Initial commit: SecPaste encrypted pastebin client

SecPaste is a Python library and CLI tool for sharing encrypted content via
public pastebin services with zero-knowledge architecture.

Features:
- Pluggable crypto backends (AES-256-GCM, ChaCha20-Poly1305, Kyber-768)
- Pluggable pastebin providers (dpaste.com, extensible)
- URL fragment key storage (key never sent to server)
- Both CLI and library usage
- Post-quantum cryptography support (experimental)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

crypto/base.py

@@ -0,0 +1,40 @@
+"""Base cryptographic interface for SecPaste."""
+
+from abc import ABC, abstractmethod
+from typing import Tuple
+
+
+class CipherBackend(ABC):
+    """Abstract base class for cipher implementations."""
+
+    @abstractmethod
+    def encrypt(self, plaintext: bytes) -> Tuple[bytes, str]:
+        """
+        Encrypt plaintext data.
+
+        Args:
+            plaintext: Raw bytes to encrypt
+
+        Returns:
+            Tuple of (ciphertext, key) where key is base64url-encoded
+        """
+        pass
+
+    @abstractmethod
+    def decrypt(self, ciphertext: bytes, key: str) -> bytes:
+        """
+        Decrypt ciphertext using the provided key.
+
+        Args:
+            ciphertext: Encrypted data
+            key: Base64url-encoded encryption key
+
+        Returns:
+            Decrypted plaintext bytes
+        """
+        pass
+
+    @abstractmethod
+    def get_name(self) -> str:
+        """Return the name/identifier of this cipher backend."""
+        pass