From 6413adfe6bcfdd8cc59a81400d61701312916977 Mon Sep 17 00:00:00 2001 From: Nicolas Williams Date: Sat, 28 Aug 2021 15:11:18 -0500 Subject: [PATCH] Describe TPM2_Policy{Secret,Signed}() --- TPM-Commands/TPM2_PolicySecret.md | 48 ++++++++++++++++++ TPM-Commands/TPM2_PolicySigned.md | 82 +++++++++++++++++++++++++++++++ 2 files changed, 130 insertions(+) create mode 100644 TPM-Commands/TPM2_PolicySecret.md create mode 100644 TPM-Commands/TPM2_PolicySigned.md diff --git a/TPM-Commands/TPM2_PolicySecret.md b/TPM-Commands/TPM2_PolicySecret.md new file mode 100644 index 0000000..4fc38fc --- /dev/null +++ b/TPM-Commands/TPM2_PolicySecret.md @@ -0,0 +1,48 @@ +# `TPM2_PolicySecret()` + +`TPM2_PolicySecret()` allows a caller to assert and prove knowledge of +the `authValue` associated with some entity. The `authValue` affects +the HMAC calculation for the affected session, so the command will fail +if the caller does not know the `authValue`. + +The `tpmNonce` input strongly binds the command to the `policySession`. +If the `Empty Buffer` is given as the `tpmNonce`, then the +`TPM2_PolicySecret()` command could be altered to refer to any other +policy on the same TPM if the object referred to by `authHandle` +requires an HMAC or policy session, or any TPM otherwise. + +If a `policyTicket` is requested and output, that ticket can be used (up +to its `expiration`), via `TPM2_PolicyTicket()`, to satisfy the same +`TPM2_PolicySecret()` that produced the ticket. + +That is, a caller can get a ticket from a `TPM2_PolicySecret()` +invocation that allows it to re-use the `authValue` proof many times +prior to the ticket's expiration without having to actually prove the +`authValue` again. For example, if the `authValue` is obtained from a +password prompt and the password and `authValue` erased from memory as +soon as the `TPM2_PolicySecret()` command is marshalled, then the caller +can keep satisfying policies containing that `TPM2_PolicySecret()` by +using `TPM2_PolicyTicket()` instead of `TPM2_PolicySecret()`. This is +useful to avoid requiring repeated password prompts in a short time +span. + +## Inputs + + - `TPMI_DH_OBJECT authHandle` (handle to the entity whose `authValue` is to be proven) + - `TPMI_SH_POLICY policySession` (handle to the session being extended) + - `TPM2B_NONCE tpmNonce` (the policy nonce for the `policySession`) + - `TPM2B_DIGEST cpHashA` (the command parameter hash of a single command to be authorized, or `Empty Buffer` to not so-limit the assertion) + - `TPM2B_NONCE policyRef` (an opaque value of the caller's choosing, possibly the `Empty Buffer`, that is used to extend the `policySession`'s `policyDigest` along with the name of `authHandle`) + - `INT32 expiration` (a positive or negative number of milliseconds which, if non-zero, sets an expiration for this assertion; if zero or positive then a `policyTicket` will not be output) + +## Outputs + + - `TPM2B_TIMEOUT timeout` (implementation-specific indication of actual timeout for the session) + - `TPMT_TK_AUTH policyTicket` + +## References + + - [TCG TPM Library part 1: Architecture, section 19.7.12](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TPM2_r1p59_Part1_Architecture_pub.pdf) + - [TCG TPM Library part 1: Architecture, section 19.7.15](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TPM2_r1p59_Part1_Architecture_pub.pdf) + - [TCG TPM Library part 3: Commands, section 23.4](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TPM2_r1p59_Part3_Commands_pub.pdf) + diff --git a/TPM-Commands/TPM2_PolicySigned.md b/TPM-Commands/TPM2_PolicySigned.md new file mode 100644 index 0000000..e331dbe --- /dev/null +++ b/TPM-Commands/TPM2_PolicySigned.md @@ -0,0 +1,82 @@ +# `TPM2_PolicySigned()` + +`TPM2_PolicySigned()` allows a caller to provide a signature of some +assertion, with the signature made by some authorizing entity identified +by its public key. + +The semantics of the signature are arbitrary and up to the entity +constructing the policies that contain this command. Possible semantics +include: + + - biometrics user identification (whereby the biometrics device signs + an assertion that a user identified by the opaque value of + `policyRef` has been identified biometrically) + + - smartcard-based user authentication (whereby access to a private + signing key on a smartcard is used to sign an assertion that the user + has been authenticated by the user's possession of the smartcard and + interactive PIN entry to unlock it) + + - assertion of attested state being trusted (whereby an attestation + server signs such an assertion) + + - etc. + +The signature made by the signed is over the following digest: + + `aHash := H(nonceTPM || expiration || cpHashA || policyRef)` + +where `H()` is the digest algorithm associated with the authorizing +entity's public key. + +When evaluating this assertion in a policy session, the TPM will check +that the signature matches the above hash as constructed by the TPM from +the `TPM2_PolicySigned()` command parameters. + +When evaluating this assertion in a trial session, the TPM will ignore +the signature and will extend the `policySession`'s `policyDigest` as if +the signature had matched the hash. + +The `nonceTPM` input strongly binds the command to the `policySession`. +If the `Empty Buffer` is given as the `nonceTPM`, then the +`TPM2_PolicySigned()` command could be altered to refer to any other +policy any TPM. For this reason it is important to use the +`policySession`'s `nonceTPM` in any call to `TPM2_PolicySigned()`. + +If a `policyTicket` is requested and output, that ticket can be used (up +to its `expiration`), via `TPM2_PolicyTicket()`, to satisfy the same +`TPM2_PolicySigned()` that produced the ticket. + +That is, a caller can get a ticket from a `TPM2_PolicySigned()` +invocation that allows it to re-use the `auth` signature many times +prior to the ticket's expiration without having to get the authorizing +entity to re-sign. For example, if the authorizing entity is a +biometrics identification device, or a smartcard, then the interactive +human identification or interactive smartcard PIN entry steps can be +eschewed by the caller up to the ticket's expiration, using the `ticket` +(via `TPM2_PolicyTicket()`) instead to satisfy the same +`TPM2_PolicySigned()` command in any policy on that TPM. This is useful +to avoid requiring repeated biometrics or PIN entry in a short time +span. + +## Inputs + + - `TPMI_DH_OBJECT authObject` (handle to the key object whose public key is the signing entity's) + - `TPMI_SH_POLICY policySession` (handle to the session being extended) + - `TPM2B_NONCE nonceTPM` (the policy nonce for the `policySession`) + - `TPM2B_DIGEST cpHashA` (the command parameter hash of a single command to be authorized, or `Empty Buffer` to not so-limit the assertion) + - `TPM2B_NONCE policyRef` (an opaque value of the caller's and/or signer's choosing that is used to limit the value of the signature and to extend the `policySession`'s `policyDigest` along with the `authObject`'s name) + - `INT32 expiration` (a positive or negative number of milliseconds which, if non-zero, sets an expiration for this assertion; if zero or positive then a `policyTicket` will not be output) + - `TPMT_SIGNATURE auth` (the signature; ignored if the `policySession` is a trial session) + +## Outputs + + - `TPM2B_TIMEOUT timeout` (implementation-specific indication of actual timeout for the session) + - `TPMT_TK_AUTH policyTicket` + +## References + + - [TCG TPM Library part 1: Architecture, section 19.7.12](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TPM2_r1p59_Part1_Architecture_pub.pdf) + - [TCG TPM Library part 1: Architecture, section 19.7.15](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TPM2_r1p59_Part1_Architecture_pub.pdf) + - [TCG TPM Library part 3: Commands, section 23.3](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TPM2_r1p59_Part3_Commands_pub.pdf) +