fixup! fixup! Support cryptography module

src/modules/crypto/mod.rs

@@ -3,6 +3,57 @@
 //!
 //! This module depends on underlying cryptography library. One crypto adaptors MUST be specified
 //! during the configuration, then it's compiled into RustyVault.
+//!
+//! # Crypto Adaptor Configurations
+//!
+//! In RustyVault, the real cryptographic operations are done via "crypto_adaptor"s.
+//!
+//! A crypto adaptor is a module that conveys and translates high level cryptography
+//! operations like encryption, signing into the APIs provided by underlying cryptography
+//! libraries such as OpenSSL, Tongsuo and so forth.
+//! 
+//! At current stage, only one crypto_adaptor can be enabled at compilation phase and later
+//! be used at run-time. "crypto_adaptor"s are configured as 'feature's in the Cargo context.
+//! 
+//! Currently, the supported feature names of crypto adaptors are as follows, you can enable
+//! them by adding one '--features crypto_adaptor_name' option when running "cargo build":
+//!
+//! 1. the OpenSSL adaptor: crypto_adaptor_openssl
+//! 2. the Tongsuo adaptor: crypto_adaptor_tongsuo
+//!
+//! If there is no explicit crypto adpator configured, then the `crypto_adaptor_openssl` is used as
+//! the default option.
+//!
+//! # Enable the Tongsuo adaptor
+//!
+//! Tongsuo is a variant of OpenSSL but with more features on SMx algorithms and protocols.
+//! RustyVault can use SM algorithms only if Tongsuo is built as the crypto adaptor.
+//!
+//! You need to build and install Tongsuo first into your local environment before building
+//! RustyVault with Tongsuo. Check the following link for detailed installation steps:
+//! [Tongsuo](https://github.com/Tongsuo-Project/Tongsuo)
+//!
+//! ~~~text
+//! $ export OPENSSL_DIR=/path/to/tongsuo/install/directory
+//! $ cargo build --features crypto_adaptor_tongsuo \
+//!    --no-default-features \
+//!    --config 'patch.crates-io.openssl.git="https://github.com/Tongsuo-Project/rust-tongsuo.git"'\
+//!    --config 'patch.crates-io.openssl-sys.git="https://github.com/Tongsuo-Project/rust-tongsuo.git"'
+//! ~~~
+//!
+//! Or you can just uncomment the following lines in Cargo.toml:
+//!
+//! ~~~text
+//! #[patch.crates-io]
+//! #openssl = { git = "https://github.com/Tongsuo-Project/rust-tongsuo.git" }
+//! #openssl-sys = { git = "https://github.com/Tongsuo-Project/rust-tongsuo.git" }
+//! ~~~
+//!
+//! and then:
+//!
+//! ~~~text
+//! $ cargo build --features crypto_adaptor_tongsuo --no-default-features
+//! ~~~
 
 use crate::errors::RvError;
 #[cfg(feature = "crypto_adaptor_openssl")]