[#2] Add methods sign and verify

frostfs_api/client/verifier.py

@@ -0,0 +1,18 @@
+import ecdsa
+from hashlib import sha256
+
+
+class Verifier:
+    def verify_rfc6979(self, public_key: bytes, message: bytes, signature: bytes) -> bool:
+        if len(public_key) == 0 or public_key is None:
+            raise ValueError(f"Incorrect public key: {public_key}")
+
+        if message is None or signature is None:
+            return False
+
+        vk = ecdsa.VerifyingKey.from_string(public_key, curve=ecdsa.SECP256k1, hashfunc=sha256)
+
+        try:
+            return vk.verify(signature, message)
+        except ecdsa.BadSignatureError:
+            return False