Security infrastructure proposal

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Security infrastructure proposal Chris Heald 2/2/13 8:32 AM
Hey folks --

I've been noodling on the state of the security infrastructure, and have
congealed my thoughts on it into a proposal. I'd like to disclose up front
that I am not a security expert per se, and don't seek to claim any kind of
authority in that regard, but I am an experienced systems architect and
hope that counts for something! :)

I'm cross-posting this to the rubygems.org group as well as the
rubygems-developers list as it would require changes to both the rubygems
installable and distribution platforms like rubygems.org.

The primary goals are:

1. Principle of least responsibility - the infrastructure consists of
multiple independent systems, with the intent that a breach of any single
system is covered by the other systems in place.
2. Verification of continuity - gem consumers should be confident that they
are consuming authorized gems every time they do an install or upgrade of a
gem.
3. Provision for multiple signers, so that group-maintained projects work
easily in the system.
4. Provision for certificate revocation, in order to revoke certificates at
any point in the chain of trust

This proposal does not attempt to verify identity beyond email address
ownership, nor does it intend to provide a vetting platform for code that
would restrict publication of gems. It would still be possible to publish
nasty gems under this system, but it would protect against backdoored gems
and illicit uploads, as well as providing a revocation mechanism for end
users to use to discover and remove compromised gems in the event of a
breach.

Additionally, I should mention that I based the proposal on X509, as Evan
previously indicated that any signing mechanism would need to work with
only the Ruby stdlib, which means OpenSSL. There has been some talk of GPG
and Web-of-trust systems for certification, and while I think that such a
system could be used with this setup, x509 gets the ball rolling faster as
it's already in place in Rubygems, and has no external dependencies like
GPG.

I'm sure there are things I haven't considered in this system, and would
very much like feedback, criticism, and flames.

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# Root Key & Signing System

A single X509 key is generated per distribution platform (Rubygems.org,
Gemfury, etc). This key is used to sign gem author requests.

A gem author may generate a certificate and request that the platform sign
it. Alice generates her x509 keypair with her email address encoded as the
x509 name field, stashes the private key somewhere safe, and submits the
pubkey to the signing system.

The signing system consists of two parts:

1. [Machine A] A web UI (or email inbox) responsible for accepting public
keys and sending emails
2. [Machine B] A signing machine with a shared data store (shared NFS
mount, redis store, whatever - it must simply be a data store to act as a
dead drop)

The UI accepts pubkeys, ensures their validity, parses the certificate for
the name field, and sends a verification email to the email specified in
the name field. The email contains a link with a cryptographic signature
(something like an HMAC of the pubkey). The email owner clicks this link
(or replies to the email) which causes Machine A to validate the response
and put the affiliated pubkey into the dead-drop inbox.

Machine B is monitoring the inbox for pubkeys. Once a key is received, it
is signed, and placed in the dead-drop outbox.

Machine A monitors the outbox for signed keys. It again parses the key for
the name field again, encrypts the signed key with itself, and emails it to
the name field in the record.

Alice retrieves the key from her email inbox, decrypts it with her private
key, and then may use it to sign her gems.

This system could have an exceptionally small attack surface, consisting of
only a minimal mailserver (A) and a local-only daemon (B) which operate on
shared storage (on either A or B, or on a third server, C).

# Gem certificate chain history server

A separate server ["Chain of trust history server"] maintains and validates
cert chain history for all gems on Rubygems.org, but which is queryable by
Rubygems-bin, allowing Rubygems-bin to obtain the last known and verified
certificate chain for a given gem when installing in the event that no
local history is known.

It must be separate from the Rubygems.org platform in order to avoid
allowing a compromise of Rubygems.org to be pivoted into a compromise of
the certificate history system, allowing an attacker to upload fraudulent
certificates.

This system would naturally serve as an automated IDS, as well, and could
raise an alarm if it ever discovered that Rubygems.org had accepted a gem
without a valid certificate chain, indicating a breach of the system's
certificate verification mechanisms.

# Gem Signing

The gem is signed with something like:

    s.signing_key = File.expand_path("~/.gem/trust/.gem-private_cert.pem")
    s.cert_chain  = ['rubygems-public_cert.pem', 'alice-public_cert.pem']

Alice may then upload her gem to Rubygems.org. Upon receipt of the gem,
Rubygems.org ensures that the gem has been signed with a cert chain
terminating in a certificate that it knows about and trusts. Additionally,
it will ensure that the gem is signed with a certificate containing an
email that matches the email on the account of the system.

Rubygems(-bin) will maintain a local history of certificate chains for a
gem. If a certificate is *removed* (without a signed authorization), then
it will refuse to install the gem, suggest review, and require a user
override to proceed. Rubygems.org will additionally maintain this
certificate chain, and refuse to accept a gem that does not include the
owning account's email as a part of the chain of trust. This ensures:

  * If an individual Rubygems.org account is compromised (but not the
legitimate owner's private key), then a malicious entity cannot upload a
modified gem into the account.
  * If an individual Rubygems.org account is comprimised, and the attacker
has been able to forge a key with the account's email, then the attacker
can upload new gems into the account, but cannot publish new versions of
existing gems, as they will fail to validate the chain of trust history.
  * If Rubygems as a whole is compromised, then the attacker may be able to
upload a malicious gem. However, Rubygems-bin will refuse to install any
newer version of it.

Rubygems will allow certificates to be *added* to the certificate chain, so
long as they are signed by a non-root certificate in the chain. This
permits for transfer of project ownership and multiple signing keys. For
example:

### Project transfer

  Alice starts a project, Foobar, signs it with her key. The chain now
looks like:

    [rubygems, alice]

  Alice then later abandons the project, and Charlie takes over as
maintainer with Alice's blessing. Alice would generate a key re-issue
signature on the project, authorizing the removal of her key, and the
addition of Charlie's. The chain now looks like:

    [rubygems, charlie] (alice removed with authorization)

  As Alice signed Charlie's key and authorized her key's removal, she is
still part of the chain of trust allowing the chain history to permit the
change, the system permits installation, with the implicit understanding
that Alice has blessed Charlie's key. Future releases will not need Alice's
blessing.

### Multi-user projects

Alice starts a popular project, which she then wants to add publishing
members to while retaining publication ability herself. Initially, the
trust chain is:

    [rubygems, alice]

Upon wanting to add a new member, Alice generate a project master key, and
authorizes key reissuance of the project using the new project master key

    [rubygems, project-master] (alice removed with authorization)

Then Alice uses the project-master key to sign Charlie's key (and perhaps
her own personal key):

    [rubygems, project-master, charlie]

Alice may continue to publish to the project while allowing Charlie to
publish to the project, without giving Charlie her personal key's trust.

### Malicious cert chain modification

  If Dave, a malicious actor, managed to wrest control of the project, he
would be able to sign the gem, but its trust chain would look like:

    [rubygems, dave]

  Thus, both Rubygems.org and Rubygems-bin would reject the gem based on
the gem's known certificate history, and Alice's unauthorized exclusion
from the certificate chain.

# Gem installation and verification

Bob, a Ruby developer, wants to use Alice's gem. Bob would install the
Rubygems.org public cert as a trusted certificate:

    gem cert --add rubygems-public_cert.pem

Bob may then download and install Alice's gem, and Rubygems(-bin)'s
HighSecurity policy will validate and accept the gem, and permit it to
install.

# Certificate revocation

Before fetching a gem, Rubygems would need to fetch any certificate
revocation lists. It would then check the trusted certificate list for
revocations, and remove any that appear on the list. This is the primary
mechanism in which a compromised CA key would be removed. Users would be
required to manually install the new key in this event.

This necessitates that the Rubygems public key must be published in a
location that is not connected to the CA, as a compromise of the CA could
allow an attacker to revoke the otherwise-legitimate root key and publish
his own for consumption.

Each time Rubygems runs a network operation, it should

  1. Check if the revocation list has changed since the last time it
validated certificates for known gems.
  2. If the list has changed, validate the certificate chains for all
installed gems. Prompt to remove any with invalid certificates.
  3. If step 2 was run, write a hash of the revocation list and the list of
gems that passed muster.
  4. Remove any entries from the local chain of trust history that contain
revoked certificates.
  5. Check for a new revocation list
  6. Run step 2 if the revocation list has changed.

This allows for certificate verifications and revocations for multiple gem
installs (RVM gemsets, bundler local installs) in a given system.

# Attack Surfaces

* Installation of a malicious certificate as a trusted root certificate on
a local machine would result in signatures becoming unreliable. However,
given that this would require some level of ownership of the machine, it
would likely be a small problem in such an event.
* Compromise of Rubygems.org's distribution platform may result in the
upload of malicious gems. Such gems would be distributed to gem installers,
which would then reject the gems due to either a local failed chain of
trust, or a failed chain of trust from the chain history server.
* Compromise of the chain history server would not be exploitable to
install malicious software, as the attacker must also have control of the
distribution platform. MITM attacks would be viable, but if you can MITM
Rubygems.org, you can MITM chain history server queries.
* Compromise of the chain history server AND Rubygems.org would allow for
attackers to upload compromised gems to Rubygems.org and distribute them to
pristine installs. Upgrades would still fail due to the local chain of
trust history.
* Compromise of the Rubygems' pubkey publication platform could result in
an attacker publishing his own public key, which would affect people
installing the certificate for the first time. However, legitimate gems
from Rubygems.org would fail to install as they were not signed with the
attacker's keypair.
* Compromise of the pubkey platform AND the Rubygems.org platform would
result in failure to install due to local or queried chain of trust
histories.
* Compromise of the CA's "Machine A" would result in people being able to
obtain signed keys for emails without validation. It would not expose the
private key for Machine B. This would permit uploading of new gems to a
compromised user account, but new versions of existing gems would fail to
upload, as the key provided would not be a part of the gem's existing chain
of trust history.
* Compromise of the CA's "Machine B" would result in disclosure of the
private key, requiring that the root key be revoked and reissued. This
would invalidate all current gem signatures. Illicit replacement of the
private key on CA's "Machine B" would result in people being issued
certificates that would fail to upload to Rubygems.org, due to failure to
validate the cert chain against the Rubygems.org private key.
* Compromise of the CA and Rubygems.org would result in pristine installs
being served malicious software. Upgrades would still fail due to local
chain of trust history.
* An author's stolen private key may be used to fraudulently sign requests.
This may be defended against by following proper key protection measures
and password-protecting the key.
* Most raised MITM attacks can be avoided by performing Rubygems.org and
chain history queries via SSL.

-- Chris
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Re: Security infrastructure proposal James Tucker 2/2/13 9:06 AM
Chris,

I'm waiting for my hiking partner to arrive in the next few minutes, so I can't review this immediately, but I have a chance to offer you my gratitude for putting something together so swiftly and presenting it properly. I look forward to reading through it later.

Cheers,

James
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