Microsoft Authentication (#29)

* a

* try to do more work on auth signing (untested)

* well auth works when i remove the d= so

* auth stuff

* sessionserver stuff

* add auth in azalea-protocol/client

* caching*

refreshing microsoft auth tokens isn't implemented yet, also i haven't tested it

* how did i not notice that i had the code duplicated

* fix cache

* add refreshing msa token

* replace some printlns with log::trace

* auth works!

* Update main.rs

* fix clippy warnings

7 files changed, 707 insertions, 2 deletions

diff --git a/azalea-auth/Cargo.toml b/azalea-auth/Cargo.toml
index 2f817354..34fec8f7 100755
--- a/azalea-auth/Cargo.toml
+++ b/azalea-auth/Cargo.toml
@@ -9,4 +9,17 @@ version = "0.1.0"
 
 [dependencies]
 azalea-buf = {path = "../azalea-buf", version = "^0.1.0"}
+azalea-crypto = {path = "../azalea-crypto", version = "^0.1.0"}
+chrono = {version = "0.4.22", default-features = false}
+log = "0.4.17"
+num-bigint = "0.4.3"
+reqwest = {version = "0.11.12", features = ["json"]}
+serde = {version = "1.0.145", features = ["derive"]}
+serde_json = "1.0.86"
+thiserror = "1.0.37"
+tokio = "1.21.2"
 uuid = "^1.1.2"
+
+[dev-dependencies]
+env_logger = "0.9.1"
+tokio = {version = "1.21.2", features = ["full"]}
diff --git a/azalea-auth/README.md b/azalea-auth/README.md
index fa87afca..aa290c94 100644
--- a/azalea-auth/README.md
+++ b/azalea-auth/README.md
@@ -1,3 +1,5 @@
 # Azalea Auth
 
-A port of Mojang's Authlib, except authentication isn't actually implemented yet.
+A port of Mojang's Authlib and launcher authentication.
+
+Thanks to [wiki.vg contributors](https://wiki.vg/Microsoft_Authentication_Scheme), [Overhash](https://gist.github.com/OverHash/a71b32846612ba09d8f79c9d775bfadf), and [prismarine-auth contributors](https://github.com/PrismarineJS/prismarine-auth).
diff --git a/azalea-auth/examples/auth.rs b/azalea-auth/examples/auth.rs
new file mode 100644
index 00000000..8f7cf7f9
--- /dev/null
+++ b/azalea-auth/examples/auth.rs
@@ -0,0 +1,19 @@
+use std::path::PathBuf;
+
+#[tokio::main]
+async fn main() {
+    env_logger::init();
+
+    let cache_file = PathBuf::from("example_cache.json");
+
+    let auth_result = azalea_auth::auth(
+        "example@example.com",
+        azalea_auth::AuthOpts {
+            cache_file: Some(cache_file),
+            ..Default::default()
+        },
+    )
+    .await
+    .unwrap();
+    println!("{:?}", auth_result);
+}
diff --git a/azalea-auth/src/auth.rs b/azalea-auth/src/auth.rs
new file mode 100644
index 00000000..5f96d4be
--- /dev/null
+++ b/azalea-auth/src/auth.rs
@@ -0,0 +1,482 @@
+//! Handle Minecraft (Xbox) authentication.
+
+use crate::cache::{self, CachedAccount, ExpiringValue};
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+use serde_json::json;
+use std::{
+    collections::HashMap,
+    path::PathBuf,
+    time::{Instant, SystemTime, UNIX_EPOCH},
+};
+use thiserror::Error;
+
+#[derive(Default)]
+pub struct AuthOpts {
+    /// Whether we should check if the user actually owns the game. This will
+    /// fail if the user has Xbox Game Pass! Note that this isn't really
+    /// necessary, since getting the user profile will check this anyways.
+    pub check_ownership: bool,
+    // /// Whether we should get the Minecraft profile data (i.e. username, uuid,
+    // /// skin, etc) for the player.
+    // pub get_profile: bool,
+    /// The directory to store the cache in. If this is not set, caching is not
+    /// done.
+    pub cache_file: Option<PathBuf>,
+}
+
+#[derive(Debug, Error)]
+pub enum AuthError {
+    #[error(
+        "The Minecraft API is indicating that you don't own the game. \
+        If you're using Xbox Game Pass, set `check_ownership` to false in the auth options."
+    )]
+    DoesNotOwnGame,
+    #[error("Error getting Microsoft auth token: {0}")]
+    GetMicrosoftAuthToken(#[from] GetMicrosoftAuthTokenError),
+    #[error("Error refreshing Microsoft auth token: {0}")]
+    RefreshMicrosoftAuthToken(#[from] RefreshMicrosoftAuthTokenError),
+    #[error("Error getting Xbox Live auth token: {0}")]
+    GetXboxLiveAuthToken(#[from] MinecraftXstsAuthError),
+    #[error("Error getting Minecraft profile: {0}")]
+    GetMinecraftProfile(#[from] GetProfileError),
+    #[error("Error checking ownership: {0}")]
+    CheckOwnership(#[from] CheckOwnershipError),
+    #[error("Error getting Minecraft auth token: {0}")]
+    GetMinecraftAuthToken(#[from] MinecraftAuthError),
+    #[error("Error authenticating with Xbox Live: {0}")]
+    GetXboxLiveAuth(#[from] XboxLiveAuthError),
+}
+
+/// Authenticate with authenticate with Microsoft. If the data isn't cached,
+/// they'll be asked to go to log into Microsoft in a web page.
+///
+/// The email is technically only used as a cache key, so it *could* be
+/// anything. You should just have it be the actual email so it's not confusing
+/// though, and in case the Microsoft API does start providing the real email.
+pub async fn auth(email: &str, opts: AuthOpts) -> Result<AuthResult, AuthError> {
+    let cached_account = if let Some(cache_file) = &opts.cache_file && let Some(account) = cache::get_account_in_cache(cache_file, email).await {
+        Some(account)
+    } else { None };
+
+    // these two MUST be set by the end, since we return them in AuthResult
+    let profile: ProfileResponse;
+    let minecraft_access_token: String;
+
+    if let Some(account) = &cached_account && !account.mca.is_expired() {
+        // the minecraft auth data is cached and not expired, so we can just
+        // use that instead of doing auth all over again :)
+        profile = account.profile.clone();
+        minecraft_access_token = account.mca.data.access_token.clone();
+    } else {
+        let client = reqwest::Client::new();
+        let mut msa = if let Some(account) = cached_account {
+            account.msa
+        } else {
+            interactive_get_ms_auth_token(&client).await?
+        };
+        if msa.is_expired() {
+            log::trace!("refreshing Microsoft auth token");
+            msa = refresh_ms_auth_token(&client, &msa.data.refresh_token).await?;
+        }
+        let ms_access_token = &msa.data.access_token;
+        log::trace!("Got access token: {}", ms_access_token);
+
+        let xbl_auth = auth_with_xbox_live(&client, ms_access_token).await?;
+
+        let xsts_token = obtain_xsts_for_minecraft(
+            &client,
+            &xbl_auth
+                .get()
+                .expect("Xbox Live auth token shouldn't have expired yet")
+                .token,
+        )
+        .await?;
+
+        // Minecraft auth
+        let mca = auth_with_minecraft(&client, &xbl_auth.data.user_hash, &xsts_token).await?;
+
+         minecraft_access_token = mca
+            .get()
+            .expect("Minecraft auth shouldn't have expired yet")
+            .access_token
+            .to_string();
+
+        if opts.check_ownership {
+            let has_game = check_ownership(&client, &minecraft_access_token).await?;
+            if !has_game {
+                return Err(AuthError::DoesNotOwnGame);
+            }
+        }
+
+        profile = get_profile(&client, &minecraft_access_token).await?;
+
+        if let Some(cache_file) = opts.cache_file {
+            if let Err(e) = cache::set_account_in_cache(
+                &cache_file,
+                email,
+                CachedAccount {
+                    email: email.to_string(),
+                    mca,
+                    msa,
+                    xbl: xbl_auth,
+                    profile: profile.clone(),
+                },
+            )
+            .await {
+                log::warn!("Error while caching auth data: {}", e);
+            }
+        }
+    }
+
+    Ok(AuthResult {
+        access_token: minecraft_access_token,
+        profile,
+    })
+}
+
+#[derive(Debug)]
+pub struct AuthResult {
+    pub access_token: String,
+    pub profile: ProfileResponse,
+}
+
+#[derive(Debug, Deserialize)]
+pub struct DeviceCodeResponse {
+    user_code: String,
+    device_code: String,
+    verification_uri: String,
+    expires_in: u64,
+    interval: u64,
+}
+
+#[allow(unused)]
+#[derive(Debug, Deserialize, Serialize)]
+pub struct AccessTokenResponse {
+    token_type: String,
+    expires_in: u64,
+    scope: String,
+    access_token: String,
+    refresh_token: String,
+    user_id: String,
+}
+
+#[allow(unused)]
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "PascalCase")]
+pub struct XboxLiveAuthResponse {
+    issue_instant: String,
+    not_after: String,
+    token: String,
+    display_claims: HashMap<String, Vec<HashMap<String, String>>>,
+}
+
+/// Just the important data
+#[derive(Serialize, Deserialize, Debug)]
+pub struct XboxLiveAuth {
+    token: String,
+    user_hash: String,
+}
+
+#[allow(unused)]
+#[derive(Debug, Deserialize, Serialize)]
+pub struct MinecraftAuthResponse {
+    username: String,
+    roles: Vec<String>,
+    access_token: String,
+    token_type: String,
+    expires_in: u64,
+}
+
+#[allow(unused)]
+#[derive(Debug, Deserialize)]
+pub struct GameOwnershipResponse {
+    items: Vec<GameOwnershipItem>,
+    signature: String,
+    key_id: String,
+}
+
+#[allow(unused)]
+#[derive(Debug, Deserialize)]
+pub struct GameOwnershipItem {
+    name: String,
+    signature: String,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ProfileResponse {
+    pub id: String,
+    pub name: String,
+    pub skins: Vec<serde_json::Value>,
+    pub capes: Vec<serde_json::Value>,
+}
+
+// nintendo switch (so it works for accounts that are under 18 years old)
+const CLIENT_ID: &str = "00000000441cc96b";
+
+#[derive(Debug, Error)]
+pub enum GetMicrosoftAuthTokenError {
+    #[error("Http error: {0}")]
+    Http(#[from] reqwest::Error),
+    #[error("Authentication timed out")]
+    Timeout,
+}
+
+/// Asks the user to go to a webpage and log in with Microsoft.
+async fn interactive_get_ms_auth_token(
+    client: &reqwest::Client,
+) -> Result<ExpiringValue<AccessTokenResponse>, GetMicrosoftAuthTokenError> {
+    let res = client
+        .post("https://login.live.com/oauth20_connect.srf")
+        .form(&vec![
+            ("scope", "service::user.auth.xboxlive.com::MBI_SSL"),
+            ("client_id", CLIENT_ID),
+            ("response_type", "device_code"),
+        ])
+        .send()
+        .await?
+        .json::<DeviceCodeResponse>()
+        .await?;
+    log::trace!("Device code response: {:?}", res);
+    println!(
+        "Go to \x1b[1m{}\x1b[m and enter the code \x1b[1m{}\x1b[m",
+        res.verification_uri, res.user_code
+    );
+
+    let login_expires_at = Instant::now() + std::time::Duration::from_secs(res.expires_in);
+
+    while Instant::now() < login_expires_at {
+        tokio::time::sleep(std::time::Duration::from_secs(res.interval)).await;
+
+        log::trace!("Polling to check if user has logged in...");
+        if let Ok(access_token_response) = client
+            .post(format!(
+                "https://login.live.com/oauth20_token.srf?client_id={}",
+                CLIENT_ID
+            ))
+            .form(&vec![
+                ("client_id", CLIENT_ID),
+                ("device_code", &res.device_code),
+                ("grant_type", "urn:ietf:params:oauth:grant-type:device_code"),
+            ])
+            .send()
+            .await?
+            .json::<AccessTokenResponse>()
+            .await
+        {
+            log::trace!("access_token_response: {:?}", access_token_response);
+            let expires_at = SystemTime::now()
+                + std::time::Duration::from_secs(access_token_response.expires_in);
+            return Ok(ExpiringValue {
+                data: access_token_response,
+                expires_at: expires_at
+                    .duration_since(UNIX_EPOCH)
+                    .expect("Time went backwards")
+                    .as_secs(),
+            });
+        }
+    }
+
+    Err(GetMicrosoftAuthTokenError::Timeout)
+}
+
+#[derive(Debug, Error)]
+pub enum RefreshMicrosoftAuthTokenError {
+    #[error("Http error: {0}")]
+    Http(#[from] reqwest::Error),
+}
+
+async fn refresh_ms_auth_token(
+    client: &reqwest::Client,
+    refresh_token: &str,
+) -> Result<ExpiringValue<AccessTokenResponse>, RefreshMicrosoftAuthTokenError> {
+    let access_token_response = client
+        .post("https://login.live.com/oauth20_token.srf")
+        .form(&vec![
+            ("scope", "service::user.auth.xboxlive.com::MBI_SSL"),
+            ("client_id", CLIENT_ID),
+            ("grant_type", "refresh_token"),
+            ("refresh_token", refresh_token),
+        ])
+        .send()
+        .await?
+        .json::<AccessTokenResponse>()
+        .await?;
+
+    let expires_at =
+        SystemTime::now() + std::time::Duration::from_secs(access_token_response.expires_in);
+    Ok(ExpiringValue {
+        data: access_token_response,
+        expires_at: expires_at
+            .duration_since(UNIX_EPOCH)
+            .expect("Time went backwards")
+            .as_secs(),
+    })
+}
+
+#[derive(Debug, Error)]
+pub enum XboxLiveAuthError {
+    #[error("Http error: {0}")]
+    Http(#[from] reqwest::Error),
+    #[error("Invalid expiry date: {0}")]
+    InvalidExpiryDate(String),
+}
+
+async fn auth_with_xbox_live(
+    client: &reqwest::Client,
+    access_token: &str,
+) -> Result<ExpiringValue<XboxLiveAuth>, XboxLiveAuthError> {
+    let auth_json = json!({
+        "Properties": {
+            "AuthMethod": "RPS",
+            "SiteName": "user.auth.xboxlive.com",
+            // i thought this was supposed to be d={} but it doesn't work for
+            // me when i add it ??????
+            "RpsTicket": format!("{}", access_token)
+        },
+        "RelyingParty": "http://auth.xboxlive.com",
+        "TokenType": "JWT"
+    });
+    let payload = auth_json.to_string();
+    log::trace!("auth_json: {:#?}", auth_json);
+    let res = client
+        .post("https://user.auth.xboxlive.com/user/authenticate")
+        .header("Content-Type", "application/json")
+        .header("Accept", "application/json")
+        .header("x-xbl-contract-version", "1")
+        // .header("Cache-Control", "no-store, must-revalidate, no-cache")
+        // .header("Signature", base64::encode(signature))
+        .body(payload)
+        .send()
+        .await?
+        .json::<XboxLiveAuthResponse>()
+        .await?;
+    log::trace!("Xbox Live auth response: {:?}", res);
+
+    // not_after looks like 2020-12-21T19:52:08.4463796Z
+    let expires_at = DateTime::parse_from_rfc3339(&res.not_after)
+        .map_err(|e| XboxLiveAuthError::InvalidExpiryDate(format!("{}: {}", res.not_after, e)))?
+        .with_timezone(&Utc)
+        .timestamp() as u64;
+    Ok(ExpiringValue {
+        data: XboxLiveAuth {
+            token: res.token,
+            user_hash: res.display_claims["xui"].get(0).unwrap()["uhs"].clone(),
+        },
+        expires_at,
+    })
+}
+
+#[derive(Debug, Error)]
+pub enum MinecraftXstsAuthError {
+    #[error("Http error: {0}")]
+    Http(#[from] reqwest::Error),
+}
+
+async fn obtain_xsts_for_minecraft(
+    client: &reqwest::Client,
+    xbl_auth_token: &str,
+) -> Result<String, MinecraftXstsAuthError> {
+    let res = client
+        .post("https://xsts.auth.xboxlive.com/xsts/authorize")
+        .header("Accept", "application/json")
+        .json(&json!({
+            "Properties": {
+                "SandboxId": "RETAIL",
+                "UserTokens": [xbl_auth_token.to_string()]
+            },
+            "RelyingParty": "rp://api.minecraftservices.com/",
+            "TokenType": "JWT"
+        }))
+        .send()
+        .await?
+        .json::<XboxLiveAuthResponse>()
+        .await?;
+    log::trace!("Xbox Live auth response (for XSTS): {:?}", res);
+
+    Ok(res.token)
+}
+
+#[derive(Debug, Error)]
+pub enum MinecraftAuthError {
+    #[error("Http error: {0}")]
+    Http(#[from] reqwest::Error),
+}
+
+async fn auth_with_minecraft(
+    client: &reqwest::Client,
+    user_hash: &str,
+    xsts_token: &str,
+) -> Result<ExpiringValue<MinecraftAuthResponse>, MinecraftAuthError> {
+    let res = client
+        .post("https://api.minecraftservices.com/authentication/login_with_xbox")
+        .header("Accept", "application/json")
+        .json(&json!({
+            "identityToken": format!("XBL3.0 x={};{}", user_hash, xsts_token)
+        }))
+        .send()
+        .await?
+        .json::<MinecraftAuthResponse>()
+        .await?;
+    log::trace!("{:?}", res);
+
+    let expires_at = SystemTime::now() + std::time::Duration::from_secs(res.expires_in);
+    Ok(ExpiringValue {
+        data: res,
+        // to seconds since epoch
+        expires_at: expires_at.duration_since(UNIX_EPOCH).unwrap().as_secs(),
+    })
+}
+
+#[derive(Debug, Error)]
+pub enum CheckOwnershipError {
+    #[error("Http error: {0}")]
+    Http(#[from] reqwest::Error),
+}
+
+async fn check_ownership(
+    client: &reqwest::Client,
+    minecraft_access_token: &str,
+) -> Result<bool, CheckOwnershipError> {
+    let res = client
+        .get("https://api.minecraftservices.com/entitlements/mcstore")
+        .header(
+            "Authorization",
+            format!("Bearer {}", minecraft_access_token),
+        )
+        .send()
+        .await?
+        .json::<GameOwnershipResponse>()
+        .await?;
+    log::trace!("{:?}", res);
+
+    // vanilla checks here to make sure the signatures are right, but it's not
+    // actually required so we just don't
+
+    Ok(!res.items.is_empty())
+}
+
+#[derive(Debug, Error)]
+pub enum GetProfileError {
+    #[error("Http error: {0}")]
+    Http(#[from] reqwest::Error),
+}
+
+async fn get_profile(
+    client: &reqwest::Client,
+    minecraft_access_token: &str,
+) -> Result<ProfileResponse, GetProfileError> {
+    let res = client
+        .get("https://api.minecraftservices.com/minecraft/profile")
+        .header(
+            "Authorization",
+            format!("Bearer {}", minecraft_access_token),
+        )
+        .send()
+        .await?
+        .json::<ProfileResponse>()
+        .await?;
+    log::trace!("{:?}", res);
+
+    Ok(res)
+}
diff --git a/azalea-auth/src/cache.rs b/azalea-auth/src/cache.rs
new file mode 100644
index 00000000..8af9e171
--- /dev/null
+++ b/azalea-auth/src/cache.rs
@@ -0,0 +1,105 @@
+//! Cache auth information
+
+use serde::{Deserialize, Serialize};
+use std::path::Path;
+use std::time::{SystemTime, UNIX_EPOCH};
+use thiserror::Error;
+use tokio::fs::File;
+use tokio::io::{AsyncReadExt, AsyncWriteExt};
+
+#[derive(Debug, Error)]
+pub enum CacheError {
+    #[error("Failed to read cache file: {0}")]
+    Read(std::io::Error),
+    #[error("Failed to write cache file: {0}")]
+    Write(std::io::Error),
+    #[error("Failed to parse cache file: {0}")]
+    Parse(serde_json::Error),
+}
+
+#[derive(Deserialize, Serialize, Debug)]
+pub struct CachedAccount {
+    pub email: String,
+    /// Microsoft auth
+    pub msa: ExpiringValue<crate::auth::AccessTokenResponse>,
+    /// Xbox Live auth
+    pub xbl: ExpiringValue<crate::auth::XboxLiveAuth>,
+    /// Minecraft auth
+    pub mca: ExpiringValue<crate::auth::MinecraftAuthResponse>,
+    /// The user's Minecraft profile (i.e. username, UUID, skin)
+    pub profile: crate::auth::ProfileResponse,
+}
+
+#[derive(Deserialize, Serialize, Debug)]
+pub struct ExpiringValue<T> {
+    /// Seconds since the UNIX epoch
+    pub expires_at: u64,
+    pub data: T,
+}
+
+impl<T> ExpiringValue<T> {
+    pub fn is_expired(&self) -> bool {
+        self.expires_at
+            < SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .unwrap()
+                .as_secs()
+    }
+
+    /// Return the data if it's not expired, otherwise return `None`
+    pub fn get(&self) -> Option<&T> {
+        if self.is_expired() {
+            None
+        } else {
+            Some(&self.data)
+        }
+    }
+}
+
+async fn get_entire_cache(cache_file: &Path) -> Result<Vec<CachedAccount>, CacheError> {
+    let mut cache: Vec<CachedAccount> = Vec::new();
+    if cache_file.exists() {
+        let mut cache_file = File::open(cache_file).await.map_err(CacheError::Read)?;
+        // read the file into a string
+        let mut contents = String::new();
+        cache_file
+            .read_to_string(&mut contents)
+            .await
+            .map_err(CacheError::Read)?;
+        cache = serde_json::from_str(&contents).map_err(CacheError::Parse)?;
+    }
+    Ok(cache)
+}
+async fn set_entire_cache(cache_file: &Path, cache: Vec<CachedAccount>) -> Result<(), CacheError> {
+    println!("saving cache: {:?}", cache);
+
+    let mut cache_file = File::create(cache_file).await.map_err(CacheError::Write)?;
+    let cache = serde_json::to_string_pretty(&cache).map_err(CacheError::Parse)?;
+    cache_file
+        .write_all(cache.as_bytes())
+        .await
+        .map_err(CacheError::Write)?;
+
+    Ok(())
+}
+
+/// Gets cached data for the given email.
+///
+/// Technically it doesn't actually have to be an email since it's only the
+/// cache key. I considered using usernames or UUIDs as the cache key, but
+/// usernames change and no one has their UUID memorized.
+pub async fn get_account_in_cache(cache_file: &Path, email: &str) -> Option<CachedAccount> {
+    let cache = get_entire_cache(cache_file).await.unwrap_or_default();
+    cache.into_iter().find(|account| account.email == email)
+}
+
+pub async fn set_account_in_cache(
+    cache_file: &Path,
+    email: &str,
+    account: CachedAccount,
+) -> Result<(), CacheError> {
+    let mut cache = get_entire_cache(cache_file).await.unwrap_or_default();
+    cache.retain(|account| account.email != email);
+    cache.push(account);
+    set_entire_cache(cache_file, cache).await
+}
diff --git a/azalea-auth/src/lib.rs b/azalea-auth/src/lib.rs
index 773ea1d9..03e15c71 100755
--- a/azalea-auth/src/lib.rs
+++ b/azalea-auth/src/lib.rs
@@ -1,3 +1,8 @@
-//! Handle Minecraft authentication.
+#![feature(let_chains)]
 
+mod auth;
+mod cache;
 pub mod game_profile;
+pub mod sessionserver;
+
+pub use auth::*;
diff --git a/azalea-auth/src/sessionserver.rs b/azalea-auth/src/sessionserver.rs
new file mode 100644
index 00000000..31857bc0
--- /dev/null
+++ b/azalea-auth/src/sessionserver.rs
@@ -0,0 +1,79 @@
+//! Tell Mojang you're joining a multiplayer server.
+//!
+use serde::Deserialize;
+use serde_json::json;
+use thiserror::Error;
+use uuid::Uuid;
+
+#[derive(Debug, Error)]
+pub enum SessionServerError {
+    #[error("Error sending HTTP request to sessionserver: {0}")]
+    HttpError(#[from] reqwest::Error),
+    #[error("Multiplayer is not enabled for this account")]
+    MultiplayerDisabled,
+    #[error("This account has been banned from multiplayer")]
+    Banned,
+    #[error("Unknown sessionserver error: {0}")]
+    Unknown(String),
+    #[error("Unexpected response from sessionserver (status code {status_code}): {body}")]
+    UnexpectedResponse { status_code: u16, body: String },
+}
+
+#[derive(Deserialize)]
+pub struct ForbiddenError {
+    pub error: String,
+    pub path: String,
+}
+
+/// Tell Mojang's servers that you are going to join a multiplayer server,
+/// which is required to join online-mode servers. The server ID is an empty
+/// string.
+pub async fn join(
+    access_token: &str,
+    public_key: &[u8],
+    private_key: &[u8],
+    uuid: &Uuid,
+    server_id: &str,
+) -> Result<(), SessionServerError> {
+    let client = reqwest::Client::new();
+
+    let server_hash = azalea_crypto::hex_digest(&azalea_crypto::digest_data(
+        server_id.as_bytes(),
+        public_key,
+        private_key,
+    ));
+
+    let mut encode_buffer = Uuid::encode_buffer();
+    let undashed_uuid = uuid.simple().encode_lower(&mut encode_buffer);
+
+    let data = json!({
+        "accessToken": access_token,
+        "selectedProfile": undashed_uuid,
+        "serverId": server_hash
+    });
+    println!("data: {:?}", data);
+    let res = client
+        .post("https://sessionserver.mojang.com/session/minecraft/join")
+        .json(&data)
+        .send()
+        .await?;
+
+    match res.status() {
+        reqwest::StatusCode::NO_CONTENT => Ok(()),
+        reqwest::StatusCode::FORBIDDEN => {
+            let forbidden = res.json::<ForbiddenError>().await?;
+            match forbidden.error.as_str() {
+                "InsufficientPrivilegesException" => Err(SessionServerError::MultiplayerDisabled),
+                "UserBannedException" => Err(SessionServerError::Banned),
+                _ => Err(SessionServerError::Unknown(forbidden.error)),
+            }
+        }
+        status_code => {
+            let body = res.text().await?;
+            Err(SessionServerError::UnexpectedResponse {
+                status_code: status_code.as_u16(),
+                body,
+            })
+        }
+    }
+}