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Password, HMAC, and Policy Sessions: What Are They?

All three types of sessions are a means of authorizing actions and, in the case of HMAC and policy sessions, configuring sessions on a per-command basis. Password sessions are the simplest type of authorization: a clear text password is passed down to the TPM to authorize an action. This has obvious security issues if the TPM is being accessed remotely; the intended use of password sessions is for local access. In the TPM, there is a single, always-available password session that is used to authorize a single TPM command with no state preserved between subsequent uses. Because of this, the password session never needs to be started.

HMAC authorizations are a way of using a simple password in a more secure manner; once the calling application and the TPM agree on the password (at the time the entity is created or its authorization value is modified), there is never a need to communicate the password again. This one-time communication of the password to the TPM can be accomplished in a secure manner: that is, the password can be communicated to the TPM in encrypted form. An HMAC session accomplishes this greater level of security by using the password (authValue, as it's called in the TPM 2.0 specification) as one of the inputs into an HMAC that is calculated on commands and responses. On a command, the calling application calculates the HMAC and inserts it in the command byte stream. When the TPM receives the command byte stream, if the TPM determines that the HMAC is calculated correctly, the action is authorized. On a response, the TPM calculates an HMAC on the response and inserts it into the response byte stream. The caller independently calculates the response HMAC and compares it to the response byte stream's HMAC field. If they match, the response data can be trusted. All this works only if both the calling application and the TPM know and agree on the authValue.

HMAC sessions use two nonces—one from the caller (nonceCaller) and one from the TPM (nonceTPM)—to prevent replay attacks. These nonces factor into the HMAC calculation. Because nonceTPM changes for every command that is sent, and the calling application can, if it wants to, change nonceCaller on every command, an attacker can't replay command byte streams. Replayed command bytes streams that use HMAC authorization will always fail because the nonces will be different on the replay.

HMAC sessions maintain state during the lifetime of the session and can be used to authorize multiple actions on TPM entities. An HMAC session is started using the TPM2_StartAuthSession command. When started, HMAC sessions can be configured as bound vs. unbound and salted vs. unsalted sessions. The combination of these two options results in four variations of HMAC sessions; these four variations determine how the session key and HMACs are calculated.

Policy sessions, otherwise known as Enhanced Authorization (EA), are built on top of HMAC sessions and add an extra level of authorization. Whereas HMAC authorizations are based only on an authorization value or password, policy authorizations enhance this with authorizations based on TPM command sequences, TPM state, and external devices such as fingerprint readers, retina scanners, and smart cards, to name a few. Many conditions can be ANDed and ORed together into complex authorization trees, providing unlimited authorization possibilities.

Table 13-1 shows a high-level summary of the various types of authorizations.

Table 13-1. Comparison of the Three Types of Sessions

With that under your belt, let's look at some important nuances in how the specification uses the terms session and authorization. Pay attention here; understanding these will greatly enhance your ability to read and understand the TPM 2.0 specification as well as the rest of this chapter.

 
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