configfile: add Validate() function, support FlagXChaCha20Poly1305

We used to do validation using lists of mandatory feature flags.

With the introduction of XChaCha20Poly1305, this became too
simplistic, as it uses a different IV length, hence disabling
GCMIV128.

Add a dedicated function, Validate(), with open-coded validation
logic.

The validation and creation logic also gets XChaCha20Poly1305
support, and gocryptfs -init -xchacha now writes the flag into
gocryptfs.conf.

1 files changed, 67 insertions, 0 deletions

diff --git a/internal/configfile/validate.go b/internal/configfile/validate.go
new file mode 100644
index 0000000..511f704
--- /dev/null
+++ b/internal/configfile/validate.go
@@ -0,0 +1,67 @@
+package configfile
+
+import (
+	"fmt"
+
+	"github.com/rfjakob/gocryptfs/v2/internal/contentenc"
+)
+
+// Validate that the combination of settings makes sense and is supported
+func (cf *ConfFile) Validate() error {
+	if cf.Version != contentenc.CurrentVersion {
+		return fmt.Errorf("Unsupported on-disk format %d", cf.Version)
+	}
+	// scrypt params ok?
+	if err := cf.ScryptObject.validateParams(); err != nil {
+		return err
+	}
+	// All feature flags that are in the config file are known?
+	for _, flag := range cf.FeatureFlags {
+		if !isFeatureFlagKnown(flag) {
+			return fmt.Errorf("Unknown feature flag %q", flag)
+		}
+	}
+	// File content encryption
+	{
+		switch {
+		case cf.IsFeatureFlagSet(FlagXChaCha20Poly1305) && cf.IsFeatureFlagSet(FlagAESSIV):
+			return fmt.Errorf("Can't have both XChaCha20Poly1305 and AESSIV feature flags")
+		case cf.IsFeatureFlagSet(FlagAESSIV):
+			if !cf.IsFeatureFlagSet(FlagGCMIV128) {
+				return fmt.Errorf("AESSIV requires GCMIV128 feature flag")
+			}
+		case cf.IsFeatureFlagSet(FlagXChaCha20Poly1305):
+			if cf.IsFeatureFlagSet(FlagGCMIV128) {
+				return fmt.Errorf("XChaCha20Poly1305 conflicts with GCMIV128 feature flag")
+			}
+			if !cf.IsFeatureFlagSet(FlagHKDF) {
+				return fmt.Errorf("XChaCha20Poly1305 requires HKDF feature flag")
+			}
+		// The absence of other flags means AES-GCM (oldest algorithm)
+		case !cf.IsFeatureFlagSet(FlagXChaCha20Poly1305) && !cf.IsFeatureFlagSet(FlagAESSIV):
+			if !cf.IsFeatureFlagSet(FlagGCMIV128) {
+				return fmt.Errorf("AES-GCM requires GCMIV128 feature flag")
+			}
+		}
+	}
+	// Filename encryption
+	{
+		switch {
+		case cf.IsFeatureFlagSet(FlagPlaintextNames) && cf.IsFeatureFlagSet(FlagEMENames):
+			return fmt.Errorf("Can't have both PlaintextNames and EMENames feature flags")
+		case cf.IsFeatureFlagSet(FlagPlaintextNames):
+			if cf.IsFeatureFlagSet(FlagDirIV) {
+				return fmt.Errorf("PlaintextNames conflicts with DirIV feature flag")
+			}
+			if cf.IsFeatureFlagSet(FlagLongNames) {
+				return fmt.Errorf("PlaintextNames conflicts with LongNames feature flag")
+			}
+			if cf.IsFeatureFlagSet(FlagRaw64) {
+				return fmt.Errorf("PlaintextNames conflicts with Raw64 feature flag")
+			}
+		case cf.IsFeatureFlagSet(FlagEMENames):
+			// All combinations of DirIV, LongNames, Raw64 allowed
+		}
+	}
+	return nil
+}