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    use std::io::{Read, Write};
    use std::ops::{Deref, DerefMut};

    use dh::Dh;
    use error::ErrorStack;
    use ssl::{
        HandshakeError, Ssl, SslContext, SslContextBuilder, SslContextRef, SslMethod, SslMode,
        SslOptions, SslRef, SslStream, SslVerifyMode,
    };
    use version;

    const FFDHE_2048: &str = "
    -----BEGIN DH PARAMETERS-----
    MIIBCAKCAQEA//////////+t+FRYortKmq/cViAnPTzx2LnFg84tNpWp4TZBFGQz
    +8yTnc4kmz75fS/jY2MMddj2gbICrsRhetPfHtXV/WVhJDP1H18GbtCFY2VVPe0a
    87VXE15/V8k1mE8McODmi3fipona8+/och3xWKE2rec1MKzKT0g6eXq8CrGCsyT7
    YdEIqUuyyOP7uWrat2DX9GgdT0Kj3jlN9K5W7edjcrsZCwenyO4KbXCeAvzhzffi
    7MA0BM0oNC9hkXL+nOmFg/+OTxIy7vKBg8P+OxtMb61zO7X8vC7CIAXFjvGDfRaD
    ssbzSibBsu/6iGtCOGEoXJf//////////wIBAg==
    -----END DH PARAMETERS-----
    ";

    #[allow(clippy::inconsistent_digit_grouping)]
    fn ctx(method: SslMethod) -> Result<SslContextBuilder, ErrorStack> {
        let mut ctx = SslContextBuilder::new(method)?;

        let mut opts = SslOptions::ALL
            | SslOptions::NO_COMPRESSION
            | SslOptions::NO_SSLV2
            | SslOptions::NO_SSLV3
            | SslOptions::SINGLE_DH_USE
            | SslOptions::SINGLE_ECDH_USE;
        opts &= !SslOptions::DONT_INSERT_EMPTY_FRAGMENTS;

        ctx.set_options(opts);

        let mut mode =
            SslMode::AUTO_RETRY | SslMode::ACCEPT_MOVING_WRITE_BUFFER | SslMode::ENABLE_PARTIAL_WRITE;

        // This is quite a useful optimization for saving memory, but historically
        // caused CVEs in OpenSSL pre-1.0.1h, according to
        // https://bugs.python.org/issue25672
        if version::number() >= 0x1_00_01_08_0 {
            mode |= SslMode::RELEASE_BUFFERS;
        }

        ctx.set_mode(mode);

        Ok(ctx)
    }

    /// A type which wraps client-side streams in a TLS session.
    ///
    /// OpenSSL's default configuration is highly insecure. This connector manages the OpenSSL
    /// structures, configuring cipher suites, session options, hostname verification, and more.
    ///
    /// OpenSSL's built in hostname verification is used when linking against OpenSSL 1.0.2 or 1.1.0,
    /// and a custom implementation is used when linking against OpenSSL 1.0.1.
    #[derive(Clone, Debug)]
    pub struct SslConnector(SslContext);

    impl SslConnector {
        /// Creates a new builder for TLS connections.
        ///
        /// The default configuration is subject to change, and is currently derived from Python.
        pub fn builder(method: SslMethod) -> Result<SslConnectorBuilder, ErrorStack> {
            let mut ctx = ctx(method)?;
            ctx.set_default_verify_paths()?;
            ctx.set_cipher_list(
                "DEFAULT:!aNULL:!eNULL:!MD5:!3DES:!DES:!RC4:!IDEA:!SEED:!aDSS:!SRP:!PSK",
            )?;
            setup_verify(&mut ctx);

            Ok(SslConnectorBuilder(ctx))
        }

        /// Initiates a client-side TLS session on a stream.
        ///
        /// The domain is used for SNI and hostname verification.
        pub fn connect<S>(&self, domain: &str, stream: S) -> Result<SslStream<S>, HandshakeError<S>>
        where
            S: Read + Write,
        {
            self.configure()?.connect(domain, stream)
        }

        /// Returns a structure allowing for configuration of a single TLS session before connection.
        pub fn configure(&self) -> Result<ConnectConfiguration, ErrorStack> {
            Ssl::new(&self.0).map(|ssl| ConnectConfiguration {
                ssl,
                sni: true,
                verify_hostname: true,
            })
        }

        /// Consumes the `SslConnector`, returning the inner raw `SslContext`.
        pub fn into_context(self) -> SslContext {
            self.0
        }

        /// Returns a shared reference to the inner raw `SslContext`.
        pub fn context(&self) -> &SslContextRef {
            &*self.0
        }
    }

    /// A builder for `SslConnector`s.
    pub struct SslConnectorBuilder(SslContextBuilder);

    impl SslConnectorBuilder {
        /// Consumes the builder, returning an `SslConnector`.
        pub fn build(self) -> SslConnector {
            SslConnector(self.0.build())
        }
    }

    impl Deref for SslConnectorBuilder {
        type Target = SslContextBuilder;

        fn deref(&self) -> &SslContextBuilder {
            &self.0
        }
    }

    impl DerefMut for SslConnectorBuilder {
        fn deref_mut(&mut self) -> &mut SslContextBuilder {
            &mut self.0
        }
    }

    /// A type which allows for configuration of a client-side TLS session before connection.
    pub struct ConnectConfiguration {
        ssl: Ssl,
        sni: bool,
        verify_hostname: bool,
    }

    impl ConnectConfiguration {
        /// A builder-style version of `set_use_server_name_indication`.
        pub fn use_server_name_indication(mut self, use_sni: bool) -> ConnectConfiguration {
            self.set_use_server_name_indication(use_sni);
            self
        }

        /// Configures the use of Server Name Indication (SNI) when connecting.
        ///
        /// Defaults to `true`.
        pub fn set_use_server_name_indication(&mut self, use_sni: bool) {
            self.sni = use_sni;
        }

        /// A builder-style version of `set_verify_hostname`.
        pub fn verify_hostname(mut self, verify_hostname: bool) -> ConnectConfiguration {
            self.set_verify_hostname(verify_hostname);
            self
        }

        /// Configures the use of hostname verification when connecting.
        ///
        /// Defaults to `true`.
        ///
        /// # Warning
        ///
        /// You should think very carefully before you use this method. If hostname verification is not
        /// used, *any* valid certificate for *any* site will be trusted for use from any other. This
        /// introduces a significant vulnerability to man-in-the-middle attacks.
        pub fn set_verify_hostname(&mut self, verify_hostname: bool) {
            self.verify_hostname = verify_hostname;
        }

        /// Returns an `Ssl` configured to connect to the provided domain.
        ///
        /// The domain is used for SNI and hostname verification if enabled.
        pub fn into_ssl(mut self, domain: &str) -> Result<Ssl, ErrorStack> {
            if self.sni {
                self.ssl.set_hostname(domain)?;
            }

            if self.verify_hostname {
                setup_verify_hostname(&mut self.ssl, domain)?;
            }

            Ok(self.ssl)
        }

        /// Initiates a client-side TLS session on a stream.
        ///
        /// The domain is used for SNI and hostname verification if enabled.
        pub fn connect<S>(self, domain: &str, stream: S) -> Result<SslStream<S>, HandshakeError<S>>
        where
            S: Read + Write,
        {
            self.into_ssl(domain)?.connect(stream)
        }
    }

    impl Deref for ConnectConfiguration {
        type Target = SslRef;

        fn deref(&self) -> &SslRef {
            &self.ssl
        }
    }

    impl DerefMut for ConnectConfiguration {
        fn deref_mut(&mut self) -> &mut SslRef {
            &mut self.ssl
        }
    }

    /// A type which wraps server-side streams in a TLS session.
    ///
    /// OpenSSL's default configuration is highly insecure. This connector manages the OpenSSL
    /// structures, configuring cipher suites, session options, and more.
    #[derive(Clone)]
    pub struct SslAcceptor(SslContext);

    impl SslAcceptor {
        /// Creates a new builder configured to connect to non-legacy clients. This should generally be
        /// considered a reasonable default choice.
        ///
        /// This corresponds to the intermediate configuration of version 5 of Mozilla's server side TLS
        /// recommendations. See its [documentation][docs] for more details on specifics.
        ///
        /// [docs]: https://wiki.mozilla.org/Security/Server_Side_TLS
        pub fn mozilla_intermediate_v5(method: SslMethod) -> Result<SslAcceptorBuilder, ErrorStack> {
            let mut ctx = ctx(method)?;
            ctx.set_options(SslOptions::NO_TLSV1 | SslOptions::NO_TLSV1_1);
            let dh = Dh::params_from_pem(FFDHE_2048.as_bytes())?;
            ctx.set_tmp_dh(&dh)?;
            setup_curves(&mut ctx)?;
            ctx.set_cipher_list(
                "ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:\
                 ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:\
                 DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384"
            )?;
            #[cfg(ossl111)]
            ctx.set_ciphersuites(
                "TLS_AES_128_GCM_SHA256:TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256",
            )?;
            Ok(SslAcceptorBuilder(ctx))
        }

        /// Creates a new builder configured to connect to modern clients.
        ///
        /// This corresponds to the modern configuration of version 5 of Mozilla's server side TLS recommendations.
        /// See its [documentation][docs] for more details on specifics.
        ///
        /// Requires OpenSSL 1.1.1 or newer.
        ///
        /// [docs]: https://wiki.mozilla.org/Security/Server_Side_TLS
        #[cfg(ossl111)]
        pub fn mozilla_modern_v5(method: SslMethod) -> Result<SslAcceptorBuilder, ErrorStack> {
            let mut ctx = ctx(method)?;
            ctx.set_options(SslOptions::NO_SSL_MASK & !SslOptions::NO_TLSV1_3);
            ctx.set_ciphersuites(
                "TLS_AES_128_GCM_SHA256:TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256",
            )?;
            Ok(SslAcceptorBuilder(ctx))
        }

        /// Creates a new builder configured to connect to non-legacy clients. This should generally be
        /// considered a reasonable default choice.
        ///
        /// This corresponds to the intermediate configuration of version 4 of Mozilla's server side TLS
        /// recommendations. See its [documentation][docs] for more details on specifics.
        ///
        /// [docs]: https://wiki.mozilla.org/Security/Server_Side_TLS
        // FIXME remove in next major version
        pub fn mozilla_intermediate(method: SslMethod) -> Result<SslAcceptorBuilder, ErrorStack> {
            let mut ctx = ctx(method)?;
            ctx.set_options(SslOptions::CIPHER_SERVER_PREFERENCE);
            #[cfg(ossl111)]
            ctx.set_options(SslOptions::NO_TLSV1_3);
            let dh = Dh::params_from_pem(FFDHE_2048.as_bytes())?;
            ctx.set_tmp_dh(&dh)?;
            setup_curves(&mut ctx)?;
            ctx.set_cipher_list(
                "ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:\
                 ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:\
                 DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:\
                 ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:\
                 ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:\
                 DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:ECDHE-ECDSA-DES-CBC3-SHA:ECDHE-RSA-DES-CBC3-SHA:\
                 EDH-RSA-DES-CBC3-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:\
                 AES256-SHA:DES-CBC3-SHA:!DSS",
            )?;
            Ok(SslAcceptorBuilder(ctx))
        }

        /// Creates a new builder configured to connect to modern clients.
        ///
        /// This corresponds to the modern configuration of version 4 of Mozilla's server side TLS recommendations.
        /// See its [documentation][docs] for more details on specifics.
        ///
        /// [docs]: https://wiki.mozilla.org/Security/Server_Side_TLS
        // FIXME remove in next major version
        pub fn mozilla_modern(method: SslMethod) -> Result<SslAcceptorBuilder, ErrorStack> {
            let mut ctx = ctx(method)?;
            ctx.set_options(
                SslOptions::CIPHER_SERVER_PREFERENCE | SslOptions::NO_TLSV1 | SslOptions::NO_TLSV1_1,
            );
            #[cfg(ossl111)]
            ctx.set_options(SslOptions::NO_TLSV1_3);
            setup_curves(&mut ctx)?;
            ctx.set_cipher_list(
                "ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:\
                 ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:\
                 ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256",
            )?;
            Ok(SslAcceptorBuilder(ctx))
        }

        /// Initiates a server-side TLS session on a stream.
        pub fn accept<S>(&self, stream: S) -> Result<SslStream<S>, HandshakeError<S>>
        where
            S: Read + Write,
        {
            let ssl = Ssl::new(&self.0)?;
            ssl.accept(stream)
        }

        /// Consumes the `SslAcceptor`, returning the inner raw `SslContext`.
        pub fn into_context(self) -> SslContext {
            self.0
        }

        /// Returns a shared reference to the inner raw `SslContext`.
        pub fn context(&self) -> &SslContextRef {
            &*self.0
        }
    }

    /// A builder for `SslAcceptor`s.
    pub struct SslAcceptorBuilder(SslContextBuilder);

    impl SslAcceptorBuilder {
        /// Consumes the builder, returning a `SslAcceptor`.
        pub fn build(self) -> SslAcceptor {
            SslAcceptor(self.0.build())
        }
    }

    impl Deref for SslAcceptorBuilder {
        type Target = SslContextBuilder;

        fn deref(&self) -> &SslContextBuilder {
            &self.0
        }
    }

    impl DerefMut for SslAcceptorBuilder {
        fn deref_mut(&mut self) -> &mut SslContextBuilder {
            &mut self.0
        }
    }

    cfg_if! {
        if #[cfg(ossl110)] {
            fn setup_curves(_: &mut SslContextBuilder) -> Result<(), ErrorStack> {
                Ok(())
            }
        } else if #[cfg(any(ossl102, libressl))] {
            fn setup_curves(ctx: &mut SslContextBuilder) -> Result<(), ErrorStack> {
                ctx.set_ecdh_auto(true)
            }
        } else {
            fn setup_curves(ctx: &mut SslContextBuilder) -> Result<(), ErrorStack> {
                use ec::EcKey;
                use nid::Nid;

                let curve = EcKey::from_curve_name(Nid::X9_62_PRIME256V1)?;
                ctx.set_tmp_ecdh(&curve)
            }
        }
    }

    cfg_if! {
        if #[cfg(any(ossl102, libressl261))] {
            fn setup_verify(ctx: &mut SslContextBuilder) {
                ctx.set_verify(SslVerifyMode::PEER);
            }

            fn setup_verify_hostname(ssl: &mut SslRef, domain: &str) -> Result<(), ErrorStack> {
                use x509::verify::X509CheckFlags;

                let param = ssl.param_mut();
                param.set_hostflags(X509CheckFlags::NO_PARTIAL_WILDCARDS);
                match domain.parse() {
                    Ok(ip) => param.set_ip(ip),
                    Err(_) => param.set_host(domain),
                }
            }
        } else {
            fn setup_verify(ctx: &mut SslContextBuilder) {
                ctx.set_verify_callback(SslVerifyMode::PEER, verify::verify_callback);
            }

            fn setup_verify_hostname(ssl: &mut Ssl, domain: &str) -> Result<(), ErrorStack> {
                let domain = domain.to_string();
                ssl.set_ex_data(*verify::HOSTNAME_IDX, domain);
                Ok(())
            }

            mod verify {
                use std::net::IpAddr;
                use std::str;

                use ex_data::Index;
                use nid::Nid;
                use ssl::Ssl;
                use stack::Stack;
                use x509::{
                    GeneralName, X509NameRef, X509Ref, X509StoreContext, X509StoreContextRef,
                    X509VerifyResult,
                };

                lazy_static! {
                    pub static ref HOSTNAME_IDX: Index<Ssl, String> = Ssl::new_ex_index().unwrap();
                }

                pub fn verify_callback(preverify_ok: bool, x509_ctx: &mut X509StoreContextRef) -> bool {
                    if !preverify_ok || x509_ctx.error_depth() != 0 {
                        return preverify_ok;
                    }

                    let ok = match (
                        x509_ctx.current_cert(),
                        X509StoreContext::ssl_idx()
                            .ok()
                            .and_then(|idx| x509_ctx.ex_data(idx))
                            .and_then(|ssl| ssl.ex_data(*HOSTNAME_IDX)),
                    ) {
                        (Some(x509), Some(domain)) => verify_hostname(domain, &x509),
                        _ => true,
                    };

                    if !ok {
                        x509_ctx.set_error(X509VerifyResult::APPLICATION_VERIFICATION);
                    }

                    ok
                }

                fn verify_hostname(domain: &str, cert: &X509Ref) -> bool {
                    match cert.subject_alt_names() {
                        Some(names) => verify_subject_alt_names(domain, names),
                        None => verify_subject_name(domain, &cert.subject_name()),
                    }
                }

                fn verify_subject_alt_names(domain: &str, names: Stack<GeneralName>) -> bool {
                    let ip = domain.parse();

                    for name in &names {
                        match ip {
                            Ok(ip) => {
                                if let Some(actual) = name.ipaddress() {
                                    if matches_ip(&ip, actual) {
                                        return true;
                                    }
                                }
                            }
                            Err(_) => {
                                if let Some(pattern) = name.dnsname() {
                                    if matches_dns(pattern, domain) {
                                        return true;
                                    }
                                }
                            }
                        }
                    }

                    false
                }

                fn verify_subject_name(domain: &str, subject_name: &X509NameRef) -> bool {
                    match subject_name.entries_by_nid(Nid::COMMONNAME).next() {
                        Some(pattern) => {
                            let pattern = match str::from_utf8(pattern.data().as_slice()) {
                                Ok(pattern) => pattern,
                                Err(_) => return false,
                            };

                            // Unlike SANs, IP addresses in the subject name don't have a
                            // different encoding.
                            match domain.parse::<IpAddr>() {
                                Ok(ip) => pattern
                                    .parse::<IpAddr>()
                                    .ok()
                                    .map_or(false, |pattern| pattern == ip),
                                Err(_) => matches_dns(pattern, domain),
                            }
                        }
                        None => false,
                    }
                }

                fn matches_dns(mut pattern: &str, mut hostname: &str) -> bool {
                    // first strip trailing . off of pattern and hostname to normalize
                    if pattern.ends_with('.') {
                        pattern = &pattern[..pattern.len() - 1];
                    }
                    if hostname.ends_with('.') {
                        hostname = &hostname[..hostname.len() - 1];
                    }

                    matches_wildcard(pattern, hostname).unwrap_or_else(|| pattern.eq_ignore_ascii_case(hostname))
                }

                fn matches_wildcard(pattern: &str, hostname: &str) -> Option<bool> {
                    let wildcard_location = match pattern.find('*') {
                        Some(l) => l,
                        None => return None,
                    };

                    let mut dot_idxs = pattern.match_indices('.').map(|(l, _)| l);
                    let wildcard_end = match dot_idxs.next() {
                        Some(l) => l,
                        None => return None,
                    };

                    // Never match wildcards if the pattern has less than 2 '.'s (no *.com)
                    //
                    // This is a bit dubious, as it doesn't disallow other TLDs like *.co.uk.
                    // Chrome has a black- and white-list for this, but Firefox (via NSS) does
                    // the same thing we do here.
                    //
                    // The Public Suffix (https://www.publicsuffix.org/) list could
                    // potentially be used here, but it's both huge and updated frequently
                    // enough that management would be a PITA.
                    if dot_idxs.next().is_none() {
                        return None;
                    }

                    // Wildcards can only be in the first component, and must be the entire first label
                    if wildcard_location != 0 || wildcard_end != wildcard_location + 1 {
                        return None;
                    }

                    let hostname_label_end = match hostname.find('.') {
                        Some(l) => l,
                        None => return None,
                    };

                    let pattern_after_wildcard = &pattern[wildcard_end..];
                    let hostname_after_wildcard = &hostname[hostname_label_end..];

                    Some(pattern_after_wildcard.eq_ignore_ascii_case(hostname_after_wildcard))
                }

                fn matches_ip(expected: &IpAddr, actual: &[u8]) -> bool {
                    match *expected {
                        IpAddr::V4(ref addr) => actual == addr.octets(),
                        IpAddr::V6(ref addr) => actual == addr.octets(),
                    }
                }

                #[test]
                fn test_dns_match() {
                    use ssl::connector::verify::matches_dns;
                    assert!(matches_dns("website.tld", "website.tld")); // A name should match itself.
                    assert!(matches_dns("website.tld", "wEbSiTe.tLd")); // DNS name matching ignores case of hostname.
                    assert!(matches_dns("wEbSiTe.TlD", "website.tld")); // DNS name matching ignores case of subject.

                    assert!(matches_dns("xn--bcher-kva.tld", "xn--bcher-kva.tld")); // Likewise, nothing special to punycode names.
                    assert!(matches_dns("xn--bcher-kva.tld", "xn--BcHer-Kva.tLd")); // And punycode must be compared similarly case-insensitively.

                    assert!(matches_dns("*.example.com", "subdomain.example.com")); // Wildcard matching works.
                    assert!(matches_dns("*.eXaMpLe.cOm", "subdomain.example.com")); // Wildcard matching ignores case of subject.
                    assert!(matches_dns("*.example.com", "sUbDoMaIn.eXaMpLe.cOm")); // Wildcard matching ignores case of hostname.

                    assert!(!matches_dns("prefix*.example.com", "p.example.com")); // Prefix longer than the label works and does not match.
                    assert!(!matches_dns("*suffix.example.com", "s.example.com")); // Suffix longer than the label works and does not match.

                    assert!(!matches_dns("prefix*.example.com", "prefix.example.com")); // Partial wildcards do not work.
                    assert!(!matches_dns("*suffix.example.com", "suffix.example.com")); // Partial wildcards do not work.

                    assert!(!matches_dns("prefix*.example.com", "prefixdomain.example.com")); // Partial wildcards do not work.
                    assert!(!matches_dns("*suffix.example.com", "domainsuffix.example.com")); // Partial wildcards do not work.

                    assert!(!matches_dns("xn--*.example.com", "subdomain.example.com")); // Punycode domains with wildcard parts do not match.
                    assert!(!matches_dns("xN--*.example.com", "subdomain.example.com")); // And we can't bypass a punycode test with weird casing.
                    assert!(!matches_dns("Xn--*.example.com", "subdomain.example.com")); // And we can't bypass a punycode test with weird casing.
                    assert!(!matches_dns("XN--*.example.com", "subdomain.example.com")); // And we can't bypass a punycode test with weird casing.
                }
            }
        }
    }