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pkg-gate / doc / signed_manifests.txt

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			Manifest signing
			----------------

Manifests in IPS contain all the packaging metadata - file
permissions, ownership, content hashes, etc, and are stored and
transmitted as a simple text file with one line per action.  During
download or when a system is checked for compliance, the manifest
contents are compared to the files to determine whether or not a
package is correctly received/installed.  Given their importance,
verifying that all manifests are correct and reflect the original
publisher's intent is an important part of system validation.
Cryptographic signatures protecting the integrity of all actions form
a Merkle hash tree that includes the delivered binaries such that
complete verification of the installed software is possible.  There
are other uses for manifest signing beyond validation; signatures can
also be used to indicate approval by other organizations or parties.
For example, the internal QA organization could sign manifests of
Sun-delivered packages once they had be determined to be qualified for
production use; policy settings could mandate such approvals prior to
installation.

As a result, it is a useful characteristic for signatures to be
independent of other signatures in a manifest; it should be possible
to add (or remove) signatures (but not other actions) in a manifest
without invalidating the other signatures that are present.  This
feature also facilitates production handoffs, with signatures used
along the path to indicate completion along the way; subsequent steps
can optionally remove previous signatures at any time without ill-effect.

The need to treat signatures differently during hash computation
suggests that the signature itself should be easily distinguished from
other sorts of package metadata; this leads us to a new signature
action, of the form:

signature <hash of certificate> algorithm=<signature algorithm> \
    value=<signature value> \
    chain="<hashes of certs needed to validate primary certificate>" \
    version=<pkg version of signature>

The payload and pkg.chain_certs attributes represent the packaging hash of the
pem file(s) containing the x.509 certificate(s) downloadable from the
originating repository; the value is the signed hash of the manifest's message
text, prepared as discussed below.  The payload certificate is the certificate
which verifies the value in pkg.sigval.  The other certificates presented need
to form a certificate path that leads from the payload certificate to the trust
anchor(s) that was established as part of the publisher configuration.

To compute the signature of a manifest, first a canonical representation of the
manifest is created.  (See "Computing the manifest's message text" below.)  The
message text is then given to the signature algorithm along with the private key
and the result is the value of the signature.

Two types of signature algorithms are currently supported.  The first is rsa
group of signature algorithms.  An example is "rsa-sha256".  The bit after the
dash specifies the hash algorithm to use to change the message text into a
single value the rsa algorithm can use.

The second type of signature algorithm is compute the hash only.  This type of
algorithm exists primarily for testing and process verification purposes and
presents the hash as the signature value.  A signature action of this type is
indicated by the lack of a payload certificate hash.  This type of signature
action is verified if the image is configured to check signatures.  Its
presence however does not count as a signature if signatures are required.

signature algorithm=<hash algorithm> value=<hash> \
    version=<pkg version of signature>

Additional signature types (pgp, for example) may be added in the future.

Additional metadata can be added to a signature if desired, as with any
other action.

Policies may be set for the image or for specific publishers.  The policies
include ignoring signatures, verifying existing signatures, requiring
signatures, and requiring that specific common names must be seen in the chain
of trust.  Other policies may be added in the future.

Computing the manifest's message text:
--------------------------------------

Manifests have an interesting property: the lines in a manifest may be
reordered without affecting the meaning of the manifest.  As a result,
manifest order is not preserved and subject to change during package
publication processing.  It is thus necessary for our manifest signing
to be independent of presented line order, or the action ordering
algorithm used for installation, as that may change over time.

Straightforward C-locale alphabetical sorting of attributes within
actions, the multiple values within those attributes, and across actions 
can be used to enforce a consistent ordering for signature purposes and 
is not subject to change for a given manifest.

In order to allow other signatures to be added or removed from a
manifest, computation of the manifest message text does not include
other signatures; in order to protect metadata on the signature
itself, the signature being produced or verified is included in the
message text at the end, aside of course from the actual signature
value itself.

For example, take the following manifest:
set name=fmri value=foo@1.0
dir path=foo/bar group=sys
signature cert1 algorithm=rsa-sha256 value=val1 random_attr=baz
signature cert2 algorithm=rsa-sha256 value=val2 another_attr=whee

The text used to compute val1 is:
dir group=sys path=foo/bar
set name=fmri value=foo@1.0
signature cert1 algorithm=rsa-sha256 value= random_attr=baz

The text used to compute val2 is:
dir group=sys path=foo/bar
set name=fmri value=foo@1.0
signature cert2 another_attr=whee algorithm=rsa-sha256 value=

Including the text of the signature action itself prevents the signature action
from being modified.  Ensuring that the text used for compute val1 contains no
mention of the second signature (or any other signature) allows signatures to be
added and removed freely.

Verification of merged signatures:
----------------------------------

We produce "fat" packages (containing variants such as different
architectures, debug vs non-debug kernel, etc) by producing manifests
for each variant, and then merging them.  Actions that are the same
between variants being merge are left unmodified; those that are
different receive a variant tag (see facets.txt). If it is considered
useful to have signatures persist and be useful across such merges,
some additional steps are required to verify such signatures after
merging.

Generally, signatures will be unique to their variant, thus they will
be tagged with variant tags after merging.  To verify signatures
post-merge, the evaluation process has three steps after other signature actions
have been removed from consideration.

1) Any variant tags present on the signature are assumed to have been added
after the merge.  Thus, all actions whose variants do not match the signature's
variants are removed from inclusion in the message text.
2) Since the variant tags were not present when the manifest was signed, they
need to be removed from the attributes of each of the remaining actions as
well.  This includes removing the set attributes which define the variants for
the package.
3) Restore any set actions in the existing_pkg_vars attribute which 
describe variants which have been removed from the text.

For example, consider the following manifest:
set name=fmri value=foo@1.0
set name=variant.arch value=sparc value=i386
set name=variant.debug value=true value=false
dir path=foo1 group=sys
dir path=foo2 variant.arch=sparc
dir path=foo2/d variant.arch=sparc variant.debug=true
dir path=foo2/nd variant.arch=sparc variant.debug=false
dir path=foo3 variant.arch=i386
signature cert1 algorithm=rsa-sha256 value=val1 random_attr=baz \
    variant.arch=sparc variant.debug=true
signature cert2 algorithm=rsa-sha256 value=val2 another_attr=whee \
    variant.arch=i386 existing_pkg_vars="variant.arch=i386"

To compute the message text for the first signature, the first step is applied
producing the following text:
set name=fmri value=foo@1.0
set name=variant.arch value=sparc value=i386
set name=variant.debug value=true value=false
dir path=foo1 group=sys
dir path=foo2 variant.arch=sparc
dir path=foo2/d variant.arch=sparc variant.debug=true
signature cert1 algorithm=rsa-sha256 value=val1 random_attr=baz \
    variant.arch=sparc variant.debug=true

After the second step is applied, the text becomes:
set name=fmri value=foo@1.0
dir path=foo1 group=sys
dir path=foo2
dir path=foo2/d
signature cert1 algorithm=rsa-sha256 value=val1 random_attr=baz

Since the third step doesn't apply to this signature, the above text becomes the
canonical message text.

Computing the message text for the second signature proceeds like this:
After the first step the text is:
set name=fmri value=foo@1.0
set name=variant.arch value=sparc value=i386
set name=variant.debug value=true value=false
dir path=foo1 group=sys
dir path=foo3 variant.arch=i386
signature cert2 algorithm=rsa-sha256 value=val2 another_attr=whee \
    variant.arch=i386 existing_pkg_vars="variant.arch=i386"

After the second step, the text is:
set name=fmri value=foo@1.0
dir path=foo1 group=sys
dir path=foo3
signature cert2 algorithm=rsa-sha256 value=val2 another_attr=whee \
    existing_pkg_vars="variant.arch=i386"

In the third step, we restore the set action which was present in the original
manifest.  The text becomes:
set name=fmri value=foo@1.0
set name=variant.arch value=i386
dir path=foo1 group=sys
dir path=foo3
signature cert2 algorithm=rsa-sha256 value=val2 another_attr=whee \
    existing_pkg_vars="variant.arch=i386"

The existing_pkg_vars attribute allows whether a package variant set
action was present or not at the time of signing to be determined 
deterministically.

Publication of signed manifests:
--------------------------------

Publishing a signed manifest is a two step process.  First the package is
published, unsigned, to a repository.  The package is then updated in place,
using pkgsign, appending a signature action to the manifest in the repository
but leaving the package, including its timestamp, intact.  This process allows a
signature action to be added by someone other than the publisher without
invalidating the publisher's signature.  For example, the QA department of a
company may want to sign all packages that are installed internally to indicate
they have been approved for use, but not republish the packages which would
create a new timestamp and invalidate the signature of the original publisher.

The disadvantage of this approach is that a fmri no longer represents a single
manifest eternally.  Eventually, this problem is solved by having the client
ensure that the hash of the manifest it loads for the fmri matches the hash in
the catalog.  Until that feature is implemented, it is imperative that
publishers ensure that no client has access to an unsigned manifest which they
plan to sign in the future, or to return to the QA example, that no client
inside the organization sees a manifest for a fmri without a signature which the
QA plans to sign in the future.

pkgsign is able to update a package in place through the use of a new publishing
operation called append.  This operation opens a transaction to modify the
manifest of a fmri which is already in the repository.  When the transaction is
closed, the signed manifest replaces the existing manifest and the catalog is 
updated to reflect the new hash of the manifest.
 
 
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