Fix F-07: pre-compute real Argon2 dummy hash via sync.Once

- auth/auth.go: add DummyHash() which uses sync.Once to compute
  HashPassword("dummy-password-for-timing-only", DefaultArgonParams())
  on first call; subsequent calls return the cached PHC string;
  add sync to imports
- auth/auth_test.go: TestDummyHashIsValidPHC verifies the hash
  parses and verifies correctly; TestDummyHashIsCached verifies
  sync.Once behaviour; TestDummyHashMatchesDefaultParams verifies
  embedded m/t/p match DefaultArgonParams()
- server/server.go, grpcserver/auth.go, ui/ui.go: replace five
  hardcoded PHC strings with auth.DummyHash() calls
- AUDIT.md: mark F-07 as fixed
Security: the previous hardcoded hash used a 6-byte salt and
  6-byte output ("testsalt"/"testhash" in base64), which Argon2id
  verifies faster than a real 16-byte-salt / 32-byte-output hash.
  This timing gap was measurable and could aid user enumeration.
  auth.DummyHash() uses identical parameters and full-length salt
  and output, so dummy verification timing matches real timing
  exactly, regardless of future parameter changes.

internal/auth/auth.go

@@ -24,6 +24,7 @@ import (
 	"math"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"golang.org/x/crypto/argon2"
@@ -54,6 +55,38 @@ func DefaultArgonParams() ArgonParams {
 	}
 }
 
+// dummyHashOnce ensures the dummy hash is computed exactly once at first use.
+// Security: computing it lazily (rather than at init time) keeps startup fast;
+// using sync.Once makes it safe to call from concurrent goroutines.
+var (
+	dummyHashOnce sync.Once
+	dummyHashVal  string
+)
+
+// DummyHash returns a pre-computed Argon2id PHC hash of a fixed dummy password
+// using DefaultArgonParams. It is computed once on first call and cached.
+//
+// Security (F-07): using a real hash with the exact same parameters as
+// production password verification ensures that dummy operations (run for
+// unknown users or inactive accounts to prevent timing-based enumeration)
+// take as long as real verifications, regardless of parameter changes.
+// The previous hardcoded string used a 6-byte salt and 6-byte hash which
+// was faster to verify than a real 16-byte-salt / 32-byte-hash record.
+func DummyHash() string {
+	dummyHashOnce.Do(func() {
+		h, err := HashPassword("dummy-password-for-timing-only", DefaultArgonParams())
+		if err != nil {
+			// This should be unreachable in production — HashPassword only fails
+			// if crypto/rand fails or the password is empty, neither of which
+			// applies here. Panic so the misconfiguration surfaces immediately
+			// rather than silently degrading security.
+			panic("auth: DummyHash: failed to pre-compute dummy hash: " + err.Error())
+		}
+		dummyHashVal = h
+	})
+	return dummyHashVal
+}
+
 // HashPassword hashes a password using Argon2id and returns a PHC-format string.
 // A random 16-byte salt is generated via crypto/rand for each call.
 //

internal/auth/auth_test.go

@@ -214,3 +214,51 @@ func TestDefaultArgonParams(t *testing.T) {
 		t.Errorf("default Threads=%d < 1", p.Threads)
 	}
 }
+
+// TestDummyHashIsValidPHC verifies DummyHash returns a non-empty string that
+// is a valid Argon2id PHC hash verifiable by VerifyPassword.
+func TestDummyHashIsValidPHC(t *testing.T) {
+	h := DummyHash()
+	if h == "" {
+		t.Fatal("DummyHash returned empty string")
+	}
+	// Must parse as a valid PHC string with OWASP-compatible parameters.
+	// VerifyPassword with the correct password must succeed.
+	ok, err := VerifyPassword("dummy-password-for-timing-only", h)
+	if err != nil {
+		t.Fatalf("VerifyPassword against DummyHash: %v", err)
+	}
+	if !ok {
+		t.Error("VerifyPassword against DummyHash returned false (hash mismatch)")
+	}
+}
+
+// TestDummyHashIsCached verifies that consecutive calls return the same string
+// (sync.Once caching).
+func TestDummyHashIsCached(t *testing.T) {
+	h1 := DummyHash()
+	h2 := DummyHash()
+	if h1 != h2 {
+		t.Errorf("DummyHash returned different values on consecutive calls:\n  first:  %q\n  second: %q", h1, h2)
+	}
+}
+
+// TestDummyHashMatchesDefaultParams verifies the embedded parameters in the
+// returned PHC string match DefaultArgonParams (m, t, p).
+func TestDummyHashMatchesDefaultParams(t *testing.T) {
+	h := DummyHash()
+	params, _, _, err := parsePHC(h)
+	if err != nil {
+		t.Fatalf("parsePHC(DummyHash()): %v", err)
+	}
+	def := DefaultArgonParams()
+	if params.Memory != def.Memory {
+		t.Errorf("Memory = %d, want %d", params.Memory, def.Memory)
+	}
+	if params.Time != def.Time {
+		t.Errorf("Time = %d, want %d", params.Time, def.Time)
+	}
+	if params.Threads != def.Threads {
+		t.Errorf("Threads = %d, want %d", params.Threads, def.Threads)
+	}
+}

internal/grpcserver/auth.go

@@ -40,14 +40,14 @@ func (a *authServiceServer) Login(ctx context.Context, req *mciasv1.LoginRequest
 	acct, err := a.s.db.GetAccountByUsername(req.Username)
 	if err != nil {
 		// Security: run dummy Argon2 to equalise timing for unknown users.
-		_, _ = auth.VerifyPassword("dummy", "$argon2id$v=19$m=65536,t=3,p=4$dGVzdHNhbHQ$dGVzdGhhc2g")
+		_, _ = auth.VerifyPassword("dummy", auth.DummyHash())
 		a.s.db.WriteAuditEvent(model.EventLoginFail, nil, nil, ip, //nolint:errcheck // audit failure is non-fatal
 			fmt.Sprintf(`{"username":%q,"reason":"unknown_user"}`, req.Username))
 		return nil, status.Error(codes.Unauthenticated, "invalid credentials")
 	}
 
 	if acct.Status != model.AccountStatusActive {
-		_, _ = auth.VerifyPassword("dummy", "$argon2id$v=19$m=65536,t=3,p=4$dGVzdHNhbHQ$dGVzdGhhc2g")
+		_, _ = auth.VerifyPassword("dummy", auth.DummyHash())
 		a.s.db.WriteAuditEvent(model.EventLoginFail, &acct.ID, nil, ip, `{"reason":"account_inactive"}`) //nolint:errcheck
 		return nil, status.Error(codes.Unauthenticated, "invalid credentials")
 	}

internal/server/server.go

@@ -174,7 +174,7 @@ func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		// Security: return a generic error whether the user exists or not.
 		// Always run a dummy Argon2 check to prevent timing-based user enumeration.
-		_, _ = auth.VerifyPassword("dummy", "$argon2id$v=19$m=65536,t=3,p=4$dGVzdHNhbHQ$dGVzdGhhc2g")
+		_, _ = auth.VerifyPassword("dummy", auth.DummyHash())
 		s.writeAudit(r, model.EventLoginFail, nil, nil, fmt.Sprintf(`{"username":%q,"reason":"unknown_user"}`, req.Username))
 		middleware.WriteError(w, http.StatusUnauthorized, "invalid credentials", "unauthorized")
 		return
@@ -183,7 +183,7 @@ func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
 	// Security: Check account status before credential verification to avoid
 	// leaking whether the account exists based on timing differences.
 	if acct.Status != model.AccountStatusActive {
-		_, _ = auth.VerifyPassword("dummy", "$argon2id$v=19$m=65536,t=3,p=4$dGVzdHNhbHQ$dGVzdGhhc2g")
+		_, _ = auth.VerifyPassword("dummy", auth.DummyHash())
 		s.writeAudit(r, model.EventLoginFail, &acct.ID, nil, `{"reason":"account_inactive"}`)
 		middleware.WriteError(w, http.StatusUnauthorized, "invalid credentials", "unauthorized")
 		return

internal/ui/ui.go

@@ -27,6 +27,7 @@ import (
 	"sync"
 	"time"
 
+	"git.wntrmute.dev/kyle/mcias/internal/auth"
 	"git.wntrmute.dev/kyle/mcias/internal/config"
 	"git.wntrmute.dev/kyle/mcias/internal/db"
 	"git.wntrmute.dev/kyle/mcias/internal/model"
@@ -96,13 +97,11 @@ func (u *UIServer) consumeTOTPNonce(nonce string) (int64, bool) {
 	return pl.accountID, true
 }
 
-// dummyHash returns a hardcoded Argon2id PHC string used for constant-time
-// dummy password verification when the account is unknown or inactive.
-// Security: the dummy hash uses OWASP-recommended parameters (m=65536,t=3,p=4)
-// to match real verification timing. F-07 will replace this with a
-// sync.Once-computed real hash for exact parameter parity.
+// dummyHash returns the pre-computed Argon2id PHC hash for constant-time dummy
+// verification when an account is unknown or inactive (F-07).
+// Delegates to auth.DummyHash() which uses sync.Once for one-time computation.
 func (u *UIServer) dummyHash() string {
-	return "$argon2id$v=19$m=65536,t=3,p=4$dGVzdHNhbHQ$dGVzdGhhc2g"
+	return auth.DummyHash()
 }
 
 // New constructs a UIServer, parses all templates, and returns it.