Add examples to crypto::sign

//! Message signatures.

//!

//! The `Signer` allows for the computation of cryptographic signatures of

//! data given a private key. The `Verifier` can then be used with the

//! corresponding public key to verify the integrity and authenticity of that

//! data given the signature.

//!

//! # Examples

//!

//! Sign and verify data given an RSA keypair:

//!

//! ```rust

//! use openssl::crypto::sign::{Signer, Verifier};

//! use openssl::crypto::rsa::RSA;

//! use openssl::crypto::pkey::PKey;

//! use openssl::crypto::hash::Type;

//!

//! // Generate a keypair

//! let keypair = RSA::generate(2048).unwrap();

//! let keypair = PKey::from_rsa(keypair).unwrap();

//!

//! let data = b"hello, world!";

//! let data2 = b"hola, mundo!";

//!

//! // Sign the data

//! let mut signer = Signer::new(Type::SHA256, &keypair).unwrap();

//! signer.update(data).unwrap();

//! signer.update(data2).unwrap();

//! let signature = signer.finish().unwrap();

//!

//! // Verify the data

//! let mut verifier = Verifier::new(Type::SHA256, &keypair).unwrap();

//! verifier.update(data).unwrap();

//! verifier.update(data2).unwrap();

//! assert!(verifier.finish(&signature).unwrap());

//! ```

//!

//! Compute an HMAC (note that `Verifier` cannot be used with HMACs):

//!

//! ```rust

//! use openssl::crypto::sign::Signer;

//! use openssl::crypto::pkey::PKey;

//! use openssl::crypto::hash::Type;

//!

//! // Create a PKey

//! let key = PKey::hmac(b"my secret").unwrap();

//!

//! let data = b"hello, world!";

//! let data2 = b"hola, mundo!";

//!

//! // Compute the HMAC

//! let mut signer = Signer::new(Type::SHA256, &key).unwrap();

//! signer.update(data).unwrap();

//! signer.update(data2).unwrap();

//! let hmac = signer.finish().unwrap();

//! ```

use ffi;

use std::io::{self, Write};

use std::marker::PhantomData;