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>

providers/base.py

@@ -0,0 +1,45 @@
+"""Base pastebin provider interface."""
+
+from abc import ABC, abstractmethod
+from typing import Optional
+
+
+class PastebinProvider(ABC):
+    """Abstract base class for pastebin service providers."""
+
+    @abstractmethod
+    def paste(self, content: bytes, **kwargs) -> str:
+        """
+        Upload content to the pastebin service.
+
+        Args:
+            content: Raw bytes to upload
+            **kwargs: Provider-specific options (e.g., expiration, syntax)
+
+        Returns:
+            The paste ID or URL (without fragment)
+        """
+        pass
+
+    @abstractmethod
+    def fetch(self, paste_id: str) -> bytes:
+        """
+        Fetch content from the pastebin service.
+
+        Args:
+            paste_id: The paste identifier or URL
+
+        Returns:
+            Raw bytes of the paste content
+        """
+        pass
+
+    @abstractmethod
+    def get_name(self) -> str:
+        """Return the name of this provider."""
+        pass
+
+    @abstractmethod
+    def get_base_url(self) -> str:
+        """Return the base URL for this provider."""
+        pass