sha256"
 	"encoding/base64"
 	"encoding/json"
 	"errors"
@@ -37,8 +48,23 @@ import (
 
 var SEALED = "B/RvQRI1EfziN3EoEY0obzrVeQsIMeN1QzBiR4Pl8PaygMUqlK1vggmbObjceeF+tmW3npiuXAvp93R18u9bejlv5M/3qL5Ix3fOpi+L5p3x90oXni7fhlPtc9Z3"
 
+// The key ~/.dnsrc is written under. Thirty-two random bytes, so there is
+// nothing to guess and no reason to slow a guesser down: hkdf, not argon2, and
+// every run opens the file in microseconds instead of a third of a second.
+//
+// A build may put another one in its place with -ldflags "-X main.FILEKEY=...".
+// Files the earlier builds wrote then no longer open, and dns says so and asks
+// for the passphrase again — which a cron run cannot do, so a key changed under
+// a running installation is changed for the hooks as well.
+var FILEKEY = "mphYib5GBHwMnKE0F3of3V8+rpS4ayUlXvaMncaZ3wE="
+
 const CREDSFILE = ".dnsrc"
 
+// The first bytes of an encrypted ~/.dnsrc. It tells the file apart from the
+// plain JSON of older versions, and leaves room to tell it apart from whatever
+// a later version writes should FILEKEY ever have to change.
+const FILETAG = "dnsrc1:"
+
 // argon2id, the second of the two settings RFC 9106 recommends: 64 MB and three
 // passes. It costs a fraction of a second here and makes an offline run through
 // a list of likely passphrases expensive on hardware built for it.
@@ -62,7 +88,18 @@ type credentials struct {
 func getcreds() (string, string) { // ----------------------------------- the login, from ~/.dnsrc or the passphrase
   path:=credspath()
 
-  if c,err:=readcreds(path); err==nil { return c.User,c.Pass }
+  c,plain,err:=readcreds(path)
+  if err==nil {
+    if (plain) {                                       // written before ~/.dnsrc was encrypted: put it away properly
+      if err:=writecreds(path,c); err!=nil { PE("cannot encrypt "+path,err.Error()) } else { PO(path+" is now encrypted") }
+    }
+    return c.User,c.Pass
+  }
+
+  // A file that is there but will not open — meddled with, truncated, written
+  // by a build with a different FILEKEY — is worth saying out loud before the
+  // passphrase is asked for and the file written afresh.
+  if (!os.IsNotExist(err)) { PE("cannot read "+path,err.Error()) }
 
   if (SEALED=="") {
     PE("this build carries no credentials","run 'dns --seal' and paste the line into creds.go")
@@ -74,7 +111,7 @@ func getcreds() (string, string) { // ----------------------------------- the lo
     os.Exit(1)
   }
 
-  c:=askpassphrase()
+  c=askpassphrase()
 
   if err:=writecreds(path,c); err!=nil {
     PE("cannot write "+path,err.Error())                        // the login still works for this one run
@@ -122,14 +159,37 @@ func sealcmd() { // --------------------------------------------- 'dns --seal':
 
 // ===================================================================================================== THE BLOCK
 
-func seal(c credentials, pass string) (string, error) { // ------------------------------------- encrypt the login
+// The block in creds.go and the file in the home directory are the same thing
+// twice, encrypted the same way and differing only in which key opens them:
+// lock and unlock do the work, and what is handed in decides whether that is
+// the shared passphrase or FILEKEY.
+
+func seal(c credentials, pass string) (string, error) { // ------------------- encrypt the login for creds.go
+  return lock(c,func(salt []byte) (cipher.AEAD,error) { return credsgcm(pass,salt) })
+}
+
+func unseal(blob string, pass string) (credentials, error) { // ------------- decrypt the login from creds.go
+  return unlock(blob,"the sealed block",func(salt []byte) (cipher.AEAD,error) { return credsgcm(pass,salt) })
+}
+
+func lockcreds(c credentials) (string, error) { // -------------------------- encrypt the login for ~/.dnsrc
+  blob,err:=lock(c,filegcm)
+  if err!=nil { return "",err }
+  return FILETAG+blob,nil
+}
+
+func opencreds(blob string) (credentials, error) { // ---------------------- decrypt the login from ~/.dnsrc
+  return unlock(strings.TrimPrefix(blob,FILETAG),"the credentials",filegcm)
+}
+
+func lock(c credentials, keyed func([]byte) (cipher.AEAD, error)) (string, error) { // -------- encrypt the login
   plain,err:=json.Marshal(c)
   if err!=nil { return "",err }
 
   salt:=make([]byte,SALTLEN)
   if _,err:=rand.Read(salt); err!=nil { return "",err }
 
-  gcm,err:=credsgcm(pass,salt)
+  gcm,err:=keyed(salt)
   if err!=nil { return "",err }
 
   nonce:=make([]byte,gcm.NonceSize())
@@ -145,27 +205,28 @@ func seal(c credentials, pass string) (string, error) { // ---------------------
   return base64.StdEncoding.EncodeToString(out),nil
 }
 
-func unseal(blob string, pass string) (credentials, error) { // -------------------------------- decrypt the login
+func unlock(blob string, what string, keyed func([]byte) (cipher.AEAD, error)) (credentials, error) { // - decrypt
   var c credentials
 
   raw,err:=base64.StdEncoding.DecodeString(strings.TrimSpace(blob))
-  if err!=nil { return c,errors.New("the sealed block is not valid base64") }
+  if err!=nil { return c,errors.New(what+" is not valid base64") }
 
-  gcm,err:=credsgcm(pass,raw[:min(SALTLEN,len(raw))])
+  gcm,err:=keyed(raw[:min(SALTLEN,len(raw))])
   if err!=nil { return c,err }
 
   if (len(raw) < SALTLEN+gcm.NonceSize()+gcm.Overhead()) {
-    return c,errors.New("the sealed block is too short")
+    return c,errors.New(what+" is too short")
   }
   nonce:=raw[SALTLEN : SALTLEN+gcm.NonceSize()]
 
-  // A wrong passphrase derives a wrong key, and the tag does not check out —
-  // the same error a block someone has meddled with produces.
+  // A wrong key — a mistyped passphrase, a FILEKEY that has moved on — and the
+  // tag does not check out: the same error a block someone has meddled with
+  // produces.
   plain,err:=gcm.Open(nil,nonce,raw[SALTLEN+gcm.NonceSize():],nil)
-  if err!=nil { return c,errors.New("cannot open the sealed block") }
+  if err!=nil { return c,errors.New("cannot open "+what) }
 
   if err:=json.Unmarshal(plain,&c); err!=nil { return c,err }
-  if (c.User=="" || c.Pass=="")             { return c,errors.New("the sealed block holds no login") }
+  if (c.User=="" || c.Pass=="")             { return c,errors.New(what+" holds no login") }
   return c,nil
 }
 
@@ -176,6 +237,14 @@ func credsgcm(pass string, salt []byte) (cipher.AEAD, error) { // --------------
   return cipher.NewGCM(block)
 }
 
+func filegcm(salt []byte) (cipher.AEAD, error) { // -------------------------------------- FILEKEY and salt to a key
+  key,err:=hkdf.Key(sha256.New,[]byte(FILEKEY),salt,CREDSFILE,KEYLEN)
+  if err!=nil { return nil,err }
+  block,err:=aes.NewCipher(key)
+  if err!=nil { return nil,err }
+  return cipher.NewGCM(block)
+}
+
 
 
 // ====================================================================================================== ~/.DNSRC
@@ -186,25 +255,36 @@ func credspath() string { // ---------------------------------------------------
   return filepath.Join(home,CREDSFILE)
 }
 
-func readcreds(path string) (credentials, error) { // ------------------------------------------------- read it
+// The second return says the file was still the plain JSON of an older dns.
+// The login in it is good, and getcreds writes it back encrypted; refusing it
+// would strand a cron run on a file it could perfectly well use.
+func readcreds(path string) (credentials, bool, error) { // ------------------------------------ read it, either form
   var c credentials
 
   st,err:=os.Stat(path)
-  if err!=nil { return c,err }
+  if err!=nil { return c,false,err }
   if (st.Mode().Perm()&0o077 != 0) { PE(path+" can be read by others",SF("chmod 600 %s",path)) }
 
   b,err:=os.ReadFile(path)
-  if err!=nil { return c,err }
+  if err!=nil { return c,false,err }
+  txt:=strings.TrimSpace(string(b))
 
-  if err:=json.Unmarshal(b,&c); err!=nil { return c,err }
-  if (c.User=="" || c.Pass=="")          { return c,errors.New("no login in "+path) }
-  return c,nil
+  if (strings.HasPrefix(txt,FILETAG)) {
+    c,err=opencreds(txt)
+    return c,false,err
+  }
+
+  if (!strings.HasPrefix(txt,"{")) { return c,false,errors.New(path+" is not a credentials file") }
+
+  if err:=json.Unmarshal([]byte(txt),&c); err!=nil { return c,true,err }
+  if (c.User=="" || c.Pass=="")                    { return c,true,errors.New("no login in "+path) }
+  return c,true,nil
 }
 
-func writecreds(path string, c credentials) error { // ----------------------------------------------- write it
-  b,err:=json.MarshalIndent(c,"","  ")
+func writecreds(path string, c credentials) error { // ------------------------------------------ write it, encrypted
+  blob,err:=lockcreds(c)
   if err!=nil { return err }
-  b=append(b,'\n')
+  b:=[]byte(blob+"\n")
 
   // Alongside first, then renamed: nobody comes upon half a file, and the login
   // is never on disk readable by others, not even for a moment — CreateTemp

creds_test.go

@@ -2,8 +2,10 @@ package main
 
 import (
 	"encoding/base64"
+	"encoding/json"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -83,24 +85,172 @@ func TestCredsFileIsPrivate(t *testing.T) {
 		t.Errorf("mode is %04o, want 0600", perm)
 	}
 
-	got, err := readcreds(path)
+	got, plain, err := readcreds(path)
 	if err != nil {
 		t.Fatalf("readcreds: %v", err)
 	}
 	if got != testCreds {
 		t.Errorf("got %+v, want %+v", got, testCreds)
 	}
+	if plain {
+		t.Error("a file dns just wrote was taken for an old plaintext one")
+	}
 
 	// Nothing written, nothing to read: the first run has to fall through to the
 	// passphrase rather than come back with an empty login.
-	if _, err := readcreds(filepath.Join(t.TempDir(), ".dnsrc")); err == nil {
+	if _, _, err := readcreds(filepath.Join(t.TempDir(), ".dnsrc")); err == nil {
 		t.Error("a missing file was accepted")
 	}
-	if err := os.WriteFile(path, []byte(`{"user":"","password":""}`), 0o600); err != nil {
+}
+
+// What lands on disk must not read out the login, and must not be the plain
+// JSON of before — that is the whole point of the exercise.
+func TestCredsFileIsEncrypted(t *testing.T) {
+	path := filepath.Join(t.TempDir(), ".dnsrc")
+
+	if err := writecreds(path, testCreds); err != nil {
+		t.Fatalf("writecreds: %v", err)
+	}
+	b, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if !strings.HasPrefix(string(b), FILETAG) {
+		t.Errorf("the file does not begin with %q", FILETAG)
+	}
+	for _, s := range []string{testCreds.User, testCreds.Pass, `"password"`} {
+		if bytesContains(b, []byte(s)) {
+			t.Errorf("%q stands in the clear in the file", s)
+		}
+	}
+
+	// Two writes of the same login differ: salt and nonce are fresh each time.
+	first := string(b)
+	if err := writecreds(path, testCreds); err != nil {
+		t.Fatalf("writecreds: %v", err)
+	}
+	if b, _ = os.ReadFile(path); string(b) == first {
+		t.Error("two writes of the same login are identical")
+	}
+}
+
+func TestCredsFileRejects(t *testing.T) {
+	dir := t.TempDir()
+
+	// A byte turned over in the ciphertext, a file that is not one of ours, and
+	// an encrypted file holding nothing: none of them may pass as a login.
+	blob, err := lockcreds(testCreds)
+	if err != nil {
+		t.Fatalf("lockcreds: %v", err)
+	}
+	raw, _ := base64.StdEncoding.DecodeString(strings.TrimPrefix(blob, FILETAG))
+	raw[len(raw)-1] ^= 0x01
+
+	for name, body := range map[string]string{
+		"tampered": FILETAG + base64.StdEncoding.EncodeToString(raw),
+		"foreign":  "just some text someone put here",
+		"empty":    "",
+		"nologin":  `{"user":"","password":""}`,
+	} {
+		path := filepath.Join(dir, name)
+		if err := os.WriteFile(path, []byte(body+"\n"), 0o600); err != nil {
+			t.Fatal(err)
+		}
+		if _, _, err := readcreds(path); err == nil {
+			t.Errorf("a %s file was accepted", name)
+		}
+	}
+}
+
+// The file an older dns wrote is still read, and flagged so getcreds writes it
+// back encrypted. Anything else would stop the certbot hooks on a home
+// directory that has not seen an interactive run yet.
+func TestCredsFileFromBefore(t *testing.T) {
+	path := filepath.Join(t.TempDir(), ".dnsrc")
+
+	b, err := json.Marshal(testCreds)
+	if err != nil {
 		t.Fatal(err)
 	}
-	if _, err := readcreds(path); err == nil {
-		t.Error("a file without a login was accepted")
+	if err := os.WriteFile(path, append(b, '\n'), 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	got, plain, err := readcreds(path)
+	if err != nil {
+		t.Fatalf("readcreds: %v", err)
+	}
+	if got != testCreds {
+		t.Errorf("got %+v, want %+v", got, testCreds)
+	}
+	if !plain {
+		t.Error("a plaintext file was not reported as one")
+	}
+
+	// And once written back it opens as an encrypted one, with the same login.
+	if err := writecreds(path, got); err != nil {
+		t.Fatalf("writecreds: %v", err)
+	}
+	got, plain, err = readcreds(path)
+	if err != nil || got != testCreds || plain {
+		t.Errorf("after rewriting: %+v, plain %v, err %v", got, plain, err)
+	}
+}
+
+// The whole way through, on a home directory holding a file from an older dns:
+// the login comes back, the file is encrypted afterwards, and the next run —
+// the one from cron, which can ask nobody anything — reads it again.
+func TestGetcredsEncryptsWhatItFinds(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("HOME", dir)
+	path := filepath.Join(dir, CREDSFILE)
+
+	b, err := json.Marshal(testCreds)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, append(b, '\n'), 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	if user, pass := getcreds(); user != testCreds.User || pass != testCreds.Pass {
+		t.Fatalf("got %q/%q, want %q/%q", user, pass, testCreds.User, testCreds.Pass)
+	}
+
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.HasPrefix(string(raw), FILETAG) {
+		t.Fatal("the file was not written back encrypted")
+	}
+
+	if user, pass := getcreds(); user != testCreds.User || pass != testCreds.Pass {
+		t.Errorf("second run: got %q/%q, want %q/%q", user, pass, testCreds.User, testCreds.Pass)
+	}
+}
+
+func TestLockcredsRoundtrip(t *testing.T) {
+	blob, err := lockcreds(testCreds)
+	if err != nil {
+		t.Fatalf("lockcreds: %v", err)
+	}
+	got, err := opencreds(blob)
+	if err != nil {
+		t.Fatalf("opencreds: %v", err)
+	}
+	if got != testCreds {
+		t.Errorf("got %+v, want %+v", got, testCreds)
+	}
+
+	// A different FILEKEY does not open it — the file is worth something only
+	// together with the binary that wrote it.
+	old := FILEKEY
+	FILEKEY = "c29tZXRoaW5nIGVsc2UgZW50aXJlbHksIHRoaXJ0eSB0d28="
+	_, err = opencreds(blob)
+	FILEKEY = old
+	if err == nil {
+		t.Error("another FILEKEY opened the file")
 	}
 }
 

version.txt

@@ -1 +1 @@
-2.4.0
+2.4.4