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/chacha.py

@@ -0,0 +1,65 @@
+"""ChaCha20-Poly1305 cipher backend."""
+
+import os
+import base64
+from typing import Tuple
+from cryptography.hazmat.primitives.ciphers.aead import ChaCha20Poly1305
+from .base import CipherBackend
+
+
+class ChaCha20Cipher(CipherBackend):
+    """ChaCha20-Poly1305 authenticated encryption."""
+
+    def encrypt(self, plaintext: bytes) -> Tuple[bytes, str]:
+        """
+        Encrypt using ChaCha20-Poly1305.
+
+        Returns:
+            Tuple of (nonce + ciphertext, base64url-encoded key)
+        """
+        # Generate random 256-bit key
+        key = ChaCha20Poly1305.generate_key()
+        chacha = ChaCha20Poly1305(key)
+
+        # Generate random 96-bit nonce
+        nonce = os.urandom(12)
+
+        # Encrypt and authenticate
+        ciphertext = chacha.encrypt(nonce, plaintext, None)
+
+        # Prepend nonce to ciphertext
+        encrypted_data = nonce + ciphertext
+
+        # Encode key for URL fragment
+        key_encoded = base64.urlsafe_b64encode(key).decode('ascii').rstrip('=')
+
+        return encrypted_data, key_encoded
+
+    def decrypt(self, ciphertext: bytes, key: str) -> bytes:
+        """
+        Decrypt using ChaCha20-Poly1305.
+
+        Args:
+            ciphertext: Nonce (12 bytes) + encrypted data
+            key: Base64url-encoded 256-bit key
+        """
+        # Decode key (add padding if needed)
+        padding = '=' * (4 - len(key) % 4) if len(key) % 4 else ''
+        key_bytes = base64.urlsafe_b64decode(key + padding)
+
+        if len(key_bytes) != 32:
+            raise ValueError("Invalid key length. Expected 32 bytes for ChaCha20.")
+
+        # Extract nonce and ciphertext
+        nonce = ciphertext[:12]
+        encrypted = ciphertext[12:]
+
+        # Decrypt
+        chacha = ChaCha20Poly1305(key_bytes)
+        plaintext = chacha.decrypt(nonce, encrypted, None)
+
+        return plaintext
+
+    def get_name(self) -> str:
+        """Return cipher name."""
+        return "chacha20-poly1305"