const builtin = @import("builtin");
const native_endian = builtin.cpu.arch.endian();

const std = @import("../../std.zig");
const tls = std.crypto.tls;
const Client = @This();
const mem = std.mem;
const crypto = std.crypto;
const assert = std.debug.assert;
const Certificate = std.crypto.Certificate;
const Reader = std.Io.Reader;
const Writer = std.Io.Writer;

const max_ciphertext_len = tls.max_ciphertext_len;
const hmacExpandLabel = tls.hmacExpandLabel;
const hkdfExpandLabel = tls.hkdfExpandLabel;
const int = tls.int;
const array = tls.array;

/// The encrypted stream from the server to the client. Bytes are pulled from
/// here via `reader`.
///
/// The buffer is asserted to have capacity at least `min_buffer_len`.
input: *Reader,
/// Decrypted stream from the server to the client.
reader: Reader,

/// The encrypted stream from the client to the server. Bytes are pushed here
/// via `writer`.
///
/// The buffer is asserted to have capacity at least `min_buffer_len`.
output: *Writer,
/// The plaintext stream from the client to the server.
writer: Writer,

/// Populated when `error.TlsAlert` is returned.
alert: ?tls.Alert = null,
read_err: ?ReadError = null,
tls_version: tls.ProtocolVersion,
read_seq: u64,
write_seq: u64,
/// When this is true, the stream may still not be at the end because there
/// may be data in the input buffer.
received_close_notify: bool,
allow_truncation_attacks: bool,
application_cipher: tls.ApplicationCipher,

/// If non-null, ssl secrets are logged to a stream. Creating such a log file
/// allows other programs with access to that file to decrypt all traffic over
/// this connection.
ssl_key_log: ?*SslKeyLog,

pub const ReadError = error{
    /// The alert description will be stored in `alert`.
    TlsAlert,
    TlsBadLength,
    TlsBadRecordMac,
    TlsConnectionTruncated,
    TlsDecodeError,
    TlsRecordOverflow,
    TlsUnexpectedMessage,
    TlsIllegalParameter,
    TlsSequenceOverflow,
};

pub const SslKeyLog = struct {
    client_key_seq: u64,
    server_key_seq: u64,
    client_random: [32]u8,
    writer: *Writer,

    fn clientCounter(key_log: *@This()) u64 {
        defer key_log.client_key_seq += 1;
        return key_log.client_key_seq;
    }

    fn serverCounter(key_log: *@This()) u64 {
        defer key_log.server_key_seq += 1;
        return key_log.server_key_seq;
    }
};

/// The `Reader` supplied to `init` requires a buffer capacity
/// at least this amount.
pub const min_buffer_len = tls.max_ciphertext_record_len;

pub const Options = struct {
    /// How to perform host verification of server certificates.
    host: union(enum) {
        /// No host verification is performed, which prevents a trusted connection from
        /// being established.
        no_verification,
        /// Verify that the server certificate was issued for a given host.
        explicit: []const u8,
    },
    /// How to verify the authenticity of server certificates.
    ca: union(enum) {
        /// No ca verification is performed, which prevents a trusted connection from
        /// being established.
        no_verification,
        /// Verify that the server certificate is a valid self-signed certificate.
        /// This provides no authorization guarantees, as anyone can create a
        /// self-signed certificate.
        self_signed,
        /// Verify that the server certificate is authorized by a given ca bundle.
        bundle: Certificate.Bundle,
    },
    /// If non-null, ssl secrets are logged to this stream. Creating such a log file allows
    /// other programs with access to that file to decrypt all traffic over this connection.
    ///
    /// Only the `writer` field is observed during the handshake (`init`).
    /// After that, the other fields are populated.
    ssl_key_log: ?*SslKeyLog = null,
    /// By default, reaching the end-of-stream when reading from the server will
    /// cause `error.TlsConnectionTruncated` to be returned, unless a close_notify
    /// message has been received. By setting this flag to `true`, instead, the
    /// end-of-stream will be forwarded to the application layer above TLS.
    ///
    /// This makes the application vulnerable to truncation attacks unless the
    /// application layer itself verifies that the amount of data received equals
    /// the amount of data expected, such as HTTP with the Content-Length header.
    allow_truncation_attacks: bool = false,
    write_buffer: []u8,
    read_buffer: []u8,
    /// Populated when `error.TlsAlert` is returned from `init`.
    alert: ?*tls.Alert = null,
};

const InitError = error{
    WriteFailed,
    ReadFailed,
    InsufficientEntropy,
    DiskQuota,
    LockViolation,
    NotOpenForWriting,
    /// The alert description will be stored in `alert`.
    TlsAlert,
    TlsUnexpectedMessage,
    TlsIllegalParameter,
    TlsDecryptFailure,
    TlsRecordOverflow,
    TlsBadRecordMac,
    CertificateFieldHasInvalidLength,
    CertificateHostMismatch,
    CertificatePublicKeyInvalid,
    CertificateExpired,
    CertificateFieldHasWrongDataType,
    CertificateIssuerMismatch,
    CertificateNotYetValid,
    CertificateSignatureAlgorithmMismatch,
    CertificateSignatureAlgorithmUnsupported,
    CertificateSignatureInvalid,
    CertificateSignatureInvalidLength,
    CertificateSignatureNamedCurveUnsupported,
    CertificateSignatureUnsupportedBitCount,
    TlsCertificateNotVerified,
    TlsBadSignatureScheme,
    TlsBadRsaSignatureBitCount,
    InvalidEncoding,
    IdentityElement,
    SignatureVerificationFailed,
    TlsDecryptError,
    TlsConnectionTruncated,
    TlsDecodeError,
    UnsupportedCertificateVersion,
    CertificateTimeInvalid,
    CertificateHasUnrecognizedObjectId,
    CertificateHasInvalidBitString,
    MessageTooLong,
    NegativeIntoUnsigned,
    TargetTooSmall,
    BufferTooSmall,
    InvalidSignature,
    NotSquare,
    NonCanonical,
    WeakPublicKey,
};

/// Initiates a TLS handshake and establishes a TLSv1.2 or TLSv1.3 session.
///
/// `host` is only borrowed during this function call.
///
/// `input` is asserted to have buffer capacity at least `min_buffer_len`.
pub fn init(input: *Reader, output: *Writer, options: Options) InitError!Client {
    assert(input.buffer.len >= min_buffer_len);
    const host = switch (options.host) {
        .no_verification => "",
        .explicit => |host| host,
    };
    const host_len: u16 = @intCast(host.len);

    var random_buffer: [176]u8 = undefined;
    crypto.random.bytes(&random_buffer);
    const client_hello_rand = random_buffer[0..32].*;
    var key_seq: u64 = 0;
    var server_hello_rand: [32]u8 = undefined;
    const legacy_session_id = random_buffer[32..64].*;

    var key_share = KeyShare.init(random_buffer[64..176].*) catch |err| switch (err) {
        // Only possible to happen if the seed is all zeroes.
        error.IdentityElement => return error.InsufficientEntropy,
    };

    const extensions_payload = tls.extension(.supported_versions, array(u8, tls.ProtocolVersion, .{
        .tls_1_3,
        .tls_1_2,
    })) ++ tls.extension(.signature_algorithms, array(u16, tls.SignatureScheme, .{
        .ecdsa_secp256r1_sha256,
        .ecdsa_secp384r1_sha384,
        .rsa_pkcs1_sha256,
        .rsa_pkcs1_sha384,
        .rsa_pkcs1_sha512,
        .rsa_pss_rsae_sha256,
        .rsa_pss_rsae_sha384,
        .rsa_pss_rsae_sha512,
        .rsa_pss_pss_sha256,
        .rsa_pss_pss_sha384,
        .rsa_pss_pss_sha512,
        .rsa_pkcs1_sha1,
        .ed25519,
    })) ++ tls.extension(.supported_groups, array(u16, tls.NamedGroup, .{
        .x25519_ml_kem768,
        .secp256r1,
        .secp384r1,
        .x25519,
    })) ++ tls.extension(.psk_key_exchange_modes, array(u8, tls.PskKeyExchangeMode, .{
        .psk_dhe_ke,
    })) ++ tls.extension(.key_share, array(
        u16,
        u8,
        int(u16, @intFromEnum(tls.NamedGroup.x25519_ml_kem768)) ++
            array(u16, u8, key_share.ml_kem768_kp.public_key.toBytes() ++ key_share.x25519_kp.public_key) ++
            int(u16, @intFromEnum(tls.NamedGroup.secp256r1)) ++
            array(u16, u8, key_share.secp256r1_kp.public_key.toUncompressedSec1()) ++
            int(u16, @intFromEnum(tls.NamedGroup.secp384r1)) ++
            array(u16, u8, key_share.secp384r1_kp.public_key.toUncompressedSec1()) ++
            int(u16, @intFromEnum(tls.NamedGroup.x25519)) ++
            array(u16, u8, key_share.x25519_kp.public_key),
    ));
    const server_name_extension = int(u16, @intFromEnum(tls.ExtensionType.server_name)) ++
        int(u16, 2 + 1 + 2 + host_len) ++ // byte length of this extension payload
        int(u16, 1 + 2 + host_len) ++ // server_name_list byte count
        .{0x00} ++ // name_type
        int(u16, host_len);
    const server_name_extension_len = switch (options.host) {
        .no_verification => 0,
        .explicit => server_name_extension.len + host_len,
    };

    const extensions_header =
        int(u16, @intCast(extensions_payload.len + server_name_extension_len)) ++
        extensions_payload ++
        server_name_extension;

    const client_hello =
        int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
        client_hello_rand ++
        [1]u8{32} ++ legacy_session_id ++
        cipher_suites ++
        array(u8, tls.CompressionMethod, .{.null}) ++
        extensions_header;

    const out_handshake = .{@intFromEnum(tls.HandshakeType.client_hello)} ++
        int(u24, @intCast(client_hello.len - server_name_extension.len + server_name_extension_len)) ++
        client_hello;

    const cleartext_header_buf = .{@intFromEnum(tls.ContentType.handshake)} ++
        int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_0)) ++
        int(u16, @intCast(out_handshake.len - server_name_extension.len + server_name_extension_len)) ++
        out_handshake;
    const cleartext_header = switch (options.host) {
        .no_verification => cleartext_header_buf[0 .. cleartext_header_buf.len - server_name_extension.len],
        .explicit => &cleartext_header_buf,
    };

    {
        var iovecs: [2][]const u8 = .{ cleartext_header, host };
        try output.writeVecAll(iovecs[0..if (host.len == 0) 1 else 2]);
        try output.flush();
    }

    var tls_version: tls.ProtocolVersion = undefined;
    // These are used for two purposes:
    // * Detect whether a certificate is the first one presented, in which case
    //   we need to verify the host name.
    var cert_index: usize = 0;
    // * Flip back and forth between the two cleartext buffers in order to keep
    //   the previous certificate in memory so that it can be verified by the
    //   next one.
    var cert_buf_index: usize = 0;
    var write_seq: u64 = 0;
    var read_seq: u64 = 0;
    var prev_cert: Certificate.Parsed = undefined;
    const CipherState = enum {
        /// No cipher is in use
        cleartext,
        /// Handshake cipher is in use
        handshake,
        /// Application cipher is in use
        application,
    };
    var pending_cipher_state: CipherState = .cleartext;
    var cipher_state = pending_cipher_state;
    const HandshakeState = enum {
        /// In this state we expect only a server hello message.
        hello,
        /// In this state we expect only an encrypted_extensions message.
        encrypted_extensions,
        /// In this state we expect certificate handshake messages.
        certificate,
        /// In this state we expect certificate or certificate_verify messages.
        /// certificate messages are ignored since the trust chain is already
        /// established.
        trust_chain_established,
        /// In this state, we expect only the server_hello_done handshake message.
        server_hello_done,
        /// In this state, we expect only the finished handshake message.
        finished,
    };
    var handshake_state: HandshakeState = .hello;
    var handshake_cipher: tls.HandshakeCipher = undefined;
    var main_cert_pub_key: CertificatePublicKey = undefined;
    const now_sec = std.time.timestamp();

    var cleartext_fragment_start: usize = 0;
    var cleartext_fragment_end: usize = 0;
    var cleartext_bufs: [2][tls.max_ciphertext_inner_record_len]u8 = undefined;
    fragment: while (true) {
        // Ensure the input buffer pointer is stable in this scope.
        input.rebase(tls.max_ciphertext_record_len) catch |err| switch (err) {
            error.EndOfStream => {}, // We have assurance the remainder of stream can be buffered.
        };
        const record_header = input.peek(tls.record_header_len) catch |err| switch (err) {
            error.EndOfStream => return error.TlsConnectionTruncated,
            error.ReadFailed => return error.ReadFailed,
        };
        const record_ct = input.takeEnumNonexhaustive(tls.ContentType, .big) catch unreachable; // already peeked
        input.toss(2); // legacy_version
        const record_len = input.takeInt(u16, .big) catch unreachable; // already peeked
        if (record_len > tls.max_ciphertext_len) return error.TlsRecordOverflow;
        const record_buffer = input.take(record_len) catch |err| switch (err) {
            error.EndOfStream => return error.TlsConnectionTruncated,
            error.ReadFailed => return error.ReadFailed,
        };
        var record_decoder: tls.Decoder = .fromTheirSlice(record_buffer);
        var ctd, const ct = content: switch (cipher_state) {
            .cleartext => .{ record_decoder, record_ct },
            .handshake => {
                assert(tls_version == .tls_1_3);
                if (record_ct != .application_data) return error.TlsUnexpectedMessage;
                try record_decoder.ensure(record_len);
                const cleartext_buf = &cleartext_bufs[cert_buf_index % 2];
                switch (handshake_cipher) {
                    inline else => |*p| {
                        const pv = &p.version.tls_1_3;
                        const P = @TypeOf(p.*).A;
                        if (record_len < P.AEAD.tag_length) return error.TlsRecordOverflow;
                        const ciphertext = record_decoder.slice(record_len - P.AEAD.tag_length);
                        const cleartext_fragment_buf = cleartext_buf[cleartext_fragment_end..];
                        if (ciphertext.len > cleartext_fragment_buf.len) return error.TlsRecordOverflow;
                        const cleartext = cleartext_fragment_buf[0..ciphertext.len];
                        const auth_tag = record_decoder.array(P.AEAD.tag_length).*;
                        const nonce = nonce: {
                            const V = @Vector(P.AEAD.nonce_length, u8);
                            const pad = [1]u8{0} ** (P.AEAD.nonce_length - 8);
                            const operand: V = pad ++ @as([8]u8, @bitCast(big(read_seq)));
                            break :nonce @as(V, pv.server_handshake_iv) ^ operand;
                        };
                        P.AEAD.decrypt(cleartext, ciphertext, auth_tag, record_header, nonce, pv.server_handshake_key) catch
                            return error.TlsBadRecordMac;
                        // TODO use scalar, non-slice version
                        cleartext_fragment_end += mem.trimEnd(u8, cleartext, "\x00").len;
                    },
                }
                read_seq += 1;
                cleartext_fragment_end -= 1;
                const ct: tls.ContentType = @enumFromInt(cleartext_buf[cleartext_fragment_end]);
                if (ct != .handshake) return error.TlsUnexpectedMessage;
                break :content .{ tls.Decoder.fromTheirSlice(@constCast(cleartext_buf[cleartext_fragment_start..cleartext_fragment_end])), ct };
            },
            .application => {
                assert(tls_version == .tls_1_2);
                if (record_ct != .handshake) return error.TlsUnexpectedMessage;
                try record_decoder.ensure(record_len);
                const cleartext_buf = &cleartext_bufs[cert_buf_index % 2];
                switch (handshake_cipher) {
                    inline else => |*p| {
                        const pv = &p.version.tls_1_2;
                        const P = @TypeOf(p.*).A;
                        if (record_len < P.record_iv_length + P.mac_length) return error.TlsRecordOverflow;
                        const message_len: u16 = record_len - P.record_iv_length - P.mac_length;
                        const cleartext_fragment_buf = cleartext_buf[cleartext_fragment_end..];
                        if (message_len > cleartext_fragment_buf.len) return error.TlsRecordOverflow;
                        const cleartext = cleartext_fragment_buf[0..message_len];
                        const ad = mem.toBytes(big(read_seq)) ++
                            record_header[0 .. 1 + 2] ++
                            mem.toBytes(big(message_len));
                        const record_iv = record_decoder.array(P.record_iv_length).*;
                        const masked_read_seq = read_seq &
                            comptime std.math.shl(u64, std.math.maxInt(u64), 8 * P.record_iv_length);
                        const nonce: [P.AEAD.nonce_length]u8 = nonce: {
                            const V = @Vector(P.AEAD.nonce_length, u8);
                            const pad = [1]u8{0} ** (P.AEAD.nonce_length - 8);
                            const operand: V = pad ++ @as([8]u8, @bitCast(big(masked_read_seq)));
                            break :nonce @as(V, pv.app_cipher.server_write_IV ++ record_iv) ^ operand;
                        };
                        const ciphertext = record_decoder.slice(message_len);
                        const auth_tag = record_decoder.array(P.mac_length);
                        P.AEAD.decrypt(cleartext, ciphertext, auth_tag.*, ad, nonce, pv.app_cipher.server_write_key) catch return error.TlsBadRecordMac;
                        cleartext_fragment_end += message_len;
                    },
                }
                read_seq += 1;
                break :content .{ tls.Decoder.fromTheirSlice(cleartext_buf[cleartext_fragment_start..cleartext_fragment_end]), record_ct };
            },
        };
        switch (ct) {
            .alert => {
                ctd.ensure(2) catch continue :fragment;
                if (options.alert) |a| a.* = .{
                    .level = ctd.decode(tls.Alert.Level),
                    .description = ctd.decode(tls.Alert.Description),
                };
                return error.TlsAlert;
            },
            .change_cipher_spec => {
                ctd.ensure(1) catch continue :fragment;
                if (ctd.decode(tls.ChangeCipherSpecType) != .change_cipher_spec) return error.TlsIllegalParameter;
                cipher_state = pending_cipher_state;
            },
            .handshake => while (true) {
                ctd.ensure(4) catch continue :fragment;
                const handshake_type = ctd.decode(tls.HandshakeType);
                const handshake_len = ctd.decode(u24);
                var hsd = ctd.sub(handshake_len) catch continue :fragment;
                const wrapped_handshake = ctd.buf[ctd.idx - handshake_len - 4 .. ctd.idx];
                switch (handshake_type) {
                    .server_hello => {
                        if (cipher_state != .cleartext) return error.TlsUnexpectedMessage;
                        if (handshake_state != .hello) return error.TlsUnexpectedMessage;
                        try hsd.ensure(2 + 32 + 1);
                        const legacy_version = hsd.decode(u16);
                        @memcpy(&server_hello_rand, hsd.array(32));
                        if (mem.eql(u8, &server_hello_rand, &tls.hello_retry_request_sequence)) {
                            // This is a HelloRetryRequest message. This client implementation
                            // does not expect to get one.
                            return error.TlsUnexpectedMessage;
                        }
                        const legacy_session_id_echo_len = hsd.decode(u8);
                        try hsd.ensure(legacy_session_id_echo_len + 2 + 1);
                        const legacy_session_id_echo = hsd.slice(legacy_session_id_echo_len);
                        const cipher_suite_tag = hsd.decode(tls.CipherSuite);
                        hsd.skip(1); // legacy_compression_method
                        var supported_version: ?u16 = null;
                        if (!hsd.eof()) {
                            try hsd.ensure(2);
                            const extensions_size = hsd.decode(u16);
                            var all_extd = try hsd.sub(extensions_size);
                            while (!all_extd.eof()) {
                                try all_extd.ensure(2 + 2);
                                const et = all_extd.decode(tls.ExtensionType);
                                const ext_size = all_extd.decode(u16);
                                var extd = try all_extd.sub(ext_size);
                                switch (et) {
                                    .supported_versions => {
                                        if (supported_version) |_| return error.TlsIllegalParameter;
                                        try extd.ensure(2);
                                        supported_version = extd.decode(u16);
                                    },
                                    .key_share => {
                                        if (key_share.getSharedSecret()) |_| return error.TlsIllegalParameter;
                                        try extd.ensure(4);
                                        const named_group = extd.decode(tls.NamedGroup);
                                        const key_size = extd.decode(u16);
                                        try extd.ensure(key_size);
                                        try key_share.exchange(named_group, extd.slice(key_size));
                                    },
                                    else => {},
                                }
                            }
                        }

                        tls_version = @enumFromInt(supported_version orelse legacy_version);
                        switch (tls_version) {
                            .tls_1_3 => if (!mem.eql(u8, legacy_session_id_echo, &legacy_session_id)) return error.TlsIllegalParameter,
                            .tls_1_2 => if (mem.eql(u8, server_hello_rand[24..31], "DOWNGRD") and
                                server_hello_rand[31] >> 1 == 0x00) return error.TlsIllegalParameter,
                            else => return error.TlsIllegalParameter,
                        }

                        switch (cipher_suite_tag) {
                            inline .AES_128_GCM_SHA256,
                            .AES_256_GCM_SHA384,
                            .CHACHA20_POLY1305_SHA256,
                            .AEGIS_256_SHA512,
                            .AEGIS_128L_SHA256,

                            .ECDHE_RSA_WITH_AES_128_GCM_SHA256,
                            .ECDHE_RSA_WITH_AES_256_GCM_SHA384,
                            .ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
                            => |tag| {
                                handshake_cipher = @unionInit(tls.HandshakeCipher, @tagName(tag.with()), .{
                                    .transcript_hash = .init(.{}),
                                    .version = undefined,
                                });
                                const p = &@field(handshake_cipher, @tagName(tag.with()));
                                p.transcript_hash.update(cleartext_header[tls.record_header_len..]); // Client Hello part 1
                                p.transcript_hash.update(host); // Client Hello part 2
                                p.transcript_hash.update(wrapped_handshake);
                            },

                            else => return error.TlsIllegalParameter,
                        }
                        switch (tls_version) {
                            .tls_1_3 => {
                                switch (cipher_suite_tag) {
                                    inline .AES_128_GCM_SHA256,
                                    .AES_256_GCM_SHA384,
                                    .CHACHA20_POLY1305_SHA256,
                                    .AEGIS_256_SHA512,
                                    .AEGIS_128L_SHA256,
                                    => |tag| {
                                        const sk = key_share.getSharedSecret() orelse return error.TlsIllegalParameter;
                                        const p = &@field(handshake_cipher, @tagName(tag.with()));
                                        const P = @TypeOf(p.*).A;
                                        const hello_hash = p.transcript_hash.peek();
                                        const zeroes = [1]u8{0} ** P.Hash.digest_length;
                                        const early_secret = P.Hkdf.extract(&[1]u8{0}, &zeroes);
                                        const empty_hash = tls.emptyHash(P.Hash);
                                        p.version = .{ .tls_1_3 = undefined };
                                        const pv = &p.version.tls_1_3;
                                        const hs_derived_secret = hkdfExpandLabel(P.Hkdf, early_secret, "derived", &empty_hash, P.Hash.digest_length);
                                        pv.handshake_secret = P.Hkdf.extract(&hs_derived_secret, sk);
                                        const ap_derived_secret = hkdfExpandLabel(P.Hkdf, pv.handshake_secret, "derived", &empty_hash, P.Hash.digest_length);
                                        pv.master_secret = P.Hkdf.extract(&ap_derived_secret, &zeroes);
                                        const client_secret = hkdfExpandLabel(P.Hkdf, pv.handshake_secret, "c hs traffic", &hello_hash, P.Hash.digest_length);
                                        const server_secret = hkdfExpandLabel(P.Hkdf, pv.handshake_secret, "s hs traffic", &hello_hash, P.Hash.digest_length);
                                        if (options.ssl_key_log) |key_log| logSecrets(key_log.writer, .{
                                            .client_random = &client_hello_rand,
                                        }, .{
                                            .SERVER_HANDSHAKE_TRAFFIC_SECRET = &server_secret,
                                            .CLIENT_HANDSHAKE_TRAFFIC_SECRET = &client_secret,
                                        });
                                        pv.client_finished_key = hkdfExpandLabel(P.Hkdf, client_secret, "finished", "", P.Hmac.key_length);
                                        pv.server_finished_key = hkdfExpandLabel(P.Hkdf, server_secret, "finished", "", P.Hmac.key_length);
                                        pv.client_handshake_key = hkdfExpandLabel(P.Hkdf, client_secret, "key", "", P.AEAD.key_length);
                                        pv.server_handshake_key = hkdfExpandLabel(P.Hkdf, server_secret, "key", "", P.AEAD.key_length);
                                        pv.client_handshake_iv = hkdfExpandLabel(P.Hkdf, client_secret, "iv", "", P.AEAD.nonce_length);
                                        pv.server_handshake_iv = hkdfExpandLabel(P.Hkdf, server_secret, "iv", "", P.AEAD.nonce_length);
                                    },
                                    else => return error.TlsIllegalParameter,
                                }
                                pending_cipher_state = .handshake;
                                handshake_state = .encrypted_extensions;
                            },
                            .tls_1_2 => switch (cipher_suite_tag) {
                                .ECDHE_RSA_WITH_AES_128_GCM_SHA256,
                                .ECDHE_RSA_WITH_AES_256_GCM_SHA384,
                                .ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
                                => handshake_state = .certificate,
                                else => return error.TlsIllegalParameter,
                            },
                            else => return error.TlsIllegalParameter,
                        }
                    },
                    .encrypted_extensions => {
                        if (tls_version != .tls_1_3) return error.TlsUnexpectedMessage;
                        if (cipher_state != .handshake) return error.TlsUnexpectedMessage;
                        if (handshake_state != .encrypted_extensions) return error.TlsUnexpectedMessage;
                        switch (handshake_cipher) {
                            inline else => |*p| p.transcript_hash.update(wrapped_handshake),
                        }
                        try hsd.ensure(2);
                        const total_ext_size = hsd.decode(u16);
                        var all_extd = try hsd.sub(total_ext_size);
                        while (!all_extd.eof()) {
                            try all_extd.ensure(4);
                            const et = all_extd.decode(tls.ExtensionType);
                            const ext_size = all_extd.decode(u16);
                            const extd = try all_extd.sub(ext_size);
                            _ = extd;
                            switch (et) {
                                .server_name => {},
                                else => {},
                            }
                        }
                        handshake_state = .certificate;
                    },
                    .certificate => cert: {
                        if (cipher_state == .application) return error.TlsUnexpectedMessage;
                        switch (handshake_state) {
                            .certificate => {},
                            .trust_chain_established => break :cert,
                            else => return error.TlsUnexpectedMessage,
                        }
                        switch (handshake_cipher) {
                            inline else => |*p| p.transcript_hash.update(wrapped_handshake),
                        }

                        switch (tls_version) {
                            .tls_1_3 => {
                                try hsd.ensure(1 + 3);
                                const cert_req_ctx_len = hsd.decode(u8);
                                if (cert_req_ctx_len != 0) return error.TlsIllegalParameter;
                            },
                            .tls_1_2 => try hsd.ensure(3),
                            else => unreachable,
                        }
                        const certs_size = hsd.decode(u24);
                        var certs_decoder = try hsd.sub(certs_size);
                        while (!certs_decoder.eof()) {
                            try certs_decoder.ensure(3);
                            const cert_size = certs_decoder.decode(u24);
                            const certd = try certs_decoder.sub(cert_size);

                            if (tls_version == .tls_1_3) {
                                try certs_decoder.ensure(2);
                                const total_ext_size = certs_decoder.decode(u16);
                                const all_extd = try certs_decoder.sub(total_ext_size);
                                _ = all_extd;
                            }

                            const subject_cert: Certificate = .{
                                .buffer = certd.buf,
                                .index = @intCast(certd.idx),
                            };
                            const subject = try subject_cert.parse();
                            if (cert_index == 0) {
                                // Verify the host on the first certificate.
                                switch (options.host) {
                                    .no_verification => {},
                                    .explicit => try subject.verifyHostName(host),
                                }

                                // Keep track of the public key for the
                                // certificate_verify message later.
                                try main_cert_pub_key.init(subject.pub_key_algo, subject.pubKey());
                            } else {
                                try prev_cert.verify(subject, now_sec);
                            }

                            switch (options.ca) {
                                .no_verification => {
                                    handshake_state = .trust_chain_established;
                                    break :cert;
                                },
                                .self_signed => {
                                    try subject.verify(subject, now_sec);
                                    handshake_state = .trust_chain_established;
                                    break :cert;
                                },
                                .bundle => |ca_bundle| if (ca_bundle.verify(subject, now_sec)) |_| {
                                    handshake_state = .trust_chain_established;
                                    break :cert;
                                } else |err| switch (err) {
                                    error.CertificateIssuerNotFound => {},
                                    else => |e| return e,
                                },
                            }

                            prev_cert = subject;
                            cert_index += 1;
                        }
                        cert_buf_index += 1;
                    },
                    .server_key_exchange => {
                        if (tls_version != .tls_1_2) return error.TlsUnexpectedMessage;
                        if (cipher_state != .cleartext) return error.TlsUnexpectedMessage;
                        switch (handshake_state) {
                            .trust_chain_established => {},
                            .certificate => return error.TlsCertificateNotVerified,
                            else => return error.TlsUnexpectedMessage,
                        }

                        switch (handshake_cipher) {
                            inline else => |*p| p.transcript_hash.update(wrapped_handshake),
                        }
                        try hsd.ensure(1 + 2 + 1);
                        const curve_type = hsd.decode(u8);
                        if (curve_type != 0x03) return error.TlsIllegalParameter; // named_curve
                        const named_group = hsd.decode(tls.NamedGroup);
                        const key_size = hsd.decode(u8);
                        try hsd.ensure(key_size);
                        const server_pub_key = hsd.slice(key_size);
                        try main_cert_pub_key.verifySignature(&hsd, &.{ &client_hello_rand, &server_hello_rand, hsd.buf[0..hsd.idx] });
                        try key_share.exchange(named_group, server_pub_key);
                        handshake_state = .server_hello_done;
                    },
                    .server_hello_done => {
                        if (tls_version != .tls_1_2) return error.TlsUnexpectedMessage;
                        if (cipher_state != .cleartext) return error.TlsUnexpectedMessage;
                        if (handshake_state != .server_hello_done) return error.TlsUnexpectedMessage;

                        const client_key_exchange_msg = .{@intFromEnum(tls.ContentType.handshake)} ++
                            int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
                            array(u16, u8, .{@intFromEnum(tls.HandshakeType.client_key_exchange)} ++
                                array(u24, u8, array(u8, u8, key_share.secp256r1_kp.public_key.toUncompressedSec1())));
                        const client_change_cipher_spec_msg = .{@intFromEnum(tls.ContentType.change_cipher_spec)} ++
                            int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
                            array(u16, tls.ChangeCipherSpecType, .{.change_cipher_spec});
                        const pre_master_secret = key_share.getSharedSecret().?;
                        switch (handshake_cipher) {
                            inline else => |*p| {
                                const P = @TypeOf(p.*).A;
                                p.transcript_hash.update(wrapped_handshake);
                                p.transcript_hash.update(client_key_exchange_msg[tls.record_header_len..]);
                                const master_secret = hmacExpandLabel(P.Hmac, pre_master_secret, &.{
                                    "master secret",
                                    &client_hello_rand,
                                    &server_hello_rand,
                                }, 48);
                                if (options.ssl_key_log) |key_log| logSecrets(key_log.writer, .{
                                    .client_random = &client_hello_rand,
                                }, .{
                                    .CLIENT_RANDOM = &master_secret,
                                });
                                const key_block = hmacExpandLabel(
                                    P.Hmac,
                                    &master_secret,
                                    &.{ "key expansion", &server_hello_rand, &client_hello_rand },
                                    @sizeOf(P.Tls_1_2),
                                );
                                const client_verify_cleartext = .{@intFromEnum(tls.HandshakeType.finished)} ++
                                    array(u24, u8, hmacExpandLabel(
                                        P.Hmac,
                                        &master_secret,
                                        &.{ "client finished", &p.transcript_hash.peek() },
                                        P.verify_data_length,
                                    ));
                                p.transcript_hash.update(&client_verify_cleartext);
                                p.version = .{ .tls_1_2 = .{
                                    .expected_server_verify_data = hmacExpandLabel(
                                        P.Hmac,
                                        &master_secret,
                                        &.{ "server finished", &p.transcript_hash.finalResult() },
                                        P.verify_data_length,
                                    ),
                                    .app_cipher = mem.bytesToValue(P.Tls_1_2, &key_block),
                                } };
                                const pv = &p.version.tls_1_2;
                                const nonce: [P.AEAD.nonce_length]u8 = nonce: {
                                    const V = @Vector(P.AEAD.nonce_length, u8);
                                    const pad = [1]u8{0} ** (P.AEAD.nonce_length - 8);
                                    const operand: V = pad ++ @as([8]u8, @bitCast(big(write_seq)));
                                    break :nonce @as(V, pv.app_cipher.client_write_IV ++ pv.app_cipher.client_salt) ^ operand;
                                };
                                var client_verify_msg = .{@intFromEnum(tls.ContentType.handshake)} ++
                                    int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
                                    array(u16, u8, nonce[P.fixed_iv_length..].* ++
                                        @as([client_verify_cleartext.len + P.mac_length]u8, undefined));
                                P.AEAD.encrypt(
                                    client_verify_msg[client_verify_msg.len - P.mac_length -
                                        client_verify_cleartext.len ..][0..client_verify_cleartext.len],
                                    client_verify_msg[client_verify_msg.len - P.mac_length ..][0..P.mac_length],
                                    &client_verify_cleartext,
                                    mem.toBytes(big(write_seq)) ++ client_verify_msg[0 .. 1 + 2] ++ int(u16, client_verify_cleartext.len),
                                    nonce,
                                    pv.app_cipher.client_write_key,
                                );
                                var all_msgs_vec: [3][]const u8 = .{
                                    &client_key_exchange_msg,
                                    &client_change_cipher_spec_msg,
                                    &client_verify_msg,
                                };
                                try output.writeVecAll(&all_msgs_vec);
                                try output.flush();
                            },
                        }
                        write_seq += 1;
                        pending_cipher_state = .application;
                        handshake_state = .finished;
                    },
                    .certificate_verify => {
                        if (tls_version != .tls_1_3) return error.TlsUnexpectedMessage;
                        if (cipher_state != .handshake) return error.TlsUnexpectedMessage;
                        switch (handshake_state) {
                            .trust_chain_established => {},
                            .certificate => return error.TlsCertificateNotVerified,
                            else => return error.TlsUnexpectedMessage,
                        }
                        switch (handshake_cipher) {
                            inline else => |*p| {
                                try main_cert_pub_key.verifySignature(&hsd, &.{
                                    " " ** 64 ++ "TLS 1.3, server CertificateVerify\x00",
                                    &p.transcript_hash.peek(),
                                });
                                p.transcript_hash.update(wrapped_handshake);
                            },
                        }
                        handshake_state = .finished;
                    },
                    .finished => {
                        if (cipher_state == .cleartext) return error.TlsUnexpectedMessage;
                        if (handshake_state != .finished) return error.TlsUnexpectedMessage;
                        // This message is to trick buggy proxies into behaving correctly.
                        const client_change_cipher_spec_msg = .{@intFromEnum(tls.ContentType.change_cipher_spec)} ++
                            int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
                            array(u16, tls.ChangeCipherSpecType, .{.change_cipher_spec});
                        const app_cipher = app_cipher: switch (handshake_cipher) {
                            inline else => |*p, tag| switch (tls_version) {
                                .tls_1_3 => {
                                    const pv = &p.version.tls_1_3;
                                    const P = @TypeOf(p.*).A;
                                    try hsd.ensure(P.Hmac.mac_length);
                                    const finished_digest = p.transcript_hash.peek();
                                    p.transcript_hash.update(wrapped_handshake);
                                    const expected_server_verify_data = tls.hmac(P.Hmac, &finished_digest, pv.server_finished_key);
                                    if (!std.crypto.timing_safe.eql([P.Hmac.mac_length]u8, expected_server_verify_data, hsd.array(P.Hmac.mac_length).*)) return error.TlsDecryptError;
                                    const handshake_hash = p.transcript_hash.finalResult();
                                    const verify_data = tls.hmac(P.Hmac, &handshake_hash, pv.client_finished_key);
                                    const out_cleartext = .{@intFromEnum(tls.HandshakeType.finished)} ++
                                        array(u24, u8, verify_data) ++
                                        .{@intFromEnum(tls.ContentType.handshake)};

                                    const wrapped_len = out_cleartext.len + P.AEAD.tag_length;

                                    var finished_msg = .{@intFromEnum(tls.ContentType.application_data)} ++
                                        int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
                                        array(u16, u8, @as([wrapped_len]u8, undefined));

                                    const ad = finished_msg[0..tls.record_header_len];
                                    const ciphertext = finished_msg[tls.record_header_len..][0..out_cleartext.len];
                                    const auth_tag = finished_msg[finished_msg.len - P.AEAD.tag_length ..];
                                    const nonce = pv.client_handshake_iv;
                                    P.AEAD.encrypt(ciphertext, auth_tag, &out_cleartext, ad, nonce, pv.client_handshake_key);

                                    var all_msgs_vec: [2][]const u8 = .{
                                        &client_change_cipher_spec_msg,
                                        &finished_msg,
                                    };
                                    try output.writeVecAll(&all_msgs_vec);
                                    try output.flush();

                                    const client_secret = hkdfExpandLabel(P.Hkdf, pv.master_secret, "c ap traffic", &handshake_hash, P.Hash.digest_length);
                                    const server_secret = hkdfExpandLabel(P.Hkdf, pv.master_secret, "s ap traffic", &handshake_hash, P.Hash.digest_length);
                                    if (options.ssl_key_log) |key_log| logSecrets(key_log.writer, .{
                                        .counter = key_seq,
                                        .client_random = &client_hello_rand,
                                    }, .{
                                        .SERVER_TRAFFIC_SECRET = &server_secret,
                                        .CLIENT_TRAFFIC_SECRET = &client_secret,
                                    });
                                    key_seq += 1;
                                    break :app_cipher @unionInit(tls.ApplicationCipher, @tagName(tag), .{ .tls_1_3 = .{
                                        .client_secret = client_secret,
                                        .server_secret = server_secret,
                                        .client_key = hkdfExpandLabel(P.Hkdf, client_secret, "key", "", P.AEAD.key_length),
                                        .server_key = hkdfExpandLabel(P.Hkdf, server_secret, "key", "", P.AEAD.key_length),
                                        .client_iv = hkdfExpandLabel(P.Hkdf, client_secret, "iv", "", P.AEAD.nonce_length),
                                        .server_iv = hkdfExpandLabel(P.Hkdf, server_secret, "iv", "", P.AEAD.nonce_length),
                                    } });
                                },
                                .tls_1_2 => {
                                    const pv = &p.version.tls_1_2;
                                    const P = @TypeOf(p.*).A;
                                    try hsd.ensure(P.verify_data_length);
                                    if (!std.crypto.timing_safe.eql([P.verify_data_length]u8, pv.expected_server_verify_data, hsd.array(P.verify_data_length).*)) return error.TlsDecryptError;
                                    break :app_cipher @unionInit(tls.ApplicationCipher, @tagName(tag), .{ .tls_1_2 = pv.app_cipher });
                                },
                                else => unreachable,
                            },
                        };
                        if (options.ssl_key_log) |ssl_key_log| ssl_key_log.* = .{
                            .client_key_seq = key_seq,
                            .server_key_seq = key_seq,
                            .client_random = client_hello_rand,
                            .writer = ssl_key_log.writer,
                        };
                        return .{
                            .input = input,
                            .reader = .{
                                .buffer = options.read_buffer,
                                .vtable = &.{
                                    .stream = stream,
                                    .readVec = readVec,
                                },
                                .seek = 0,
                                .end = 0,
                            },
                            .output = output,
                            .writer = .{
                                .buffer = options.write_buffer,
                                .vtable = &.{
                                    .drain = drain,
                                    .flush = flush,
                                },
                            },
                            .tls_version = tls_version,
                            .read_seq = switch (tls_version) {
                                .tls_1_3 => 0,
                                .tls_1_2 => read_seq,
                                else => unreachable,
                            },
                            .write_seq = switch (tls_version) {
                                .tls_1_3 => 0,
                                .tls_1_2 => write_seq,
                                else => unreachable,
                            },
                            .received_close_notify = false,
                            .allow_truncation_attacks = options.allow_truncation_attacks,
                            .application_cipher = app_cipher,
                            .ssl_key_log = options.ssl_key_log,
                        };
                    },
                    else => return error.TlsUnexpectedMessage,
                }
                if (ctd.eof()) break;
                cleartext_fragment_start = ctd.idx;
            },
            else => return error.TlsUnexpectedMessage,
        }
        cleartext_fragment_start = 0;
        cleartext_fragment_end = 0;
    }
}

fn drain(w: *Writer, data: []const []const u8, splat: usize) Writer.Error!usize {
    const c: *Client = @alignCast(@fieldParentPtr("writer", w));
    const output = c.output;
    const ciphertext_buf = try output.writableSliceGreedy(min_buffer_len);
    var ciphertext_end: usize = 0;
    var total_clear: usize = 0;
    done: {
        {
            const buf = w.buffered();
            const prepared = prepareCiphertextRecord(c, ciphertext_buf[ciphertext_end..], buf, .application_data);
            total_clear += prepared.cleartext_len;
            ciphertext_end += prepared.ciphertext_end;
            if (prepared.cleartext_len < buf.len) break :done;
        }
        for (data[0 .. data.len - 1]) |buf| {
            if (buf.len < min_buffer_len) break :done;
            const prepared = prepareCiphertextRecord(c, ciphertext_buf[ciphertext_end..], buf, .application_data);
            total_clear += prepared.cleartext_len;
            ciphertext_end += prepared.ciphertext_end;
            if (prepared.cleartext_len < buf.len) break :done;
        }
        const buf = data[data.len - 1];
        for (0..splat) |_| {
            if (buf.len < min_buffer_len) break :done;
            const prepared = prepareCiphertextRecord(c, ciphertext_buf[ciphertext_end..], buf, .application_data);
            total_clear += prepared.cleartext_len;
            ciphertext_end += prepared.ciphertext_end;
            if (prepared.cleartext_len < buf.len) break :done;
        }
    }
    output.advance(ciphertext_end);
    return w.consume(total_clear);
}

fn flush(w: *Writer) Writer.Error!void {
    const c: *Client = @alignCast(@fieldParentPtr("writer", w));
    const output = c.output;
    const ciphertext_buf = try output.writableSliceGreedy(min_buffer_len);
    const prepared = prepareCiphertextRecord(c, ciphertext_buf, w.buffered(), .application_data);
    output.advance(prepared.ciphertext_end);
    w.end = 0;
}

/// Sends a `close_notify` alert, which is necessary for the server to
/// distinguish between a properly finished TLS session, or a truncation
/// attack.
pub fn end(c: *Client) Writer.Error!void {
    try flush(&c.writer);
    const output = c.output;
    const ciphertext_buf = try output.writableSliceGreedy(min_buffer_len);
    const prepared = prepareCiphertextRecord(c, ciphertext_buf, &tls.close_notify_alert, .alert);
    output.advance(prepared.ciphertext_end);
}

fn prepareCiphertextRecord(
    c: *Client,
    ciphertext_buf: []u8,
    bytes: []const u8,
    inner_content_type: tls.ContentType,
) struct {
    ciphertext_end: usize,
    cleartext_len: usize,
} {
    // Due to the trailing inner content type byte in the ciphertext, we need
    // an additional buffer for storing the cleartext into before encrypting.
    var cleartext_buf: [max_ciphertext_len]u8 = undefined;
    var ciphertext_end: usize = 0;
    var bytes_i: usize = 0;
    switch (c.application_cipher) {
        inline else => |*p| switch (c.tls_version) {
            .tls_1_3 => {
                const pv = &p.tls_1_3;
                const P = @TypeOf(p.*);
                const overhead_len = tls.record_header_len + P.AEAD.tag_length + 1;
                while (true) {
                    const encrypted_content_len: u16 = @min(
                        bytes.len - bytes_i,
                        tls.max_ciphertext_inner_record_len,
                        ciphertext_buf.len -| (overhead_len + ciphertext_end),
                    );
                    if (encrypted_content_len == 0) return .{
                        .ciphertext_end = ciphertext_end,
                        .cleartext_len = bytes_i,
                    };

                    @memcpy(cleartext_buf[0..encrypted_content_len], bytes[bytes_i..][0..encrypted_content_len]);
                    cleartext_buf[encrypted_content_len] = @intFromEnum(inner_content_type);
                    bytes_i += encrypted_content_len;
                    const ciphertext_len = encrypted_content_len + 1;
                    const cleartext = cleartext_buf[0..ciphertext_len];

                    const ad = ciphertext_buf[ciphertext_end..][0..tls.record_header_len];
                    ad.* = .{@intFromEnum(tls.ContentType.application_data)} ++
                        int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
                        int(u16, ciphertext_len + P.AEAD.tag_length);
                    ciphertext_end += ad.len;
                    const ciphertext = ciphertext_buf[ciphertext_end..][0..ciphertext_len];
                    ciphertext_end += ciphertext_len;
                    const auth_tag = ciphertext_buf[ciphertext_end..][0..P.AEAD.tag_length];
                    ciphertext_end += auth_tag.len;
                    const nonce = nonce: {
                        const V = @Vector(P.AEAD.nonce_length, u8);
                        const pad = [1]u8{0} ** (P.AEAD.nonce_length - 8);
                        const operand: V = pad ++ mem.toBytes(big(c.write_seq));
                        break :nonce @as(V, pv.client_iv) ^ operand;
                    };
                    P.AEAD.encrypt(ciphertext, auth_tag, cleartext, ad, nonce, pv.client_key);
                    c.write_seq += 1; // TODO send key_update on overflow
                }
            },
            .tls_1_2 => {
                const pv = &p.tls_1_2;
                const P = @TypeOf(p.*);
                const overhead_len = tls.record_header_len + P.record_iv_length + P.mac_length;
                while (true) {
                    const message_len: u16 = @min(
                        bytes.len - bytes_i,
                        tls.max_ciphertext_inner_record_len,
                        ciphertext_buf.len -| (overhead_len + ciphertext_end),
                    );
                    if (message_len == 0) return .{
                        .ciphertext_end = ciphertext_end,
                        .cleartext_len = bytes_i,
                    };

                    @memcpy(cleartext_buf[0..message_len], bytes[bytes_i..][0..message_len]);
                    bytes_i += message_len;
                    const cleartext = cleartext_buf[0..message_len];

                    const record_header = ciphertext_buf[ciphertext_end..][0..tls.record_header_len];
                    ciphertext_end += tls.record_header_len;
                    record_header.* = .{@intFromEnum(inner_content_type)} ++
                        int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
                        int(u16, P.record_iv_length + message_len + P.mac_length);
                    const ad = mem.toBytes(big(c.write_seq)) ++ record_header[0 .. 1 + 2] ++ int(u16, message_len);
                    const record_iv = ciphertext_buf[ciphertext_end..][0..P.record_iv_length];
                    ciphertext_end += P.record_iv_length;
                    const nonce: [P.AEAD.nonce_length]u8 = nonce: {
                        const V = @Vector(P.AEAD.nonce_length, u8);
                        const pad = [1]u8{0} ** (P.AEAD.nonce_length - 8);
                        const operand: V = pad ++ @as([8]u8, @bitCast(big(c.write_seq)));
                        break :nonce @as(V, pv.client_write_IV ++ pv.client_salt) ^ operand;
                    };
                    record_iv.* = nonce[P.fixed_iv_length..].*;
                    const ciphertext = ciphertext_buf[ciphertext_end..][0..message_len];
                    ciphertext_end += message_len;
                    const auth_tag = ciphertext_buf[ciphertext_end..][0..P.mac_length];
                    ciphertext_end += P.mac_length;
                    P.AEAD.encrypt(ciphertext, auth_tag, cleartext, ad, nonce, pv.client_write_key);
                    c.write_seq += 1; // TODO send key_update on overflow
                }
            },
            else => unreachable,
        },
    }
}

pub fn eof(c: Client) bool {
    return c.received_close_notify;
}

fn stream(r: *Reader, w: *Writer, limit: std.Io.Limit) Reader.StreamError!usize {
    // This function writes exclusively to the buffer.
    _ = w;
    _ = limit;
    const c: *Client = @alignCast(@fieldParentPtr("reader", r));
    return readIndirect(c);
}

fn readVec(r: *Reader, data: [][]u8) Reader.Error!usize {
    // This function writes exclusively to the buffer.
    _ = data;
    const c: *Client = @alignCast(@fieldParentPtr("reader", r));
    return readIndirect(c);
}

fn readIndirect(c: *Client) Reader.Error!usize {
    const r = &c.reader;
    if (c.eof()) return error.EndOfStream;
    const input = c.input;
    // If at least one full encrypted record is not buffered, read once.
    const record_header = input.peek(tls.record_header_len) catch |err| switch (err) {
        error.EndOfStream => {
            // This is either a truncation attack, a bug in the server, or an
            // intentional omission of the close_notify message due to truncation
            // detection handled above the TLS layer.
            if (c.allow_truncation_attacks) {
                c.received_close_notify = true;
                return error.EndOfStream;
            } else {
                return failRead(c, error.TlsConnectionTruncated);
            }
        },
        error.ReadFailed => return error.ReadFailed,
    };
    const ct: tls.ContentType = @enumFromInt(record_header[0]);
    const legacy_version = mem.readInt(u16, record_header[1..][0..2], .big);
    _ = legacy_version;
    const record_len = mem.readInt(u16, record_header[3..][0..2], .big);
    if (record_len > max_ciphertext_len) return failRead(c, error.TlsRecordOverflow);
    const record_end = 5 + record_len;
    if (record_end > input.buffered().len) {
        input.fillMore() catch |err| switch (err) {
            error.EndOfStream => return failRead(c, error.TlsConnectionTruncated),
            error.ReadFailed => return error.ReadFailed,
        };
        if (record_end > input.buffered().len) return 0;
    }

    const cleartext_len, const inner_ct: tls.ContentType = cleartext: switch (c.application_cipher) {
        inline else => |*p| switch (c.tls_version) {
            .tls_1_3 => {
                const pv = &p.tls_1_3;
                const P = @TypeOf(p.*);
                const ad = input.take(tls.record_header_len) catch unreachable; // already peeked
                const ciphertext_len = record_len - P.AEAD.tag_length;
                const ciphertext = input.take(ciphertext_len) catch unreachable; // already peeked
                const auth_tag = (input.takeArray(P.AEAD.tag_length) catch unreachable).*; // already peeked
                const nonce = nonce: {
                    const V = @Vector(P.AEAD.nonce_length, u8);
                    const pad = [1]u8{0} ** (P.AEAD.nonce_length - 8);
                    const operand: V = pad ++ mem.toBytes(big(c.read_seq));
                    break :nonce @as(V, pv.server_iv) ^ operand;
                };
                rebase(r, ciphertext.len);
                const cleartext = r.buffer[r.end..][0..ciphertext.len];
                P.AEAD.decrypt(cleartext, ciphertext, auth_tag, ad, nonce, pv.server_key) catch
                    return failRead(c, error.TlsBadRecordMac);
                // TODO use scalar, non-slice version
                const msg = mem.trimRight(u8, cleartext, "\x00");
                break :cleartext .{ msg.len - 1, @enumFromInt(msg[msg.len - 1]) };
            },
            .tls_1_2 => {
                const pv = &p.tls_1_2;
                const P = @TypeOf(p.*);
                const message_len: u16 = record_len - P.record_iv_length - P.mac_length;
                const ad_header = input.take(tls.record_header_len) catch unreachable; // already peeked
                const ad = mem.toBytes(big(c.read_seq)) ++
                    ad_header[0 .. 1 + 2] ++
                    mem.toBytes(big(message_len));
                const record_iv = (input.takeArray(P.record_iv_length) catch unreachable).*; // already peeked
                const masked_read_seq = c.read_seq &
                    comptime std.math.shl(u64, std.math.maxInt(u64), 8 * P.record_iv_length);
                const nonce: [P.AEAD.nonce_length]u8 = nonce: {
                    const V = @Vector(P.AEAD.nonce_length, u8);
                    const pad = [1]u8{0} ** (P.AEAD.nonce_length - 8);
                    const operand: V = pad ++ @as([8]u8, @bitCast(big(masked_read_seq)));
                    break :nonce @as(V, pv.server_write_IV ++ record_iv) ^ operand;
                };
                const ciphertext = input.take(message_len) catch unreachable; // already peeked
                const auth_tag = (input.takeArray(P.mac_length) catch unreachable).*; // already peeked
                rebase(r, ciphertext.len);
                const cleartext = r.buffer[r.end..][0..ciphertext.len];
                P.AEAD.decrypt(cleartext, ciphertext, auth_tag, ad, nonce, pv.server_write_key) catch
                    return failRead(c, error.TlsBadRecordMac);
                break :cleartext .{ cleartext.len, ct };
            },
            else => unreachable,
        },
    };
    const cleartext = r.buffer[r.end..][0..cleartext_len];
    c.read_seq = std.math.add(u64, c.read_seq, 1) catch return failRead(c, error.TlsSequenceOverflow);
    switch (inner_ct) {
        .alert => {
            if (cleartext.len != 2) return failRead(c, error.TlsDecodeError);
            const alert: tls.Alert = .{
                .level = @enumFromInt(cleartext[0]),
                .description = @enumFromInt(cleartext[1]),
            };
            switch (alert.description) {
                .close_notify => {
                    c.received_close_notify = true;
                    return 0;
                },
                .user_canceled => {
                    // TODO: handle server-side closures
                    return failRead(c, error.TlsUnexpectedMessage);
                },
                else => {
                    c.alert = alert;
                    return failRead(c, error.TlsAlert);
                },
            }
        },
        .handshake => {
            var ct_i: usize = 0;
            while (true) {
                const handshake_type: tls.HandshakeType = @enumFromInt(cleartext[ct_i]);
                ct_i += 1;
                const handshake_len = mem.readInt(u24, cleartext[ct_i..][0..3], .big);
                ct_i += 3;
                const next_handshake_i = ct_i + handshake_len;
                if (next_handshake_i > cleartext.len) return failRead(c, error.TlsBadLength);
                const handshake = cleartext[ct_i..next_handshake_i];
                switch (handshake_type) {
                    .new_session_ticket => {
                        // This client implementation ignores new session tickets.
                    },
                    .key_update => {
                        switch (c.application_cipher) {
                            inline else => |*p| {
                                const pv = &p.tls_1_3;
                                const P = @TypeOf(p.*);
                                const server_secret = hkdfExpandLabel(P.Hkdf, pv.server_secret, "traffic upd", "", P.Hash.digest_length);
                                if (c.ssl_key_log) |key_log| logSecrets(key_log.writer, .{
                                    .counter = key_log.serverCounter(),
                                    .client_random = &key_log.client_random,
                                }, .{
                                    .SERVER_TRAFFIC_SECRET = &server_secret,
                                });
                                pv.server_secret = server_secret;
                                pv.server_key = hkdfExpandLabel(P.Hkdf, server_secret, "key", "", P.AEAD.key_length);
                                pv.server_iv = hkdfExpandLabel(P.Hkdf, server_secret, "iv", "", P.AEAD.nonce_length);
                            },
                        }
                        c.read_seq = 0;

                        switch (@as(tls.KeyUpdateRequest, @enumFromInt(handshake[0]))) {
                            .update_requested => {
                                switch (c.application_cipher) {
                                    inline else => |*p| {
                                        const pv = &p.tls_1_3;
                                        const P = @TypeOf(p.*);
                                        const client_secret = hkdfExpandLabel(P.Hkdf, pv.client_secret, "traffic upd", "", P.Hash.digest_length);
                                        if (c.ssl_key_log) |key_log| logSecrets(key_log.writer, .{
                                            .counter = key_log.clientCounter(),
                                            .client_random = &key_log.client_random,
                                        }, .{
                                            .CLIENT_TRAFFIC_SECRET = &client_secret,
                                        });
                                        pv.client_secret = client_secret;
                                        pv.client_key = hkdfExpandLabel(P.Hkdf, client_secret, "key", "", P.AEAD.key_length);
                                        pv.client_iv = hkdfExpandLabel(P.Hkdf, client_secret, "iv", "", P.AEAD.nonce_length);
                                    },
                                }
                                c.write_seq = 0;
                            },
                            .update_not_requested => {},
                            _ => return failRead(c, error.TlsIllegalParameter),
                        }
                    },
                    else => return failRead(c, error.TlsUnexpectedMessage),
                }
                ct_i = next_handshake_i;
                if (ct_i >= cleartext.len) break;
            }
            return 0;
        },
        .application_data => {
            r.end += cleartext.len;
            return 0;
        },
        else => return failRead(c, error.TlsUnexpectedMessage),
    }
}

fn rebase(r: *Reader, capacity: usize) void {
    if (r.buffer.len - r.end >= capacity) return;
    const data = r.buffer[r.seek..r.end];
    @memmove(r.buffer[0..data.len], data);
    r.seek = 0;
    r.end = data.len;
    assert(r.buffer.len - r.end >= capacity);
}

fn failRead(c: *Client, err: ReadError) error{ReadFailed} {
    c.read_err = err;
    return error.ReadFailed;
}

fn logSecrets(w: *Writer, context: anytype, secrets: anytype) void {
    inline for (@typeInfo(@TypeOf(secrets)).@"struct".fields) |field| w.print("{s}" ++
        (if (@hasField(@TypeOf(context), "counter")) "_{d}" else "") ++ " {x} {x}\n", .{field.name} ++
        (if (@hasField(@TypeOf(context), "counter")) .{context.counter} else .{}) ++ .{
        context.client_random,
        @field(secrets, field.name),
    }) catch {};
}

fn big(x: anytype) @TypeOf(x) {
    return switch (native_endian) {
        .big => x,
        .little => @byteSwap(x),
    };
}

const KeyShare = struct {
    ml_kem768_kp: crypto.kem.ml_kem.MLKem768.KeyPair,
    secp256r1_kp: crypto.sign.ecdsa.EcdsaP256Sha256.KeyPair,
    secp384r1_kp: crypto.sign.ecdsa.EcdsaP384Sha384.KeyPair,
    x25519_kp: crypto.dh.X25519.KeyPair,
    sk_buf: [sk_max_len]u8,
    sk_len: std.math.IntFittingRange(0, sk_max_len),

    const sk_max_len = @max(
        crypto.dh.X25519.shared_length + crypto.kem.ml_kem.MLKem768.shared_length,
        crypto.ecc.P256.scalar.encoded_length,
        crypto.ecc.P384.scalar.encoded_length,
        crypto.dh.X25519.shared_length,
    );

    fn init(seed: [112]u8) error{IdentityElement}!KeyShare {
        return .{
            .ml_kem768_kp = .generate(),
            .secp256r1_kp = try .generateDeterministic(seed[0..32].*),
            .secp384r1_kp = try .generateDeterministic(seed[32..80].*),
            .x25519_kp = try .generateDeterministic(seed[80..112].*),
            .sk_buf = undefined,
            .sk_len = 0,
        };
    }

    fn exchange(
        ks: *KeyShare,
        named_group: tls.NamedGroup,
        server_pub_key: []const u8,
    ) error{ TlsIllegalParameter, TlsDecryptFailure }!void {
        switch (named_group) {
            .x25519_ml_kem768 => {
                const hksl = crypto.kem.ml_kem.MLKem768.ciphertext_length;
                const xksl = hksl + crypto.dh.X25519.public_length;
                if (server_pub_key.len != xksl) return error.TlsIllegalParameter;

                const hsk = ks.ml_kem768_kp.secret_key.decaps(server_pub_key[0..hksl]) catch
                    return error.TlsDecryptFailure;
                const xsk = crypto.dh.X25519.scalarmult(ks.x25519_kp.secret_key, server_pub_key[hksl..xksl].*) catch
                    return error.TlsDecryptFailure;
                @memcpy(ks.sk_buf[0..hsk.len], &hsk);
                @memcpy(ks.sk_buf[hsk.len..][0..xsk.len], &xsk);
                ks.sk_len = hsk.len + xsk.len;
            },
            .secp256r1 => {
                const PublicKey = crypto.sign.ecdsa.EcdsaP256Sha256.PublicKey;
                const pk = PublicKey.fromSec1(server_pub_key) catch return error.TlsDecryptFailure;
                const mul = pk.p.mulPublic(ks.secp256r1_kp.secret_key.bytes, .big) catch
                    return error.TlsDecryptFailure;
                const sk = mul.affineCoordinates().x.toBytes(.big);
                @memcpy(ks.sk_buf[0..sk.len], &sk);
                ks.sk_len = sk.len;
            },
            .secp384r1 => {
                const PublicKey = crypto.sign.ecdsa.EcdsaP384Sha384.PublicKey;
                const pk = PublicKey.fromSec1(server_pub_key) catch return error.TlsDecryptFailure;
                const mul = pk.p.mulPublic(ks.secp384r1_kp.secret_key.bytes, .big) catch
                    return error.TlsDecryptFailure;
                const sk = mul.affineCoordinates().x.toBytes(.big);
                @memcpy(ks.sk_buf[0..sk.len], &sk);
                ks.sk_len = sk.len;
            },
            .x25519 => {
                const ksl = crypto.dh.X25519.public_length;
                if (server_pub_key.len != ksl) return error.TlsIllegalParameter;
                const sk = crypto.dh.X25519.scalarmult(ks.x25519_kp.secret_key, server_pub_key[0..ksl].*) catch
                    return error.TlsDecryptFailure;
                @memcpy(ks.sk_buf[0..sk.len], &sk);
                ks.sk_len = sk.len;
            },
            else => return error.TlsIllegalParameter,
        }
    }

    fn getSharedSecret(ks: *const KeyShare) ?[]const u8 {
        return if (ks.sk_len > 0) ks.sk_buf[0..ks.sk_len] else null;
    }
};

fn SchemeEcdsa(comptime scheme: tls.SignatureScheme) type {
    return switch (scheme) {
        .ecdsa_secp256r1_sha256 => crypto.sign.ecdsa.EcdsaP256Sha256,
        .ecdsa_secp384r1_sha384 => crypto.sign.ecdsa.EcdsaP384Sha384,
        else => @compileError("bad scheme"),
    };
}

fn SchemeRsa(comptime scheme: tls.SignatureScheme) type {
    return switch (scheme) {
        .rsa_pkcs1_sha256,
        .rsa_pkcs1_sha384,
        .rsa_pkcs1_sha512,
        .rsa_pkcs1_sha1,
        => Certificate.rsa.PKCS1v1_5Signature,
        .rsa_pss_rsae_sha256,
        .rsa_pss_rsae_sha384,
        .rsa_pss_rsae_sha512,
        .rsa_pss_pss_sha256,
        .rsa_pss_pss_sha384,
        .rsa_pss_pss_sha512,
        => Certificate.rsa.PSSSignature,
        else => @compileError("bad scheme"),
    };
}

fn SchemeEddsa(comptime scheme: tls.SignatureScheme) type {
    return switch (scheme) {
        .ed25519 => crypto.sign.Ed25519,
        else => @compileError("bad scheme"),
    };
}

fn SchemeHash(comptime scheme: tls.SignatureScheme) type {
    return switch (scheme) {
        .rsa_pkcs1_sha256,
        .ecdsa_secp256r1_sha256,
        .rsa_pss_rsae_sha256,
        .rsa_pss_pss_sha256,
        => crypto.hash.sha2.Sha256,
        .rsa_pkcs1_sha384,
        .ecdsa_secp384r1_sha384,
        .rsa_pss_rsae_sha384,
        .rsa_pss_pss_sha384,
        => crypto.hash.sha2.Sha384,
        .rsa_pkcs1_sha512,
        .ecdsa_secp521r1_sha512,
        .rsa_pss_rsae_sha512,
        .rsa_pss_pss_sha512,
        => crypto.hash.sha2.Sha512,
        .rsa_pkcs1_sha1,
        .ecdsa_sha1,
        => crypto.hash.Sha1,
        else => @compileError("bad scheme"),
    };
}

const CertificatePublicKey = struct {
    algo: Certificate.AlgorithmCategory,
    buf: [600]u8,
    len: u16,

    fn init(
        cert_pub_key: *CertificatePublicKey,
        algo: Certificate.AlgorithmCategory,
        pub_key: []const u8,
    ) error{CertificatePublicKeyInvalid}!void {
        if (pub_key.len > cert_pub_key.buf.len) return error.CertificatePublicKeyInvalid;
        cert_pub_key.algo = algo;
        @memcpy(cert_pub_key.buf[0..pub_key.len], pub_key);
        cert_pub_key.len = @intCast(pub_key.len);
    }

    const VerifyError = error{ TlsDecodeError, TlsBadSignatureScheme, InvalidEncoding } ||
        // ecdsa
        crypto.errors.EncodingError ||
        crypto.errors.NotSquareError ||
        crypto.errors.NonCanonicalError ||
        SchemeEcdsa(.ecdsa_secp256r1_sha256).Signature.VerifyError ||
        SchemeEcdsa(.ecdsa_secp384r1_sha384).Signature.VerifyError ||
        // rsa
        error{TlsBadRsaSignatureBitCount} ||
        Certificate.rsa.PublicKey.ParseDerError ||
        Certificate.rsa.PublicKey.FromBytesError ||
        Certificate.rsa.PSSSignature.VerifyError ||
        Certificate.rsa.PKCS1v1_5Signature.VerifyError ||
        // eddsa
        SchemeEddsa(.ed25519).Signature.VerifyError;

    fn verifySignature(
        cert_pub_key: *const CertificatePublicKey,
        sigd: *tls.Decoder,
        msg: []const []const u8,
    ) VerifyError!void {
        const pub_key = cert_pub_key.buf[0..cert_pub_key.len];

        try sigd.ensure(2 + 2);
        const scheme = sigd.decode(tls.SignatureScheme);
        const sig_len = sigd.decode(u16);
        try sigd.ensure(sig_len);
        const encoded_sig = sigd.slice(sig_len);

        if (cert_pub_key.algo != @as(Certificate.AlgorithmCategory, switch (scheme) {
            .ecdsa_secp256r1_sha256,
            .ecdsa_secp384r1_sha384,
            => .X9_62_id_ecPublicKey,
            .rsa_pkcs1_sha256,
            .rsa_pkcs1_sha384,
            .rsa_pkcs1_sha512,
            .rsa_pss_rsae_sha256,
            .rsa_pss_rsae_sha384,
            .rsa_pss_rsae_sha512,
            .rsa_pkcs1_sha1,
            => .rsaEncryption,
            .rsa_pss_pss_sha256,
            .rsa_pss_pss_sha384,
            .rsa_pss_pss_sha512,
            => .rsassa_pss,
            else => return error.TlsBadSignatureScheme,
        })) return error.TlsBadSignatureScheme;

        switch (scheme) {
            inline .ecdsa_secp256r1_sha256,
            .ecdsa_secp384r1_sha384,
            => |comptime_scheme| {
                const Ecdsa = SchemeEcdsa(comptime_scheme);
                const sig = try Ecdsa.Signature.fromDer(encoded_sig);
                const key = try Ecdsa.PublicKey.fromSec1(pub_key);
                var ver = try sig.verifier(key);
                for (msg) |part| ver.update(part);
                try ver.verify();
            },
            inline .rsa_pkcs1_sha256,
            .rsa_pkcs1_sha384,
            .rsa_pkcs1_sha512,
            .rsa_pss_rsae_sha256,
            .rsa_pss_rsae_sha384,
            .rsa_pss_rsae_sha512,
            .rsa_pss_pss_sha256,
            .rsa_pss_pss_sha384,
            .rsa_pss_pss_sha512,
            .rsa_pkcs1_sha1,
            => |comptime_scheme| {
                const RsaSignature = SchemeRsa(comptime_scheme);
                const Hash = SchemeHash(comptime_scheme);
                const PublicKey = Certificate.rsa.PublicKey;
                const components = try PublicKey.parseDer(pub_key);
                const exponent = components.exponent;
                const modulus = components.modulus;
                switch (modulus.len) {
                    inline 128, 256, 384, 512 => |modulus_len| {
                        const key: PublicKey = try .fromBytes(exponent, modulus);
                        const sig = RsaSignature.fromBytes(modulus_len, encoded_sig);
                        try RsaSignature.concatVerify(modulus_len, sig, msg, key, Hash);
                    },
                    else => return error.TlsBadRsaSignatureBitCount,
                }
            },
            inline .ed25519 => |comptime_scheme| {
                const Eddsa = SchemeEddsa(comptime_scheme);
                if (encoded_sig.len != Eddsa.Signature.encoded_length) return error.InvalidEncoding;
                const sig = Eddsa.Signature.fromBytes(encoded_sig[0..Eddsa.Signature.encoded_length].*);
                if (pub_key.len != Eddsa.PublicKey.encoded_length) return error.InvalidEncoding;
                const key = try Eddsa.PublicKey.fromBytes(pub_key[0..Eddsa.PublicKey.encoded_length].*);
                var ver = try sig.verifier(key);
                for (msg) |part| ver.update(part);
                try ver.verify();
            },
            else => unreachable,
        }
    }
};

/// The priority order here is chosen based on what crypto algorithms Zig has
/// available in the standard library as well as what is faster. Following are
/// a few data points on the relative performance of these algorithms.
///
/// Measurement taken with 0.11.0-dev.810+c2f5848fe
/// on x86_64-linux Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz:
/// zig run .lib/std/crypto/benchmark.zig -OReleaseFast
///       aegis-128l:      15382 MiB/s
///        aegis-256:       9553 MiB/s
///       aes128-gcm:       3721 MiB/s
///       aes256-gcm:       3010 MiB/s
/// chacha20Poly1305:        597 MiB/s
///
/// Measurement taken with 0.11.0-dev.810+c2f5848fe
/// on x86_64-linux Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz:
/// zig run .lib/std/crypto/benchmark.zig -OReleaseFast -mcpu=baseline
///       aegis-128l:        629 MiB/s
/// chacha20Poly1305:        529 MiB/s
///        aegis-256:        461 MiB/s
///       aes128-gcm:        138 MiB/s
///       aes256-gcm:        120 MiB/s
const cipher_suites = if (crypto.core.aes.has_hardware_support)
    array(u16, tls.CipherSuite, .{
        .AEGIS_128L_SHA256,
        .AEGIS_256_SHA512,
        .AES_128_GCM_SHA256,
        .ECDHE_RSA_WITH_AES_128_GCM_SHA256,
        .AES_256_GCM_SHA384,
        .ECDHE_RSA_WITH_AES_256_GCM_SHA384,
        .CHACHA20_POLY1305_SHA256,
        .ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
    })
else
    array(u16, tls.CipherSuite, .{
        .CHACHA20_POLY1305_SHA256,
        .ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
        .AEGIS_128L_SHA256,
        .AEGIS_256_SHA512,
        .AES_128_GCM_SHA256,
        .ECDHE_RSA_WITH_AES_128_GCM_SHA256,
        .AES_256_GCM_SHA384,
        .ECDHE_RSA_WITH_AES_256_GCM_SHA384,
    });
