Expand quickstart - add basic operations

Author: tomato42

docs/source/quickstart.rst

@@ -6,6 +6,12 @@ The library has just one mandatory dependency: ``six``.
 If you install ``python-ecdsa`` through pip, it should automatically
 install ``six`` too.
 
+To install it you can run the following command:
+
+.. code:: bash
+
+    pip install ecdsa
+
 The high level API provided by the library is primarily in the
 :py:class:`~ecdsa.keys` module.
 There you will find the :py:class:`~ecdsa.keys.SigningKey` (the class
@@ -19,3 +25,154 @@ is used.
 Finally, in case use of custom elliptic curves is necessary, the
 :py:class:`~ecdsa.curves.Curve` class may be needed.
 
+Key generation
+==============
+
+To generate a key, import the :py:class:`~ecdsa.keys.SigningKey` and
+call the :py:func:`~ecdsa.keys.SigningKey.generate` function in it:
+
+.. code:: python
+
+    from ecdsa.keys import SigningKey
+
+    key = SigningKey.generate()
+
+By default, that will create a key that uses the NIST P-192 curve. To
+select a more secure curve, like NIST P-256, import it from the
+:py:mod:`ecdsa.curves` or from the :py:mod:`ecdsa` module:
+
+.. code:: python
+
+    from ecdsa import SigningKey, NIST256p
+
+    key = SigningKey.generate(curve=NIST256p)
+
+Private key storage and retrieval
+=================================
+
+To store a key as string or file, you can serialise it using many formats,
+in general we recommend the PKCS#8 PEM encoding.
+
+If you have a :py:class:`~ecdsa.keys.SigningKey` object in ``key`` and
+want to save it to a file like ``priv_key.pem`` you can run the following
+code:
+
+.. code:: python
+
+    with open("priv_key.pem", "wb") as f:
+        f.write(key.to_pem(format="pkcs8"))
+
+.. warning::
+
+    Not specifying the ``format=pkcs8`` will create a file that uses the legacy
+    ``ssleay`` file format which is most commonly used by applications
+    that use OpenSSL, as that was originally the only format supported by it.
+    For a long time though OpenSSL supports the PKCS# 8 format too.
+
+To read that file back, you can run code like this:
+
+.. code:: python
+
+    from ecdsa import SigningKey
+
+    with open("priv_key.pem") as f:
+        key = SigningKey.from_pem(f.read())
+
+.. tip::
+
+    As the format is self-describing, the parser will automatically detect
+    if the provided file is in the ``ssleay`` or the ``pkcs8`` format
+    and process it accordingly.
+
+Public key derivation
+=====================
+
+To get the public key associated with the given private key, either
+call the :py:func:`~ecdsa.keys.SigningKey.get_verifying_key` method or
+access the ``verifying_key`` attribute in
+:py:class:`~ecdsa.keys.SigningKey` directly:
+
+.. code:: python
+
+    from ecdsa import SigningKey, NIST256p
+
+    private_key = SigningKey.generate(curve=NIST256p)
+
+    public_key = private_key.verifying_key
+
+Public key storage and retrieval
+================================
+
+Similarly to private keys, public keys can be stored in files:
+
+.. code:: python
+
+    from ecdsa import SigningKey
+
+    private_key = SigningKey.generate()
+
+    public_key = private_key.verifying_key
+
+    with open("pub_key.pem", "wb") as f:
+        f.write(public_key.to_pem())
+
+And read from files:
+
+.. code:: python
+
+    from ecdsa import VerifyingKey
+
+    with open("pub_key.pem") as f:
+        public_key = VerifyingKey.from_pem(f.read())
+
+Signing
+=======
+
+To sign a byte string stored in variable ``message`` using SigningKey in
+``private_key``, SHA-256, get a signature in the DER format and save it to a
+file, you can use the following code:
+
+.. code:: python
+
+    from hashlib import sha256
+    from ecdsa.util import sigencode_der
+
+    sig = private_key.sign_deterministic(
+        message,
+        hashfunc=sha256,
+        sigencode=sigencode_der
+    )
+
+    with open("message.sig", "wb") as f:
+        f.write(sig)
+
+.. note::
+
+    As cryptographic hashes (SHA-256, SHA3-256, etc.) operate on *bytes* not
+    text strings, any text needs to be serialised into *bytes* before it can
+    be signed. This is because encoding of string "text" results in very
+    different bytes when it's encoded using UTF-8 and when it's encoded using
+    UCS-2.
+
+Verifying
+=========
+
+To verify a signature of a byte string in ``message`` using a VerifyingKey
+in ``public_key``, SHA-256 and a DER signature in a ``message.sig`` file,
+you can use the following code:
+
+.. code:: python
+
+    from hashlib import sha256
+    from ecdsa import BadSignatureError
+    from ecdsa.util import sigdecode_der
+
+    with open("message.sig", "rb") as f:
+        sig = f.read()
+
+    try:
+        ret = public_key.verify(sig, message, sha256, sigdecode=sigdecode_der)
+        assert ret
+        print("Valid signature")
+    except BadSignatureError:
+        print("Incorrect signature")