Writes and verify image

clitools/pkg/catalog/checksums.go

@@ -0,0 +1,30 @@
+package catalog
+
+import (
+	"encoding/hex"
+	"fmt"
+	"strings"
+)
+
+func (c *CatalogImage) SHA256() (string, error) {
+	if c.Checksum == "" {
+		return "", fmt.Errorf("checksum is empty")
+	}
+
+	const prefix = "sha256:"
+	if !strings.HasPrefix(c.Checksum, prefix) {
+		return "", fmt.Errorf("unsupported checksum format (expected sha256:...)")
+	}
+
+	hash := strings.TrimPrefix(c.Checksum, prefix)
+
+	if len(hash) != 64 {
+		return "", fmt.Errorf("invalid sha256 length: got %d, want 64", len(hash))
+	}
+
+	if _, err := hex.DecodeString(hash); err != nil {
+		return "", fmt.Errorf("invalid sha256 hex: %w", err)
+	}
+
+	return hash, nil
+}