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Hash Extend

The term extend is not a common cryptographic term, but this operation is used throughout the TPM. An extend operation consists of the following:

1. Start with an existing value, A.

2. Concatenate another value, B (the value to be extended) to it, creating the message, A || B.

3. Hash the resulting message to create a new hash, hash(A || B).

4. This new hash replaces the original value, A.

The whole process can be summarized as: A ← hash (original A || B).

As an example, using SHA-1, extending the digest of the message 'abc' to an initial value of all zero yields this result:

ccd5bd41458de644ac34a2478b58ff819bef5acf

The extend value that results from a series of extend operations effectively records a history of the messages extended into it. Because of the properties of a secure hash, it is infeasible to back up to a previous value. Thus, once a message is extended, there is no “undo” to go backward—to reverse the calculation and erase past history. However, the actual size of the value, such as 32 bytes for the SHA-256 hash algorithm, never changes, no matter how many messages are extended. The fixed length of an extend value coupled with its ability to record a virtually unlimited history of extend operations is critical in the memory-constrained TPM.

Extend is used to update platform configuration register (PCR) values. A PCR is a TPM register holding a hash value. The values extended into the PCR can represent the platform state. Suppose the PCR indicates an untrusted state, but an attacker wants to change the PCR to a trusted value. To do this, the attacker would have to construct another message, starting with the current PCR value, whose resulting hash was a trusted value. The properties of a secure hash dictate that this is infeasible.

Extend is also used in TPM audit logs. The audit logs record TPM commands and responses. Because of the extend properties, an item cannot be removed from the log once it has been added, and the size of the log in the TPM remains constant no matter how many commands are audited.

In addition, extend is used in creating policies that represent how a TPM can be authenticated. This is described in the chapter on extended authorization.

Hashes are also used in a simpler form of authorization, which was also used in the TPM 1.1 and TPM 1.2, called an HMAC.

HMAC: Message Authentication Code

An HMAC is a keyed hash. That is, it performs a secure-hash operation but mixes in a shared secret key, the HMAC key. Because of the properties of a secure hash, given a message, only a party with knowledge of the HMAC key can calculate the result. Applying a key to a message in this way is known as “HMACing” the message.

TPM 1.2 used HMACs throughout to prove to the TPM that the user knew a TPM entity's authorization data, which was used as the HMAC key. TPM 2.0 can also authorize entities this way. The HMAC key is a shared secret between the TPM and the caller. As an example, a TPM object, such as a signing key, may have an associated authorization value that is known to both the TPM and an authorized user of the key. The user constructs a TPM command to use the object and calculates an HMAC over the command message using an HMAC key that is derived in part from the object's authorization value. The TPM receives the command message and performs the same HMAC operation using the same HMAC key. If the results are the same, the TPM knows the command has not been altered (integrity) and that the caller knew the object's HMAC key, which in turn means the caller knew the authorization value. Thus the caller was authorized.

The TPM also uses HMAC for integrity of structures that may at times be stored externally—in other words, proof that an attacker has not altered a value. Here, the HMAC key is a secret known only to the TPM, not shared with any party outside the TPM. When the TPM outputs a value to be stored for later use, it includes an HMAC using its secret.

When the value is later input, the TPM verifies the HMAC to detect any external alteration of the value. Essentially, the HMAC key is a shared secret between the TPM and itself across a time interval: from the time the value is stored externally to the time the value is input to the TPM. One example of this use of HMACs is an authentication ticket. The TPM uses these tickets to prove to itself that it did an operation by producing an HMAC of a digest of the result of the operation, using an internal secret that only it knows.

HMACs can also be used to derive keys, using something called a key derivation function.

 
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