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Bitfrost ist die Sicherheitsplattform von OLPC. In diesem Artikel versuchen wir eine nicht-technische Einführung zu Sicherheitsproblemen zu geben. Ansätze und Prinzipien zur deren Behebung folgen danach. Diese Lösungsansätze wurden der vollständigen Bitfrost-Spezifikation (oder seinem Wiki) entnommen. Wir laden dazu ein, diese Artikel genauch durchzusehen und auf der öffentlichen, von OLPC bereitgestellten Mailing-Liste für Sicherheitsfragen zu diskutieren.

Einführung und Zusammenfassung

Im Jahre 1971 veröffentlichten die Programmierer Ken Thompson und Dennis Ritchie von der Firma AT&T die erste Version von Unix. Dieses Betriebssystem, welches im Jahre 1969 als unbezahltes Projekt namens UNICS begann, wurde ein anderer Name gegeben und erhielt öffentlich von den Bell Labs finanziert, als die Programmierer anboten, Unterstützung zur Textverarbeitung hinzuzufügen. Viele dieser großen Designideen hinter UNIX bestehen noch heute: Populäre Server-Betriebssysteme wie Linux, FreeBSD und viele andere teilen all dieses grundsätzliche UNIX-Design.

Die Unix-Version aus dem Jahr 1971 unterstützte nach dem folgenden Modell Zugang zu sicherheitsrelevanten Benutzerdateien:

• Nicht-Besitzer kann Datei verändern (schreiben)
• Nicht-Besitzer kann Datei lesen
• Besitzer kann Datei verändern (schreiben)
• Besitzer kann Datei lesen
• Datei kann ausgeführt werden
• Datei erhält den Status "uid"

Diese Zugangsrechte sollten für jeden bekannt sein, denn sie sind sehr ähnlich zu den heutigen Zugangsmodellen, den ein Benutzer für seine Dateien, bei seinenm Betriebssystem seiner Wahl, heute einstellen kann. Was sehr beunruhigend - fast geglaubt werden kann - diese Rechte ist das fast das Einzige, was bis heute ein User als wahrer Kontrollmechanismus über seine Dateien hat: Ein Benutzer hat sich zwar dafür entscheiden, seine Dateien vor anderen Personen zu schützen, aber er hat keine wie auch immer geartete Kontrolle, was sein eigenes Programm mit seinen Dateien anstellen kann.

Im Jahre 1971 war dies vielleicht noch akzeptierbar: Dies war 20 Jahre vor Entstehung des Webs und das Bedrohungs-Szenario für die meisten Computer war vollkommen anders, als das, was heutige Anwendungen ausgesetzt sind. Wenn dies so ist, ist es dann eine Überraschung, dass wir Viren und Malware heute nicht stoppen können, wenn unsere Art der Verteidigung seit über 35 Jahren unverändert ist?

Die Krux liegt in der Annahme, dass jedes, durch den Benutzer gestartete Programm auf einem System,

The crux of the problem lies in the assumption that any program executing on a system on the user's behalf should have the exact same abilities and permissions as any other program executing on behalf of the same user. 1971 was seven years before the first ever international packet-switched network came into existence. And the first wide-area network using TCP/IP, the communication suite used by the modern Internet, wasn't created until 1983, twelve years after Thompson and Ritchie designed the file permissions we're discussing. The bottom line is that in 1971, there was almost no conceivable way a program could "come to exist" on a computer except if the account owner — the user — physically transported it to a machine (for instance, on punched tape), or entered it there manually. And so the "all or nothing" security approach, where executing programs have full control over their owner's account, made quite a lot of sense: any code the user executed, she ipso facto trusted for all practical purposes.

Fast forward to today, and the situation couldn't be more different: the starkest contrast is perhaps the Web, where a user's web browser executes untrusted scripting code on just about every web page she visits! Browsers are growing increasingly complex sandboxing systems that try to restrict the abilities of such web scripts, but even the latest browser versions are still fixing bugs in their scripting engine implementations. And don't forget e-mail: anyone can send a user an executable program, and for many years the users' instinctive reaction was to open the attachment and run the program. Untrusted code is everywhere, and the only defense seems to be tedious user training and anti-virus software — the latter assuming it's fully updated, and assuming the anti-virus makers have had time to deconstruct each latest virus and construct a defense for it.

Most technologies and approaches that constitute the Bitfrost platform do not represent original research: they have been known in the security literature for years, some of them have been deployed in the field, and others are being tested in the lab. What makes the OLPC XO laptops notable, however, is that they represent the first time that all these security measures have been carefully put together on a system slated to be introduced to tens or hundreds of millions of users. The laptops are also possibly the first time that a mainstream computing product has been willing to give up compatibility with legacy programs in order to achieve strong security. As an example, you'll find that talk about anti-virus and anti-spyware technology is conspicuously absent from the Bitfrost specification, because the security platform on the XO laptops largely renders these issues moot.

We have set out to create a system that is both drastically more secure and provides drastically more usable security than any mainstream system currently on the market. One result of the dedication to usability is that there is only one protection provided by the Bitfrost platform that requires user response, and even then, it's a simple 'yes or no' question understandable even by young children. The remainder of the security is provided behind the scenes. But pushing the envelope on both security and usability is a tall order, and it's important to note that we have neither tried to create, nor do we believe we have created, a "perfectly secure" system. Notions of perfect security in the real world are foolish, and we distance ourselves up front from any such claims.

The Bitfrost approach

Principles

• Open design

 The laptop's security must not depend upon a secret design implemented in
 hardware or software.

• No lockdown

 Though in their default settings, the laptop's security systems may impose
 various prohibitions on the user's actions, there must exist a way for these
 security systems to be disabled. When that is the case, the machine will
 grant the user complete control.

• No reading required

 Security cannot depend upon the user's ability to read a message from the
 computer and act in an informed and sensible manner. While disabling a
 particular security mechanism may require reading, a machine must be secure
 out of the factory if given to a user who cannot yet read.

• Unobtrusive security

 Whenever possible, the security on the machines must be behind the scenes,
 making its presence known only through subtle visual or audio cues, and never
 getting in the user's way. Whenever in conflict with slight user convenience,
 strong unobtrusive security is to take precedence, though utmost care must be
 taken to ensure such allowances do not seriously or conspicuously reduce the
 usability of the machines. As an example, if a program is found attempting to
 violate a security setting, the user will not be prompted to permit the action;
 the action will simply be denied. If the user wishes to grant permission for
 such an action, she can do so through the graphical security center interface.

Goals

• No user passwords

 With users as young as 5 years old, the security of the laptop cannot depend
 on the user's ability to remember a password. Users cannot be expected to
 choose passwords when they first receive computers.

• No unencrypted authentication

 Authentication of laptops or users will not depend upon identifiers that are
 sent unencrypted over the network.  This means no cleartext passwords of any
 kind will be used in any OLPC protocol and Ethernet MAC addresses will never
 be used for authentication.

• Out-of-the-box security

 The laptop should be both usable and secure out-of-the-box, without the need
 to download security updates when at all possible.

• Limited institutional PKI

 The laptop will be supplied with public keys from OLPC and the country or
 regional authority (e.g. the ministry or department of education), but these
 keys will not be used to validate the identity of laptop users. The sole
 purpose of these keys will be to verify the integrity of bundled software and
 content. Users will be identified through an organically-grown PKI without a
 certified chain of trust — in other words, our approach to PKI is KCM, or
 key continuity management.

• No permanent data loss

 Information on the laptop will be replicated to some centralized storage
 place so that the student can recover it in the event that the laptop is lost,
 stolen or destroyed.

If this subject matter interests you, please read the complete Bitfrost specification, join the OLPC security mailing list, share your thoughts, and join the discussion.