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    //! SSH host key validation support.
    //!
    //! The only public item in this module is [`certificate_check`],
    //! which provides a callback to [`git2::RemoteCallbacks::certificate_check`].
    //!
    //! A primary goal with this implementation is to provide user-friendly error
    //! messages, guiding them to understand the issue and how to resolve it.
    //!
    //! Note that there are a lot of limitations here. This reads OpenSSH
    //! known_hosts files from well-known locations, but it does not read OpenSSH
    //! config files. The config file can change the behavior of how OpenSSH
    //! handles known_hosts files. For example, some things we don't handle:
    //!
    //! - `GlobalKnownHostsFile` — Changes the location of the global host file.
    //! - `UserKnownHostsFile` — Changes the location of the user's host file.
    //! - `KnownHostsCommand` — A command to fetch known hosts.
    //! - `CheckHostIP` — DNS spoofing checks.
    //! - `VisualHostKey` — Shows a visual ascii-art key.
    //! - `VerifyHostKeyDNS` — Uses SSHFP DNS records to fetch a host key.
    //!
    //! There's also a number of things that aren't supported but could be easily
    //! added (it just adds a little complexity). For example, hostname patterns,
    //! and revoked markers. See "FIXME" comments littered in this file.

    use crate::util::context::{Definition, GlobalContext, Value};
    use base64::engine::general_purpose::STANDARD;
    use base64::engine::general_purpose::STANDARD_NO_PAD;
    use base64::Engine as _;
    use git2::cert::{Cert, SshHostKeyType};
    use git2::CertificateCheckStatus;
    use hmac::Mac;
    use std::collections::HashSet;
    use std::fmt::{Display, Write};
    use std::path::{Path, PathBuf};

    /// These are host keys that are hard-coded in cargo to provide convenience.
    ///
    /// If GitHub ever publishes new keys, the user can add them to their own
    /// configuration file to use those instead.
    ///
    /// The GitHub keys are sourced from <https://api.github.com/meta> or
    /// <https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/githubs-ssh-key-fingerprints>.
    ///
    /// These will be ignored if the user adds their own entries for `github.com`,
    /// which can be useful if GitHub ever revokes their old keys.
    static BUNDLED_KEYS: &[(&str, &str, &str)] = &[
        ("github.com", "ssh-ed25519", "AAAAC3NzaC1lZDI1NTE5AAAAIOMqqnkVzrm0SdG6UOoqKLsabgH5C9okWi0dh2l9GKJl"),
        ("github.com", "ecdsa-sha2-nistp256", "AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBEmKSENjQEezOmxkZMy7opKgwFB9nkt5YRrYMjNuG5N87uRgg6CLrbo5wAdT/y6v0mKV0U2w0WZ2YB/++Tpockg="),
        ("github.com", "ssh-rsa", " "),
    ];

    /// List of keys that public hosts have rotated away from.
    ///
    /// We explicitly distrust these keys as users with the old key in their
    /// local configuration will otherwise be vulnerable to MITM attacks if the
    /// attacker has access to the old key. As there is no other way to distribute
    /// revocations of ssh host keys, we need to bundle them with the client.
    ///
    /// Unlike [`BUNDLED_KEYS`], these revocations will not be ignored if the user
    /// has their own entries: we *know* that these keys are bad.
    static BUNDLED_REVOCATIONS: &[(&str, &str, &str)] = &[
        // Used until March 24, 2023: https://github.blog/2023-03-23-we-updated-our-rsa-ssh-host-key/
        ("github.com", "ssh-rsa", "AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ=="),
    ];

    enum KnownHostError {
        /// Some general error happened while validating the known hosts.
        CheckError(anyhow::Error),
        /// The host key was not found.
        HostKeyNotFound {
            hostname: String,
            key_type: SshHostKeyType,
            remote_host_key: String,
            remote_fingerprint: String,
            other_hosts: Vec<KnownHost>,
        },
        /// The host key was found, but does not match the remote's key.
        HostKeyHasChanged {
            hostname: String,
            key_type: SshHostKeyType,
            old_known_host: KnownHost,
            remote_host_key: String,
            remote_fingerprint: String,
        },
        /// The host key was found with a @revoked marker, it must not be accepted.
        HostKeyRevoked {
            hostname: String,
            key_type: SshHostKeyType,
            remote_host_key: String,
            location: KnownHostLocation,
        },
        /// The host key was not found, but there was a matching known host with a
        /// @cert-authority marker (which Cargo doesn't yet support).
        HostHasOnlyCertAuthority {
            hostname: String,
            location: KnownHostLocation,
        },
    }

    impl From<anyhow::Error> for KnownHostError {
        fn from(err: anyhow::Error) -> KnownHostError {
            KnownHostError::CheckError(err.into())
        }
    }

    /// The location where a host key was located.
    #[derive(Clone)]
    enum KnownHostLocation {
        /// Loaded from a file from disk.
        File { path: PathBuf, lineno: u32 },
        /// Loaded from cargo's config system.
        Config { definition: Definition },
        /// Part of the hard-coded bundled keys in Cargo.
        Bundled,
    }

    impl Display for KnownHostLocation {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            let loc = match self {
                KnownHostLocation::File { path, lineno } => {
                    format!("{} line {lineno}", path.display())
                }
                KnownHostLocation::Config { definition } => {
                    format!("config value from {definition}")
                }
                KnownHostLocation::Bundled => format!("bundled with cargo"),
            };
            f.write_str(&loc)
        }
    }

    /// The git2 callback used to validate a certificate (only ssh known hosts are validated).
    pub fn certificate_check(
        gctx: &GlobalContext,
        cert: &Cert<'_>,
        host: &str,
        port: Option<u16>,
        config_known_hosts: Option<&Vec<Value<String>>>,
        diagnostic_home_config: &str,
    ) -> Result<CertificateCheckStatus, git2::Error> {
        let Some(host_key) = cert.as_hostkey() else {
            // Return passthrough for TLS X509 certificates to use whatever validation
            // was done in git2.
            return Ok(CertificateCheckStatus::CertificatePassthrough);
        };
        // If a nonstandard port is in use, check for that first.
        // The fallback to check without a port is handled in the HostKeyNotFound handler.
        let host_maybe_port = match port {
            Some(port) if port != 22 => format!("[{host}]:{port}"),
            _ => host.to_string(),
        };
        // The error message must be constructed as a string to pass through the libgit2 C API.
        let err_msg = match check_ssh_known_hosts(gctx, host_key, &host_maybe_port, config_known_hosts)
        {
            Ok(()) => {
                return Ok(CertificateCheckStatus::CertificateOk);
            }
            Err(KnownHostError::CheckError(e)) => {
                format!("error: failed to validate host key:\n{:#}", e)
            }
            Err(KnownHostError::HostKeyNotFound {
                hostname,
                key_type,
                remote_host_key,
                remote_fingerprint,
                other_hosts,
            }) => {
                // Try checking without the port.
                if port.is_some()
                    && !matches!(port, Some(22))
                    && check_ssh_known_hosts(gctx, host_key, host, config_known_hosts).is_ok()
                {
                    return Ok(CertificateCheckStatus::CertificateOk);
                }
                let key_type_short_name = key_type.short_name();
                let key_type_name = key_type.name();
                let known_hosts_location = user_known_host_location_to_add(diagnostic_home_config);
                let other_hosts_message = if other_hosts.is_empty() {
                    String::new()
                } else {
                    let mut msg = String::from(
                        "Note: This host key was found, \
                        but is associated with a different host:\n",
                    );
                    for known_host in other_hosts {
                        write!(
                            msg,
                            "    {loc}: {patterns}\n",
                            loc = known_host.location,
                            patterns = known_host.patterns
                        )
                        .unwrap();
                    }
                    msg
                };
                format!("error: unknown SSH host key\n\
                    The SSH host key for `{hostname}` is not known and cannot be validated.\n\
                    \n\
                    To resolve this issue, add the host key to {known_hosts_location}\n\
                    \n\
                    The key to add is:\n\
                    \n\
                    {hostname} {key_type_name} {remote_host_key}\n\
                    \n\
                    The {key_type_short_name} key fingerprint is: SHA256:{remote_fingerprint}\n\
                    This fingerprint should be validated with the server administrator that it is correct.\n\
                    {other_hosts_message}\n\
                    See https://doc.rust-lang.org/stable/cargo/appendix/git-authentication.html#ssh-known-hosts \
                    for more information.\n\
                    ")
            }
            Err(KnownHostError::HostKeyHasChanged {
                hostname,
                key_type,
                old_known_host,
                remote_host_key,
                remote_fingerprint,
            }) => {
                let key_type_short_name = key_type.short_name();
                let key_type_name = key_type.name();
                let known_hosts_location = user_known_host_location_to_add(diagnostic_home_config);
                let old_key_resolution = match old_known_host.location {
                    KnownHostLocation::File { path, lineno } => {
                        let old_key_location = path.display();
                        format!(
                            "removing the old {key_type_name} key for `{hostname}` \
                            located at {old_key_location} line {lineno}, \
                            and adding the new key to {known_hosts_location}",
                        )
                    }
                    KnownHostLocation::Config { definition } => {
                        format!(
                            "removing the old {key_type_name} key for `{hostname}` \
                            loaded from Cargo's config at {definition}, \
                            and adding the new key to {known_hosts_location}"
                        )
                    }
                    KnownHostLocation::Bundled => {
                        format!(
                            "adding the new key to {known_hosts_location}\n\
                            The current host key is bundled as part of Cargo."
                        )
                    }
                };
                format!("error: SSH host key has changed for `{hostname}`\n\
                    *********************************\n\
                    * WARNING: HOST KEY HAS CHANGED *\n\
                    *********************************\n\
                    This may be caused by a man-in-the-middle attack, or the \
                    server may have changed its host key.\n\
                    \n\
                    The {key_type_short_name} fingerprint for the key from the remote host is:\n\
                        SHA256:{remote_fingerprint}\n\
                    \n\
                    You are strongly encouraged to contact the server \
                    administrator for `{hostname}` to verify that this new key is \
                    correct.\n\
                    \n\
                    If you can verify that the server has a new key, you can \
                    resolve this error by {old_key_resolution}\n\
                    \n\
                    The key provided by the remote host is:\n\
                    \n\
                    {hostname} {key_type_name} {remote_host_key}\n\
                    \n\
                    See https://doc.rust-lang.org/stable/cargo/appendix/git-authentication.html#ssh-known-hosts \
                    for more information.\n\
                    ")
            }
            Err(KnownHostError::HostKeyRevoked {
                hostname,
                key_type,
                remote_host_key,
                location,
            }) => {
                let key_type_short_name = key_type.short_name();
                format!(
                    "error: Key has been revoked for `{hostname}`\n\
                    **************************************\n\
                    * WARNING: REVOKED HOST KEY DETECTED *\n\
                    **************************************\n\
                    This may indicate that the key provided by this host has been\n\
                    compromised and should not be accepted.
                    \n\
                    The host key {key_type_short_name} {remote_host_key} is revoked\n\
                    in {location} and has been rejected.\n\
                    "
                )
            }
            Err(KnownHostError::HostHasOnlyCertAuthority { hostname, location }) => {
                format!("error: Found a `@cert-authority` marker for `{hostname}`\n\
                    \n\
                    Cargo doesn't support certificate authorities for host key verification. It is\n\
                    recommended that the command line Git client is used instead. This can be achieved\n\
                    by setting `net.git-fetch-with-cli` to `true` in the Cargo config.\n\
                    \n
                    The `@cert-authority` line was found in {location}.\n\
                    \n\
                    See https://doc.rust-lang.org/stable/cargo/appendix/git-authentication.html#ssh-known-hosts \
                    for more information.\n\
                    ")
            }
        };
        Err(git2::Error::new(
            git2::ErrorCode::GenericError,
            git2::ErrorClass::Callback,
            err_msg,
        ))
    }

    /// Checks if the given host/host key pair is known.
    fn check_ssh_known_hosts(
        gctx: &GlobalContext,
        cert_host_key: &git2::cert::CertHostkey<'_>,
        host: &str,
        config_known_hosts: Option<&Vec<Value<String>>>,
    ) -> Result<(), KnownHostError> {
        let Some(remote_host_key) = cert_host_key.hostkey() else {
            return Err(anyhow::format_err!("remote host key is not available").into());
        };
        let remote_key_type = cert_host_key.hostkey_type().unwrap();

        // Collect all the known host entries from disk.
        let mut known_hosts = Vec::new();
        for path in known_host_files(gctx) {
            if !path.exists() {
                continue;
            }
            let hosts = load_hostfile(&path)?;
            known_hosts.extend(hosts);
        }
        if let Some(config_known_hosts) = config_known_hosts {
            // Format errors aren't an error in case the format needs to change in
            // the future, to retain forwards compatibility.
            for line_value in config_known_hosts {
                let location = KnownHostLocation::Config {
                    definition: line_value.definition.clone(),
                };
                match parse_known_hosts_line(&line_value.val, location) {
                    Some(known_host) => known_hosts.push(known_host),
                    None => tracing::warn!(
                        "failed to parse known host {} from {}",
                        line_value.val,
                        line_value.definition
                    ),
                }
            }
        }
        // Load the bundled keys. Don't add keys for hosts that the user has
        // configured, which gives them the option to override them. This could be
        // useful if the keys are ever revoked.
        let configured_hosts: HashSet<_> = known_hosts
            .iter()
            .flat_map(|known_host| {
                known_host
                    .patterns
                    .split(',')
                    .map(|pattern| pattern.to_lowercase())
            })
            .collect();
        for (patterns, key_type, key) in BUNDLED_KEYS {
            if !configured_hosts.contains(*patterns) {
                let key = STANDARD.decode(key).unwrap();
                known_hosts.push(KnownHost {
                    location: KnownHostLocation::Bundled,
                    patterns: patterns.to_string(),
                    key_type: key_type.to_string(),
                    key,
                    line_type: KnownHostLineType::Key,
                });
            }
        }
        for (patterns, key_type, key) in BUNDLED_REVOCATIONS {
            let key = STANDARD.decode(key).unwrap();
            known_hosts.push(KnownHost {
                location: KnownHostLocation::Bundled,
                patterns: patterns.to_string(),
                key_type: key_type.to_string(),
                key,
                line_type: KnownHostLineType::Revoked,
            });
        }
        check_ssh_known_hosts_loaded(&known_hosts, host, remote_key_type, remote_host_key)
    }

    /// Checks a host key against a loaded set of known hosts.
    fn check_ssh_known_hosts_loaded(
        known_hosts: &[KnownHost],
        host: &str,
        remote_key_type: SshHostKeyType,
        remote_host_key: &[u8],
    ) -> Result<(), KnownHostError> {
        // `latent_error` keeps track of a potential error that will be returned
        // in case a matching host key isn't found.
        let mut latent_errors: Vec<KnownHostError> = Vec::new();

        // `other_hosts` keeps track of any entries that have an identical key,
        // but a different hostname.
        let mut other_hosts = Vec::new();

        // `accepted_known_host_found` keeps track of whether we've found a matching
        // line in the `known_hosts` file that we would accept. We can't return that
        // immediately, because there may be a subsequent @revoked key.
        let mut accepted_known_host_found = false;

        // Older versions of OpenSSH (before 6.8, March 2015) showed MD5
        // fingerprints (see FingerprintHash ssh config option). Here we only
        // support SHA256.
        let mut remote_fingerprint = cargo_util::Sha256::new();
        remote_fingerprint.update(remote_host_key);
        let remote_fingerprint = STANDARD_NO_PAD.encode(remote_fingerprint.finish());
        let remote_host_key_encoded = STANDARD.encode(remote_host_key);

        for known_host in known_hosts {
            // The key type from libgit2 needs to match the key type from the host file.
            if known_host.key_type != remote_key_type.name() {
                continue;
            }
            let key_matches = known_host.key == remote_host_key;
            if !known_host.host_matches(host) {
                if key_matches {
                    other_hosts.push(known_host.clone());
                }
                continue;
            }
            match known_host.line_type {
                KnownHostLineType::Key => {
                    if key_matches {
                        accepted_known_host_found = true;
                    } else {
                        // The host and key type matched, but the key itself did not.
                        // This indicates the key has changed.
                        // This is only reported as an error if no subsequent lines have a
                        // correct key.
                        latent_errors.push(KnownHostError::HostKeyHasChanged {
                            hostname: host.to_string(),
                            key_type: remote_key_type,
                            old_known_host: known_host.clone(),
                            remote_host_key: remote_host_key_encoded.clone(),
                            remote_fingerprint: remote_fingerprint.clone(),
                        });
                    }
                }
                KnownHostLineType::Revoked => {
                    if key_matches {
                        return Err(KnownHostError::HostKeyRevoked {
                            hostname: host.to_string(),
                            key_type: remote_key_type,
                            remote_host_key: remote_host_key_encoded,
                            location: known_host.location.clone(),
                        });
                    }
                }
                KnownHostLineType::CertAuthority => {
                    // The host matches a @cert-authority line, which is unsupported.
                    latent_errors.push(KnownHostError::HostHasOnlyCertAuthority {
                        hostname: host.to_string(),
                        location: known_host.location.clone(),
                    });
                }
            }
        }

        // We have an accepted host key and it hasn't been revoked.
        if accepted_known_host_found {
            return Ok(());
        }

        if latent_errors.is_empty() {
            // FIXME: Ideally the error message should include the IP address of the
            // remote host (to help the user validate that they are connecting to the
            // host they were expecting to). However, I don't see a way to obtain that
            // information from libgit2.
            Err(KnownHostError::HostKeyNotFound {
                hostname: host.to_string(),
                key_type: remote_key_type,
                remote_host_key: remote_host_key_encoded,
                remote_fingerprint,
                other_hosts,
            })
        } else {
            // We're going to take the first HostKeyHasChanged error if
            // we find one, otherwise we'll take the first error (which
            // we expect to be a CertAuthority error).
            if let Some(index) = latent_errors
                .iter()
                .position(|e| matches!(e, KnownHostError::HostKeyHasChanged { .. }))
            {
                return Err(latent_errors.remove(index));
            } else {
                // Otherwise, we take the first error (which we expect to be
                // a CertAuthority error).
                Err(latent_errors.pop().unwrap())
            }
        }
    }

    /// Returns a list of files to try loading OpenSSH-formatted known hosts.
    fn known_host_files(gctx: &GlobalContext) -> Vec<PathBuf> {
        let mut result = Vec::new();
        if gctx
            .get_env_os("__CARGO_TEST_DISABLE_GLOBAL_KNOWN_HOST")
            .is_some()
        {
        } else if cfg!(unix) {
            result.push(PathBuf::from("/etc/ssh/ssh_known_hosts"));
        } else if cfg!(windows) {
            // The msys/cygwin version of OpenSSH uses `/etc` from the posix root
            // filesystem there (such as `C:\msys64\etc\ssh\ssh_known_hosts`).
            // However, I do not know of a way to obtain that location from
            // Windows-land. The ProgramData version here is what the PowerShell
            // port of OpenSSH does.
            if let Some(progdata) = gctx.get_env_os("ProgramData") {
                let mut progdata = PathBuf::from(progdata);
                progdata.push("ssh");
                progdata.push("ssh_known_hosts");
                result.push(progdata)
            }
        }
        result.extend(user_known_host_location());
        result
    }

    /// The location of the user's known_hosts file.
    fn user_known_host_location() -> Option<PathBuf> {
        // NOTE: This is a potentially inaccurate prediction of what the user
        // actually wants. The actual location depends on several factors:
        //
        // - Windows OpenSSH Powershell version: I believe this looks up the home
        //   directory via ProfileImagePath in the registry, falling back to
        //   `GetWindowsDirectoryW` if that fails.
        // - OpenSSH Portable (under msys): This is very complicated. I got lost
        //   after following it through some ldap/active directory stuff.
        // - OpenSSH (most unix platforms): Uses `pw->pw_dir` from `getpwuid()`.
        //
        // This doesn't do anything close to that. home_dir's behavior is:
        // - Windows: $USERPROFILE, or SHGetKnownFolderPath()
        // - Unix: $HOME, or getpwuid_r()
        //
        // Since there is a mismatch here, the location returned here might be
        // different than what the user's `ssh` CLI command uses. We may want to
        // consider trying to align it better.
        home::home_dir().map(|mut home| {
            home.push(".ssh");
            home.push("known_hosts");
            home
        })
    }

    /// The location to display in an error message instructing the user where to
    /// add the new key.
    fn user_known_host_location_to_add(diagnostic_home_config: &str) -> String {
        // Note that we don't bother with the legacy known_hosts2 files.
        let user = user_known_host_location();
        let openssh_loc = match &user {
            Some(path) => path.to_str().expect("utf-8 home"),
            None => "~/.ssh/known_hosts",
        };
        format!(
            "the `net.ssh.known-hosts` array in your Cargo configuration \
            (such as {diagnostic_home_config}) \
            or in your OpenSSH known_hosts file at {openssh_loc}"
        )
    }

    const HASH_HOSTNAME_PREFIX: &str = "|1|";

    #[derive(Clone)]
    enum KnownHostLineType {
        Key,
        CertAuthority,
        Revoked,
    }

    /// A single known host entry.
    #[derive(Clone)]
    struct KnownHost {
        location: KnownHostLocation,
        /// The hostname. May be comma separated to match multiple hosts.
        patterns: String,
        key_type: String,
        key: Vec<u8>,
        line_type: KnownHostLineType,
    }

    impl KnownHost {
        /// Returns whether or not the given host matches this known host entry.
        fn host_matches(&self, host: &str) -> bool {
            let mut match_found = false;
            let host = host.to_lowercase();
            if let Some(hashed) = self.patterns.strip_prefix(HASH_HOSTNAME_PREFIX) {
                return hashed_hostname_matches(&host, hashed);
            }
            for pattern in self.patterns.split(',') {
                let pattern = pattern.to_lowercase();
                // FIXME: support * and ? wildcards
                if let Some(pattern) = pattern.strip_prefix('!') {
                    if pattern == host {
                        return false;
                    }
                } else {
                    match_found |= pattern == host;
                }
            }
            match_found
        }
    }

    fn hashed_hostname_matches(host: &str, hashed: &str) -> bool {
        let Some((b64_salt, b64_host)) = hashed.split_once('|') else {
            return false;
        };
        let Ok(salt) = STANDARD.decode(b64_salt) else {
            return false;
        };
        let Ok(hashed_host) = STANDARD.decode(b64_host) else {
            return false;
        };
        let Ok(mut mac) = hmac::Hmac::<sha1::Sha1>::new_from_slice(&salt) else {
            return false;
        };
        mac.update(host.as_bytes());
        let result = mac.finalize().into_bytes();
        hashed_host == &result[..]
    }

    /// Loads an OpenSSH known_hosts file.
    fn load_hostfile(path: &Path) -> Result<Vec<KnownHost>, anyhow::Error> {
        let contents = cargo_util::paths::read(path)?;
        Ok(load_hostfile_contents(path, &contents))
    }

    fn load_hostfile_contents(path: &Path, contents: &str) -> Vec<KnownHost> {
        let entries = contents
            .lines()
            .enumerate()
            .filter_map(|(lineno, line)| {
                let location = KnownHostLocation::File {
                    path: path.to_path_buf(),
                    lineno: lineno as u32 + 1,
                };
                parse_known_hosts_line(line, location)
            })
            .collect();
        entries
    }

    fn parse_known_hosts_line(line: &str, location: KnownHostLocation) -> Option<KnownHost> {
        let line = line.trim();
        if line.is_empty() || line.starts_with('#') {
            return None;
        }
        let mut parts = line.split([' ', '\t']).filter(|s| !s.is_empty());

        let line_type = if line.starts_with("@") {
            let line_type = parts.next()?;

            if line_type == "@cert-authority" {
                KnownHostLineType::CertAuthority
            } else if line_type == "@revoked" {
                KnownHostLineType::Revoked
            } else {
                // No other markers are defined
                return None;
            }
        } else {
            KnownHostLineType::Key
        };

        let patterns = parts.next()?;
        let key_type = parts.next()?;
        let key = parts.next().map(|p| STANDARD.decode(p))?.ok()?;
        Some(KnownHost {
            line_type,
            location,
            patterns: patterns.to_string(),
            key_type: key_type.to_string(),
            key,
        })
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        static COMMON_CONTENTS: &str = r#"
            # Comments allowed at start of line

            example.com,rust-lang.org ssh-rsa   eric@host
            Example.net ssh-dss   eric@host
            [example.net]:2222 ssh-dss   eric@host
            nistp256.example.org ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBJ4iYGCcJrUIfrHfzlsv8e8kaF36qpcUpe3VNAKVCZX/BDptIdlEe8u8vKNRTPgUO9jqS0+tjTcPiQd8/8I9qng= eric@host
            nistp384.example.org ecdsa-sha2-nistp384 AAAAE2VjZHNhLXNoYTItbmlzdHAzODQAAAAIbmlzdHAzODQAAABhBNuGT3TqMz2rcwOt2ZqkiNqq7dvWPE66W2qPCoZsh0pQhVU3BnhKIc6nEr6+Wts0Z3jdF3QWwxbbTjbVTVhdr8fMCFhDCWiQFm9xLerYPKnu9qHvx9K87/fjc5+0pu4hLA== eric@host
            nistp521.example.org ecdsa-sha2-nistp521 AAAAE2VjZHNhLXNoYTItbmlzdHA1MjEAAAAIbmlzdHA1MjEAAACFBAD35HH6OsK4DN75BrKipVj/GvZaUzjPNa1F8wMjUdPB1JlVcUfgzJjWSxrhmaNN3u0soiZw8WNRFINsGPCw5E7DywF1689WcIj2Ye2rcy99je15FknScTzBBD04JgIyOI50mCUaPCBoF14vFlN6BmO00cFo+yzy5N8GuQ2sx9kr21xmFQ== eric@host
            # Revoked is supported, but without Cert-Authority support, it will only negate some other fixed key.
            @revoked revoked.example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKtQsi+KPYispwm2rkMidQf30fG1Niy8XNkvASfePoca eric@host
            # Cert-Authority is not supported (below key should not be valid anyway)
            @cert-authority ca.example.com ssh-rsa AABBB5Wm
            example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAWkjI6XT2SZh3xNk5NhisA3o3sGzWR+VAKMSqHtI0aY eric@host
            192.168.42.12 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKVYJpa0yUGaNk0NXQTPWa0tHjqRpx+7hl2diReH6DtR eric@host
            |1|QxzZoTXIWLhUsuHAXjuDMIV3FjQ=|M6NCOIkjiWdCWqkh5+Q+/uFLGjs= ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIHgN3O21U4LWtP5OzjTzPnUnSDmCNDvyvlaj6Hi65JC eric@host
            # Negation isn't terribly useful without globs.
            neg.example.com,!neg.example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOXfUnaAHTlo1Qi//rNk26OcmHikmkns1Z6WW/UuuS3K eric@host
        "#;

        #[test]
        fn known_hosts_parse() {
            let kh_path = Path::new("/home/abc/.known_hosts");
            let khs = load_hostfile_contents(kh_path, COMMON_CONTENTS);
            assert_eq!(khs.len(), 12);
            match &khs[0].location {
                KnownHostLocation::File { path, lineno } => {
                    assert_eq!(path, kh_path);
                    assert_eq!(*lineno, 4);
                }
                _ => panic!("unexpected"),
            }
            assert_eq!(khs[0].patterns, "example.com,rust-lang.org");
            assert_eq!(khs[0].key_type, "ssh-rsa");
            assert_eq!(khs[0].key.len(), 407);
            assert_eq!(&khs[0].key[..30], b"\x00\x00\x00\x07ssh-rsa\x00\x00\x00\x03\x01\x00\x01\x00\x00\x01\x81\x00\xb935\x88\xa5\x9c)");
            match &khs[1].location {
                KnownHostLocation::File { path, lineno } => {
                    assert_eq!(path, kh_path);
                    assert_eq!(*lineno, 5);
                }
                _ => panic!("unexpected"),
            }
            assert_eq!(khs[2].patterns, "[example.net]:2222");
            assert_eq!(khs[3].patterns, "nistp256.example.org");
            assert_eq!(khs[9].patterns, "192.168.42.12");
        }

        #[test]
        fn host_matches() {
            let kh_path = Path::new("/home/abc/.known_hosts");
            let khs = load_hostfile_contents(kh_path, COMMON_CONTENTS);
            assert!(khs[0].host_matches("example.com"));
            assert!(khs[0].host_matches("rust-lang.org"));
            assert!(khs[0].host_matches("EXAMPLE.COM"));
            assert!(khs[1].host_matches("example.net"));
            assert!(!khs[0].host_matches("example.net"));
            assert!(khs[2].host_matches("[example.net]:2222"));
            assert!(!khs[2].host_matches("example.net"));
            assert!(khs[10].host_matches("hashed.example.com"));
            assert!(!khs[10].host_matches("example.com"));
            assert!(!khs[11].host_matches("neg.example.com"));
        }

        #[test]
        fn check_match() {
            let kh_path = Path::new("/home/abc/.known_hosts");
            let khs = load_hostfile_contents(kh_path, COMMON_CONTENTS);

            assert!(check_ssh_known_hosts_loaded(
                &khs,
                "example.com",
                SshHostKeyType::Rsa,
                &khs[0].key
            )
            .is_ok());

            match check_ssh_known_hosts_loaded(&khs, "example.com", SshHostKeyType::Dss, &khs[0].key) {
                Err(KnownHostError::HostKeyNotFound {
                    hostname,
                    remote_fingerprint,
                    other_hosts,
                    ..
                }) => {
                    assert_eq!(
                        remote_fingerprint,
                        "yn+pONDn0EcgdOCVptgB4RZd/wqmsVKrPnQMLtrvhw8"
                    );
                    assert_eq!(hostname, "example.com");
                    assert_eq!(other_hosts.len(), 0);
                }
                _ => panic!("unexpected"),
            }

            match check_ssh_known_hosts_loaded(
                &khs,
                "foo.example.com",
                SshHostKeyType::Rsa,
                &khs[0].key,
            ) {
                Err(KnownHostError::HostKeyNotFound { other_hosts, .. }) => {
                    assert_eq!(other_hosts.len(), 1);
                    assert_eq!(other_hosts[0].patterns, "example.com,rust-lang.org");
                }
                _ => panic!("unexpected"),
            }

            let mut modified_key = khs[0].key.clone();
            modified_key[0] = 1;
            match check_ssh_known_hosts_loaded(&khs, "example.com", SshHostKeyType::Rsa, &modified_key)
            {
                Err(KnownHostError::HostKeyHasChanged { old_known_host, .. }) => {
                    assert!(matches!(
                        old_known_host.location,
                        KnownHostLocation::File { lineno: 4, .. }
                    ));
                }
                _ => panic!("unexpected"),
            }
        }

        #[test]
        fn revoked() {
            let kh_path = Path::new("/home/abc/.known_hosts");
            let khs = load_hostfile_contents(kh_path, COMMON_CONTENTS);

            match check_ssh_known_hosts_loaded(
                &khs,
                "revoked.example.com",
                SshHostKeyType::Ed255219,
                &khs[6].key,
            ) {
                Err(KnownHostError::HostKeyRevoked {
                    hostname, location, ..
                }) => {
                    assert_eq!("revoked.example.com", hostname);
                    assert!(matches!(
                        location,
                        KnownHostLocation::File { lineno: 11, .. }
                    ));
                }
                _ => panic!("Expected key to be revoked for revoked.example.com."),
            }
        }

        #[test]
        fn cert_authority() {
            let kh_path = Path::new("/home/abc/.known_hosts");
            let khs = load_hostfile_contents(kh_path, COMMON_CONTENTS);

            match check_ssh_known_hosts_loaded(
                &khs,
                "ca.example.com",
                SshHostKeyType::Rsa,
                &khs[0].key, // The key should not matter
            ) {
                Err(KnownHostError::HostHasOnlyCertAuthority {
                    hostname, location, ..
                }) => {
                    assert_eq!("ca.example.com", hostname);
                    assert!(matches!(
                        location,
                        KnownHostLocation::File { lineno: 13, .. }
                    ));
                }
                Err(KnownHostError::HostKeyNotFound { hostname, .. }) => {
                    panic!("host key not found... {}", hostname);
                }
                _ => panic!("Expected host to only have @cert-authority line (which is unsupported)."),
            }
        }

        #[test]
        fn multiple_errors() {
            let contents = r#"
            not-used.example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAWkjI6XT2SZh3xNk5NhisA3o3sGzWR+VAKMSqHtI0aY eric@host
            # Cert-authority and changed key for the same host - changed key error should prevail
            @cert-authority example.com ssh-ed25519 AABBB5Wm
            example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKVYJpa0yUGaNk0NXQTPWa0tHjqRpx+7hl2diReH6DtR eric@host
            "#;

            let kh_path = Path::new("/home/abc/.known_hosts");
            let khs = load_hostfile_contents(kh_path, contents);

            match check_ssh_known_hosts_loaded(
                &khs,
                "example.com",
                SshHostKeyType::Ed255219,
                &khs[0].key,
            ) {
                Err(KnownHostError::HostKeyHasChanged {
                    hostname,
                    old_known_host,
                    remote_host_key,
                    ..
                }) => {
                    assert_eq!("example.com", hostname);
                    assert_eq!(
                        "AAAAC3NzaC1lZDI1NTE5AAAAIAWkjI6XT2SZh3xNk5NhisA3o3sGzWR+VAKMSqHtI0aY",
                        remote_host_key
                    );
                    assert!(matches!(
                        old_known_host.location,
                        KnownHostLocation::File { lineno: 5, .. }
                    ));
                }
                _ => panic!("Expected error to be of type HostKeyHasChanged."),
            }
        }

        #[test]
        fn known_host_and_revoked() {
            let contents = r#"
            example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKVYJpa0yUGaNk0NXQTPWa0tHjqRpx+7hl2diReH6DtR eric@host
            # Later in the file the same host key is revoked
            @revoked example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKVYJpa0yUGaNk0NXQTPWa0tHjqRpx+7hl2diReH6DtR eric@host
            "#;

            let kh_path = Path::new("/home/abc/.known_hosts");
            let khs = load_hostfile_contents(kh_path, contents);

            match check_ssh_known_hosts_loaded(
                &khs,
                "example.com",
                SshHostKeyType::Ed255219,
                &khs[0].key,
            ) {
                Err(KnownHostError::HostKeyRevoked {
                    hostname,
                    remote_host_key,
                    location,
                    ..
                }) => {
                    assert_eq!("example.com", hostname);
                    assert_eq!(
                        "AAAAC3NzaC1lZDI1NTE5AAAAIKVYJpa0yUGaNk0NXQTPWa0tHjqRpx+7hl2diReH6DtR",
                        remote_host_key
                    );
                    assert!(matches!(
                        location,
                        KnownHostLocation::File { lineno: 4, .. }
                    ));
                }
                _ => panic!("Expected host key to be reject with error HostKeyRevoked."),
            }
        }
    }