Implement SASL ANONYMOUS on the server side

Fixes #11.

sasl/src/server/mechanisms/anonymous.rs

@@ -0,0 +1,28 @@
+use crate::common::Identity;
+use crate::server::{Mechanism, MechanismError, Response};
+use getrandom::getrandom;
+
+pub struct Anonymous;
+
+impl Anonymous {
+    pub fn new() -> Anonymous {
+        Anonymous
+    }
+}
+
+impl Mechanism for Anonymous {
+    fn name(&self) -> &str {
+        "ANONYMOUS"
+    }
+
+    fn respond(&mut self, payload: &[u8]) -> Result<Response, MechanismError> {
+        if !payload.is_empty() {
+            return Err(MechanismError::FailedToDecodeMessage);
+        }
+        let mut rand = [0u8; 16];
+        getrandom(&mut rand)?;
+        let username = format!("{:02x?}", rand);
+        let ident = Identity::Username(username);
+        Ok(Response::Success(ident, Vec::new()))
+    }
+}

sasl/src/server/mechanisms/mod.rs

@@ -1,7 +1,9 @@
+mod anonymous;
 mod plain;
 #[cfg(feature = "scram")]
 mod scram;
 
+pub use self::anonymous::Anonymous;
 pub use self::plain::Plain;
 #[cfg(feature = "scram")]
 pub use self::scram::Scram;