diff --git a/backup/fingerprint.go b/backup/fingerprint.go
new file mode 100644
index 0000000..5437653
--- /dev/null
+++ b/backup/fingerprint.go
@@ -0,0 +1,57 @@
+// Package backup dumps and restores a whole Vantage MongoDB database.
+//
+// The archive never contains KEY_ENCRYPTION_KEY. It contains a fingerprint of
+// it, which is enough to answer "will this archive restore into this
+// deployment" and is not a hint at the value.
+package backup
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+
+	"gitea.hostxtra.co.uk/mrhid6/vantage/shared/cryptobox"
+)
+
+// ErrNoKey is returned when no key was supplied at all. It is distinct from
+// ErrBadKey because the operator remedies are different: one is "set the
+// variable", the other is "the value you set is wrong".
+var ErrNoKey = errors.New("KEY_ENCRYPTION_KEY is not set")
+
+// ErrBadKey is returned when a key was supplied but is not 64 hex characters.
+var ErrBadKey = errors.New("KEY_ENCRYPTION_KEY must be a 64-character hex string (32 bytes)")
+
+// ParseKey decodes the hex form used by KEY_ENCRYPTION_KEY.
+func ParseKey(hexKey string) ([]byte, error) {
+	if hexKey == "" {
+		return nil, ErrNoKey
+	}
+	key, err := hex.DecodeString(hexKey)
+	if err != nil {
+		return nil, fmt.Errorf("%w: not hexadecimal", ErrBadKey)
+	}
+	if len(key) != cryptobox.KeySize {
+		return nil, fmt.Errorf("%w: decoded to %d bytes", ErrBadKey, len(key))
+	}
+	return key, nil
+}
+
+// Fingerprint is the SHA-256 of the raw key bytes, hex encoded.
+//
+// Of the raw bytes rather than of the hex string, so an operator who writes the
+// key in uppercase in one deployment and lowercase in another still gets one
+// fingerprint for one key.
+func Fingerprint(key []byte) string {
+	sum := sha256.Sum256(key)
+	return hex.EncodeToString(sum[:])
+}
+
+// FingerprintHex parses and fingerprints in one step.
+func FingerprintHex(hexKey string) (string, error) {
+	key, err := ParseKey(hexKey)
+	if err != nil {
+		return "", err
+	}
+	return Fingerprint(key), nil
+}
diff --git a/backup/fingerprint_test.go b/backup/fingerprint_test.go
new file mode 100644
index 0000000..d05fc7a
--- /dev/null
+++ b/backup/fingerprint_test.go
@@ -0,0 +1,61 @@
+package backup
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+const validKeyHex = "0000000000000000000000000000000000000000000000000000000000000001"
+
+func TestFingerprintIsStableAndNotTheKey(t *testing.T) {
+	fp, err := FingerprintHex(validKeyHex)
+	if err != nil {
+		t.Fatalf("FingerprintHex: %v", err)
+	}
+	if len(fp) != 64 {
+		t.Fatalf("fingerprint is %d chars, want 64", len(fp))
+	}
+	if strings.EqualFold(fp, validKeyHex) {
+		t.Fatal("fingerprint equals the key")
+	}
+	again, err := FingerprintHex(validKeyHex)
+	if err != nil {
+		t.Fatalf("FingerprintHex: %v", err)
+	}
+	if fp != again {
+		t.Fatal("fingerprint is not stable across calls")
+	}
+}
+
+func TestFingerprintDiffersPerKey(t *testing.T) {
+	other := "0000000000000000000000000000000000000000000000000000000000000002"
+	a, err := FingerprintHex(validKeyHex)
+	if err != nil {
+		t.Fatalf("FingerprintHex: %v", err)
+	}
+	b, err := FingerprintHex(other)
+	if err != nil {
+		t.Fatalf("FingerprintHex: %v", err)
+	}
+	if a == b {
+		t.Fatal("two different keys produced the same fingerprint")
+	}
+}
+
+func TestParseKeyRejections(t *testing.T) {
+	if _, err := ParseKey(""); !errors.Is(err, ErrNoKey) {
+		t.Fatalf("empty key: got %v, want ErrNoKey", err)
+	}
+	for _, bad := range []string{"zz", validKeyHex[:62], validKeyHex + "00"} {
+		if _, err := ParseKey(bad); !errors.Is(err, ErrBadKey) {
+			t.Fatalf("key %q: got %v, want ErrBadKey", bad, err)
+		}
+	}
+}
+
+func TestParseKeyAcceptsUppercase(t *testing.T) {
+	if _, err := ParseKey(strings.ToUpper(validKeyHex)); err != nil {
+		t.Fatalf("uppercase hex rejected: %v", err)
+	}
+}