test(02): RED — failing argon2id password tests

- Six tests: HashVerify, VerifyWrong, VerifyMalformed, VerifyVersion, DistinctSaltsPerCall, DefaultParamsShape
- Tests use auth.TestParams for fast wall time (Pitfall 4 guard)
- Guards ErrInvalidHash + ErrIncompatibleVersion sentinel errors
- All tests fail with undefined: auth.Hash / auth.TestParams / auth.Verify (implementation missing)

backend/internal/auth/password_test.go

@@ -0,0 +1,134 @@
+package auth_test
+
+import (
+	"strings"
+	"testing"
+
+	"backend/internal/auth"
+)
+
+// TestPassword_HashVerify verifies that Hash returns a valid PHC string
+// starting with the expected argon2id prefix and that Verify confirms the
+// original password.
+func TestPassword_HashVerify(t *testing.T) {
+	const pw = "correct-horse-battery-staple"
+	phc, err := auth.Hash(pw, auth.TestParams)
+	if err != nil {
+		t.Fatalf("Hash returned error: %v", err)
+	}
+	if phc == "" {
+		t.Fatal("Hash returned empty string")
+	}
+
+	// TestParams: Memory=8192, Iterations=1, Parallelism=2
+	const wantPrefix = "$argon2id$v=19$m=8192,t=1,p=2$"
+	if !strings.HasPrefix(phc, wantPrefix) {
+		t.Errorf("PHC string does not start with expected prefix\n  got:  %s\n  want prefix: %s", phc, wantPrefix)
+	}
+
+	ok, err := auth.Verify(phc, pw)
+	if err != nil {
+		t.Fatalf("Verify returned unexpected error: %v", err)
+	}
+	if !ok {
+		t.Fatal("Verify returned false for the original password")
+	}
+}
+
+// TestPassword_VerifyWrong ensures that Verify returns (false, nil) — not an
+// error — when the supplied password does not match the hash.
+func TestPassword_VerifyWrong(t *testing.T) {
+	const pw = "correct-horse-battery-staple"
+	phc, err := auth.Hash(pw, auth.TestParams)
+	if err != nil {
+		t.Fatalf("Hash returned error: %v", err)
+	}
+
+	ok, err := auth.Verify(phc, pw+"x")
+	if err != nil {
+		t.Fatalf("Verify returned an unexpected error on wrong password: %v", err)
+	}
+	if ok {
+		t.Fatal("Verify returned true for a wrong password")
+	}
+}
+
+// TestPassword_VerifyMalformed checks that malformed or foreign-algorithm PHC
+// strings return (false, ErrInvalidHash).
+func TestPassword_VerifyMalformed(t *testing.T) {
+	cases := []struct {
+		name    string
+		encoded string
+	}{
+		{"plain string", "not-a-phc-string"},
+		{"too few segments", "$argon2id$v=19$m=8192,t=1,p=2$only-three-segments"},
+		{"wrong algorithm", "$bcrypt$v=19$m=8192,t=1,p=2$saltb64$hashb64"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			ok, err := auth.Verify(tc.encoded, "anypassword")
+			if ok {
+				t.Error("Verify returned true for malformed input")
+			}
+			if err != auth.ErrInvalidHash {
+				t.Errorf("Verify error = %v, want ErrInvalidHash", err)
+			}
+		})
+	}
+}
+
+// TestPassword_VerifyVersion ensures that a synthetic PHC string with v=18
+// (one less than argon2.Version == 19) returns (false, ErrIncompatibleVersion).
+func TestPassword_VerifyVersion(t *testing.T) {
+	// Craft a syntactically valid PHC string with an old version number.
+	// The segment count must be correct (6 after split on "$"):
+	// $argon2id$v=18$m=8192,t=1,p=2$<validb64salt>$<validb64hash>
+	synthetic := "$argon2id$v=18$m=8192,t=1,p=2$c29tZXNhbHQxNjBiYXNl$c29tZWhhc2gzMmJ5dGVzMDAwMDAwMDAwMDAwMDA"
+
+	ok, err := auth.Verify(synthetic, "anypassword")
+	if ok {
+		t.Error("Verify returned true for incompatible version")
+	}
+	if err != auth.ErrIncompatibleVersion {
+		t.Errorf("Verify error = %v, want ErrIncompatibleVersion", err)
+	}
+}
+
+// TestPassword_DistinctSaltsPerCall ensures that two Hash calls for the same
+// password produce different PHC strings (random salt per call — Pitfall guard).
+func TestPassword_DistinctSaltsPerCall(t *testing.T) {
+	const pw = "correct-horse-battery-staple"
+	phc1, err := auth.Hash(pw, auth.TestParams)
+	if err != nil {
+		t.Fatalf("first Hash returned error: %v", err)
+	}
+	phc2, err := auth.Hash(pw, auth.TestParams)
+	if err != nil {
+		t.Fatalf("second Hash returned error: %v", err)
+	}
+	if phc1 == phc2 {
+		t.Error("Two Hash calls for the same password returned identical PHC strings (salt reuse detected)")
+	}
+}
+
+// TestPassword_DefaultParamsShape guards against accidental drift in DefaultParams.
+// Values must match OWASP 2024 baseline as locked in D-08.
+func TestPassword_DefaultParamsShape(t *testing.T) {
+	p := auth.DefaultParams
+	if p.Memory != 64*1024 {
+		t.Errorf("DefaultParams.Memory = %d, want %d", p.Memory, 64*1024)
+	}
+	if p.Iterations != 1 {
+		t.Errorf("DefaultParams.Iterations = %d, want 1", p.Iterations)
+	}
+	if p.Parallelism != 4 {
+		t.Errorf("DefaultParams.Parallelism = %d, want 4", p.Parallelism)
+	}
+	if p.SaltLength != 16 {
+		t.Errorf("DefaultParams.SaltLength = %d, want 16", p.SaltLength)
+	}
+	if p.KeyLength != 32 {
+		t.Errorf("DefaultParams.KeyLength = %d, want 32", p.KeyLength)
+	}
+}