tmputil: introduce non-global encoding logic

[ericchiang]

Right now it's not possible to use the tpm and tpm2 packages
simultaneously. Refactor the encoding logic into separate structs, so
using one doesn't impact the other, while maintaining the top level API.

As a follow up, refactor the tpm and tpm2 packages to use the encoders.
For example instead of:

tpmutil.Pack(digest)

Use:

tpmutil.TPM2.Pack(digest)

cc @awly

[awly]

cc @twitchy-jsonp @DenisKarch
We were just talking about the same issue.

[twitchy-jsonp]

We mentioned this in the team chat and Eric mentioned he had a fix lying around for it. Thanks for digging it up!

LGTM. Happy to do the big refactor to switch all le methods into using the more specific encoder types.

[ericchiang]

@twitchy-jsonp said you'd discussed introducing a new interface instead. If there are bigger issues with the encoding between 1.2/2.0 that need fixing, that seems like a better way to go.

This was intended to be as small a diff as possible. I imagine a follow up would look like:

diff --git a/tpm2/constants.go b/tpm2/constants.go
index ea954e7..eb16aba 100644
--- a/tpm2/constants.go
+++ b/tpm2/constants.go
@@ -25,9 +25,7 @@ import (
        "github.com/google/go-tpm/tpmutil"
 )
 
-func init() {
-       tpmutil.UseTPM20LengthPrefixSize()
-}
+var encoding = tpmutil.TPM2
 
 // MAX_DIGEST_BUFFER is the maximum size of []byte request or response fields.
 // Typically used for chunking of big blobs of data (such as for hashing or
diff --git a/tpm2/tpm2.go b/tpm2/tpm2.go
index 898f1de..dd61c79 100644
--- a/tpm2/tpm2.go
+++ b/tpm2/tpm2.go
@@ -37,7 +37,7 @@ func GetRandom(rw io.ReadWriter, size uint16) ([]byte, error) {
        }
 
        var randBytes []byte
-       if _, err := tpmutil.Unpack(resp, &randBytes); err != nil {
+       if _, err := encoding.Unpack(resp, &randBytes); err != nil {
                return nil, err
        }
        return randBytes, nil
@@ -53,7 +53,7 @@ func FlushContext(rw io.ReadWriter, handle tpmutil.Handle) error {
 
 func encodeTPMLPCRSelection(sel PCRSelection) ([]byte, error) {
        if len(sel.PCRs) == 0 {
-               return tpmutil.Pack(uint32(0))
+               return encoding.Pack(uint32(0))
        }
 
        // PCR selection is a variable-size bitmask, where position of a set bit is

[awly]

Rename to UseTPM12Encoding

[ericchiang]

If we're okay with backward incompatible changes in tpmutil, should we just remove the global encoder?

[awly]

@ericchiang there already is a new type for marshaling customization https://github.com/google/go-tpm/blob/master/tpmutil/structures.go#L54-L60
And at least one case where a byte slice in 1.2 is encoded differently than others https://github.com/google/go-tpm/blob/master/tpm/structures.go#L245-L287

I think both approaches will be equally noisy to implement. Since custom interface is needed regardless for that one use case, maybe we can cut out any knowledge of protocol version from tpmutil and instead let version-specific packages define byte slice encoding?

[ericchiang]

I think both approaches will be equally noisy to implement. Since custom interface is needed regardless for that one use case, maybe we can cut out any knowledge of protocol version from tpmutil and instead let version-specific packages define byte slice encoding?

Maybe internal packages that implement this encoding?

That sounds good to me. Though it's going to be a pain to have to iterate through every use of a []byte and replace it with the TPM 1.2 and TPM 2.0 wrappers.