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How TPM 2.0 Developed from TPM 1.2

In early 2000, when the TCG was faced with the choice of a hash algorithm, it had two choices: MD5, which was most widely deployed; and SHA-1, which was stronger and was deployed widely, although not as widely as MD5. SHA-1 was the strongest commercial algorithm at the time and could feasibly be used in a small, cheap package. This was a fortunate choice, because MD5 weaknesses became apparent shortly afterward.

Around 2005, cryptographers published the first significant attack on the SHA-1 digest algorithm. The TPM 1.2 architecture relied heavily on the SHA-1 algorithm and had hard-coded SHA-1 everywhere. Although an analysis of the attack showed that it did not apply to the ways SHA-1 was used in the TPM, a common axiom in cryptography is that cryptographic algorithms only become weaker over time, never stronger. The TCG immediately began work on a TPM 2.0 specification that would be agile with respect to digest algorithms. That is, the specification would not hard-code SHA-1 or any other algorithm, but rather would incorporate an algorithm identifier that would permit design of a TPM using any algorithm without changing the specification. With this change (and other changes allowing all cryptographic algorithms to be agile), the TCG hoped the new specification could be the last major TPM specification to be released.

The original mandate of the TPM 2.0 Work Group within the TCG was only that: digest agility. However, even a cursory look at the TPM 2.0 specification shows that it's far more than TPM 1.2 plus an algorithm identifier. How did that happen?

TPM 1.1b had carefully crafted structures so that serialized versions (the structures translated into byte streams) were compact enough to be encrypted with a 2,048-bit RSA key in a single encryption. That meant there were only 2,048 bits (256 bytes) to work with. No symmetric encryption algorithms were required in the design, which kept the cost down and avoided problems when exporting designs that could do bulk symmetric encryption. RSA was the only required asymmetric algorithm, and performance required that structures be encrypted in one operation.

TPM 2.0 had to provide for digests that were larger than SHA-1's 20 bytes, so it was clear the existing structures were too large. An RSA key could not directly encrypt the serialized structures in one operation. Using multiple operations to encrypt the structures in blocks was impractical, because RSA operations are slow. Increasing the size of the RSA key would mean using key sizes that were not widely used in the industry and would also increase the cost, change the key structures, and slow the chip. Instead, the TPM Work Group decided the specification had to use the common practice of encrypting a symmetric key with the asymmetric key and the data with the symmetric key. Symmetric key operations are well suited for encrypting large byte streams, because they are much faster than asymmetric operations. Symmetric key encryption thus removed the barrier on the size of structures. This freed the specification developers to use their creativity to architect several functions that were distinct from TPM 1.2.

It is said that the best designs come about when the architects make a design and then, having learned all the problems they will encounter, throw away the first design and start over with a second. TPM 2.0 gave the architects the opportunity to do this. However, they still had to make sure that the opportunities for software development that were enabled by the TPM 1.2 design were not lost with the new architecture.

 
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