diff --git a/demo/__init__.py b/demo/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/demo/httpsig.py b/demo/httpsig.py
new file mode 100644
index 0000000..979c235
--- /dev/null
+++ b/demo/httpsig.py
@@ -0,0 +1,95 @@
+#!/usr/bin/env python3
+"""
+Verify and build HTTP signatures
+"""
+import base64
+
+from Crypto.Hash import SHA256
+from Crypto.Signature import PKCS1_v1_5
+from Crypto.PublicKey import RSA
+
+
+class HttpSignature:
+    """
+    calculation and verification of HTTP signatures
+    """
+    @classmethod
+    def calculation_digest(
+            cls,
+            body : bytes,
+            algorithm : str ="sha-256"
+    ) -> str:
+        """
+        Calculates the digest header value for a given HTTP body
+        """
+        if "sha-256" == algorithm:
+            body_digest = SHA256.new()
+            body_digest.update(body)
+            return "SHA-256=" + \
+                base64.b64encode(body_digest.digest()
+                                 ).decode("utf-8")
+
+        raise ValueError(f"No support algorithm {algorithm}")
+
+    @classmethod
+    def verify_signature(
+            cls,
+            signature_string : str,
+            signature : bytes,
+            pubkey,
+    ) -> bool:
+        """
+        Verify signatur that returns bool
+        """
+        pubkey = RSA.importKey(pubkey)
+        signer = PKCS1_v1_5.new(pubkey)
+        digest = SHA256.new()
+        digest.update(signature_string.encode("utf-8"))
+        return signer.verify(digest, signature)
+
+
+    @classmethod
+    def parse_signature(
+            cls,
+            signature : str
+    ) -> dict:
+        """
+        Parse signature string in headers
+        """
+        detail = {}
+        for item in signature.split(','):
+            name, value = item.split('=', 1)
+            value = value.strip('"')
+            detail[name.lower()] = value
+        signature_details = {
+            "headers": detail["headers"].split(),
+            "signature": base64.b64decode(detail["signature"]),
+            "algorithm": detail["algorithm"],
+            "keyid": detail["keyid"],
+        }
+        return signature_details
+
+    @classmethod
+    def build_signature_string(
+            cls,
+            method : str,
+            path : str,
+            signed_headers : list,
+            body_digest : str | None,
+            headers,
+    ) -> str:
+        """
+        Build signature string
+        """
+        signed_string = []
+        for signed_header in signed_headers:
+            if signed_header == "(request-target)":
+                signed_string.append("(request-target): "
+                                     + method.lower() + ' ' + path)
+            elif signed_header == "digest" and body_digest:
+                signed_string.append("digest: " + body_digest)
+            else:
+                signed_string.append(signed_header + ": "
+                                     + headers[signed_header])
+
+        return "\n".join(signed_string)