[#2] Add methods sign and verify

frostfs_api/client/signer.py

@@ -0,0 +1,15 @@
+import ecdsa
+from hashlib import sha256
+
+
+class Signer:
+    def sign_rfc6979(self, private_key: bytes, message: bytes) -> bytes:
+        if len(private_key) == 0 or private_key is None:
+            raise ValueError(f"Incorrect private_key: {private_key}")
+
+        # SECP256k1 is the Bitcoin elliptic curve
+        sk = ecdsa.SigningKey.from_string(private_key, curve=ecdsa.SECP256k1, hashfunc=sha256)
+
+        signature = sk.sign(message, sigencode=ecdsa.util.sigencode_string)
+
+        return signature