prepareCiphertextRecord

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] = @backingInt(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.* = .{@backingInt(tls.ContentType.application_data)} ++
                        int(u16, @backingInt(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: [P.AEAD.nonce_length - 8]u8 = @splat(0);
                        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.* = .{@backingInt(inner_content_type)} ++
                        int(u16, @backingInt(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: [P.AEAD.nonce_length - 8]u8 = @splat(0);
                        const operand: V = pad ++ @as([8]u8, @bitCast(@byteSwap(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,
        },
    }
}