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Security Model

Cryptographic Peculiarities

IBM Security


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RPOW Crypto Peculiarities

Because the 4758 was designed in the early 1990s, it uses some cryptographic formats and algorithms which are different from the ones which went on to become standards. The basic cryptographic algorithms are RSA, with 1024 bit keys, and triple DES, with 168 bit keys. These are adequate choices which are still considered strong today, although a new system would probably use larger RSA key sizes and would use AES in place of triple DES.

Certificate Format

The 4758's certificate format is ideosyncratic, designed specifically for its own needs. Today, X.509 certificates with custom extension fields would probably be used. But the important aspect of a certificate is not the details of the format, but what is certified, and the 4758's certificate formats are perfectly adequate for its (and our) needs.

Hash Padding

One choice which may raise some cryptographic eyebrows is the use of ISO 9796 as the hash padding mechanism within the RSA signatures used in the certificates. ISO 9796 is widely viewed as broken. However, the manner in which it is used in the IBM 4758 certificate is not affected by known attacks. Those attacks require varying the input to the ISO 9796 padding at will. IBM only uses the output of a SHA-1 hash function as input to the ISO 9796 padding function. This removes the ability of an attacker to control the inputs in the required manner, and allows ISO 9796 to be used securely.

Published Root Key Errors

Certifications made by the IBM 4758 start with IBM's root key. This is published on the IBM web page; unfortunately, that page has a typo. The RSA modulus published there has small divisors, including 3604209483533471833, which a true RSA modulus would never have. The correct IBM root key is published in an appendix to the document Custom Software Interface Reference, available from IBM. The modulus on the IBM web page has two digits transposed (the 6C at the end of the third-to-last line should be a C6).

Certificate Chain Details

4758 certification chains are a little more complicated than might be expected, but there are good reasons for the complexity. One might expect the IBM root key to sign the device key, which would then sign an attestation about the code which is running. Instead, it works like this: The IBM root key signs a "class key", of which there is one for each model of 4758 card. The class key then signs the public part of the card's device key. The private part of the device key, as mentioned above, is secret and never leaves the card.

The next part of the cert chain is rather interesting and surprising, and illustrates the careful thought which the IBM security engineers paid to the design of the card.

First a small digression. There are three main software components in the IBM 4758, identified by layer numbers. Layer 1 is the so-called Miniboot. This is analogous to the BIOS in a PC. It is held in flash ROM and almost never changes. The Miniboot is responsible for performing some power-on self-tests and for loading the other layers. Layer 2 is the operating system, which IBM calls CP/Q++. It is a fairly conventional real-time OS. (There has been some work done on porting Linux as the native OS on the 4758, but IBM has not yet made this code available.) Layer 3 is the application program; in our case, the RPOW server.

The security problem is caused by the fact that we want to securely hash each of these three components; but in fact it is the Miniboot (the 4758 "BIOS") which is responsible for doing the hashes. This means that, among other things, the Miniboot has to hash itself. The problem is that this isn't really persuasive from the security perspective. If somehow a bogus Miniboot got into the card, or someone found a bug in the Miniboot which they were able to exploit to make it misbehave, it could lie about its own hash.

IBM solves this by making sure that the first Miniboot loaded into the card by the factory is reliable, although not completely functional. Then, as the card is configured for use in the field, a new and more functional Miniboot image is loaded. Over time, a card may be updated with subsequent Miniboots as new ones are released by IBM. The key to making this work is that the load of a new Miniboot goes through a distinctive process. Each Miniboot hashes the next one before it loads, generates a new public/private key for the new Miniboot, certifies that key and its hash using the old Miniboot's secret key, and then permanently deletes the old secret key. The new Miniboot then takes over.

The result of this complex process is that instead of a single device key certificate, there is a chain of certificates corresponding to each version or revision of the Miniboot which has existed in the card. Each Miniboot has certified the next one before deleting the old private key. If at any time in this process a bogus or buggy Miniboot got loaded into the card, its hash will show up in the certificate chain, and the card can't hide that fact. The bad Miniboot got its hash taken by the earlier one, which was a good Miniboot. And the only certificate chain which goes back to IBM's root key includes that hash measurement.

In practice, my understanding is that none of the Miniboots which IBM has released so far have had the kind of problems which this mechanism was intended to defend against. The main impact is that the certificate chain includes two or more Miniboot certificates, the last one representing the current contents of the Miniboot.