In Files

  • rubygems/install_update_options.rb
  • rubygems/package.rb
  • rubygems/package/old.rb
  • rubygems/security.rb
  • rubygems/security/policies.rb
  • rubygems/security/policy.rb
  • rubygems/security/signer.rb
  • rubygems/security/trust_dir.rb

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Gem::Security

Signing gems

The Gem::Security implements cryptographic signatures for gems. The section below is a step-by-step guide to using signed gems and generating your own.

Walkthrough

Building your certificate

In order to start signing your gems, you’ll need to build a private key and a self-signed certificate. Here’s how:

# build a private key and certificate for yourself:
$ gem cert --build you@example.com

This could take anywhere from a few seconds to a minute or two, depending on the speed of your computer (public key algorithms aren’t exactly the speediest crypto algorithms in the world). When it’s finished, you’ll see the files “gem-private_key.pem” and “gem-public_cert.pem” in the current directory.

First things first: Move both files to ~/.gem if you don’t already have a key and certificate in that directory. Ensure the file permissions make the key unreadable by others (by default the file is saved securely).

Keep your private key hidden; if it’s compromised, someone can sign packages as you (note: PKI has ways of mitigating the risk of stolen keys; more on that later).

Signing Gems

In RubyGems 2 and newer there is no extra work to sign a gem. RubyGems will automatically find your key and certificate in your home directory and use them to sign newly packaged gems.

If your certificate is not self-signed (signed by a third party) RubyGems will attempt to load the certificate chain from the trusted certificates. Use gem cert --add signing_cert.pem to add your signers as trusted certificates. See below for further information on certificate chains.

If you build your gem it will automatically be signed. If you peek inside your gem file, you’ll see a couple of new files have been added:

$ tar tf your-gem-1.0.gem
metadata.gz
metadata.gz.sum
metadata.gz.sig # metadata signature
data.tar.gz
data.tar.gz.sum
data.tar.gz.sig # data signature

Manually signing gems

If you wish to store your key in a separate secure location you’ll need to set your gems up for signing by hand. To do this, set the signing_key and cert_chain in the gemspec before packaging your gem:

s.signing_key = '/secure/path/to/gem-private_key.pem'
s.cert_chain = %w[/secure/path/to/gem-public_cert.pem]

When you package your gem with these options set RubyGems will automatically load your key and certificate from the secure paths.

Signed gems and security policies

Now let’s verify the signature. Go ahead and install the gem, but add the following options: -P HighSecurity, like this:

# install the gem with using the security policy "HighSecurity"
$ sudo gem install your.gem -P HighSecurity

The -P option sets your security policy – we’ll talk about that in just a minute. Eh, what’s this?

$ gem install -P HighSecurity your-gem-1.0.gem
ERROR:  While executing gem ... (Gem::Security::Exception)
    root cert /CN=you/DC=example is not trusted

The culprit here is the security policy. RubyGems has several different security policies. Let’s take a short break and go over the security policies. Here’s a list of the available security policies, and a brief description of each one:

  • NoSecurity - Well, no security at all. Signed packages are treated like unsigned packages.

  • LowSecurity - Pretty much no security. If a package is signed then RubyGems will make sure the signature matches the signing certificate, and that the signing certificate hasn't expired, but that's it. A malicious user could easily circumvent this kind of security.

  • MediumSecurity - Better than LowSecurity and NoSecurity, but still fallible. Package contents are verified against the signing certificate, and the signing certificate is checked for validity, and checked against the rest of the certificate chain (if you don't know what a certificate chain is, stay tuned, we'll get to that). The biggest improvement over LowSecurity is that MediumSecurity won't install packages that are signed by untrusted sources. Unfortunately, MediumSecurity still isn't totally secure -- a malicious user can still unpack the gem, strip the signatures, and distribute the gem unsigned.

  • HighSecurity - Here's the bugger that got us into this mess. The HighSecurity policy is identical to the MediumSecurity policy, except that it does not allow unsigned gems. A malicious user doesn't have a whole lot of options here; he can't modify the package contents without invalidating the signature, and he can't modify or remove signature or the signing certificate chain, or RubyGems will simply refuse to install the package. Oh well, maybe he'll have better luck causing problems for CPAN users instead :).

The reason RubyGems refused to install your shiny new signed gem was because it was from an untrusted source. Well, your code is infallible (naturally), so you need to add yourself as a trusted source:

# add trusted certificate
gem cert --add ~/.gem/gem-public_cert.pem

You’ve now added your public certificate as a trusted source. Now you can install packages signed by your private key without any hassle. Let’s try the install command above again:

# install the gem with using the HighSecurity policy (and this time
# without any shenanigans)
$ gem install -P HighSecurity your-gem-1.0.gem
Successfully installed your-gem-1.0
1 gem installed

This time RubyGems will accept your signed package and begin installing.

While you’re waiting for RubyGems to work it’s magic, have a look at some of the other security commands by running gem help cert:

Options:
  -a, --add CERT                   Add a trusted certificate.
  -l, --list [FILTER]              List trusted certificates where the
                                   subject contains FILTER
  -r, --remove FILTER              Remove trusted certificates where the
                                   subject contains FILTER
  -b, --build EMAIL_ADDR           Build private key and self-signed
                                   certificate for EMAIL_ADDR
  -C, --certificate CERT           Signing certificate for --sign
  -K, --private-key KEY            Key for --sign or --build
  -s, --sign CERT                  Signs CERT with the key from -K
                                   and the certificate from -C

We’ve already covered the --build option, and the --add, --list, and --remove commands seem fairly straightforward; they allow you to add, list, and remove the certificates in your trusted certificate list. But what’s with this --sign option?

Certificate chains

To answer that question, let’s take a look at “certificate chains”, a concept I mentioned earlier. There are a couple of problems with self-signed certificates: first of all, self-signed certificates don’t offer a whole lot of security. Sure, the certificate says Yukihiro Matsumoto, but how do I know it was actually generated and signed by matz himself unless he gave me the certificate in person?

The second problem is scalability. Sure, if there are 50 gem authors, then I have 50 trusted certificates, no problem. What if there are 500 gem authors? 1000? Having to constantly add new trusted certificates is a pain, and it actually makes the trust system less secure by encouraging RubyGems users to blindly trust new certificates.

Here’s where certificate chains come in. A certificate chain establishes an arbitrarily long chain of trust between an issuing certificate and a child certificate. So instead of trusting certificates on a per-developer basis, we use the PKI concept of certificate chains to build a logical hierarchy of trust. Here’s a hypothetical example of a trust hierarchy based (roughly) on geography:

                    --------------------------
                    | rubygems@rubygems.org |
                    --------------------------
                                |
              -----------------------------------
              |                                 |
  ----------------------------    -----------------------------
  |  seattlerb@seattlerb.org |    | dcrubyists@richkilmer.com |
  ----------------------------    -----------------------------
       |                |                 |             |
---------------   ----------------   -----------   --------------
|   drbrain   |   |   zenspider  |   | pabs@dc |   | tomcope@dc |
---------------   ----------------   -----------   --------------

Now, rather than having 4 trusted certificates (one for drbrain, zenspider, pabs@dc, and tomecope@dc), a user could actually get by with one certificate, the “rubygems@rubygems.org” certificate.

Here’s how it works:

I install “rdoc-3.12.gem”, a package signed by “drbrain”. I’ve never heard of “drbrain”, but his certificate has a valid signature from the “seattle.rb@seattlerb.org” certificate, which in turn has a valid signature from the “rubygems@rubygems.org” certificate. Voila! At this point, it’s much more reasonable for me to trust a package signed by “drbrain”, because I can establish a chain to “rubygems@rubygems.org”, which I do trust.

Signing certificates

The --sign option allows all this to happen. A developer creates their build certificate with the --build option, then has their certificate signed by taking it with them to their next regional Ruby meetup (in our hypothetical example), and it’s signed there by the person holding the regional RubyGems signing certificate, which is signed at the next RubyConf by the holder of the top-level RubyGems certificate. At each point the issuer runs the same command:

# sign a certificate with the specified key and certificate
# (note that this modifies client_cert.pem!)
$ gem cert -K /mnt/floppy/issuer-priv_key.pem -C issuer-pub_cert.pem
   --sign client_cert.pem

Then the holder of issued certificate (in this case, your buddy “drbrain”), can start using this signed certificate to sign RubyGems. By the way, in order to let everyone else know about his new fancy signed certificate, “drbrain” would save his newly signed certificate as ~/.gem/gem-public_cert.pem

Obviously this RubyGems trust infrastructure doesn’t exist yet. Also, in the “real world”, issuers actually generate the child certificate from a certificate request, rather than sign an existing certificate. And our hypothetical infrastructure is missing a certificate revocation system. These are that can be fixed in the future…

At this point you should know how to do all of these new and interesting things:

  • build a gem signing key and certificate

  • adjust your security policy

  • modify your trusted certificate list

  • sign a certificate

Manually verifying signatures

In case you don’t trust RubyGems you can verify gem signatures manually:

  1. Fetch and unpack the gem

    gem fetch some_signed_gem
    tar -xf some_signed_gem-1.0.gem
  2. Grab the public key from the gemspec

    gem spec some_signed_gem-1.0.gem cert_chain | \
      ruby -ryaml -e 'puts YAML.load_documents($stdin)' > public_key.crt
  3. Generate a SHA1 hash of the data.tar.gz

    openssl dgst -sha1 < data.tar.gz > my.hash
  4. Verify the signature

    openssl rsautl -verify -inkey public_key.crt -certin \
      -in data.tar.gz.sig > verified.hash
  5. Compare your hash to the verified hash

    diff -s verified.hash my.hash
  6. Repeat 5 and 6 with metadata.gz

OpenSSL Reference

The .pem files generated by –build and –sign are PEM files. Here’s a couple of useful OpenSSL commands for manipulating them:

# convert a PEM format X509 certificate into DER format:
# (note: Windows .cer files are X509 certificates in DER format)
$ openssl x509 -in input.pem -outform der -out output.der

# print out the certificate in a human-readable format:
$ openssl x509 -in input.pem -noout -text

And you can do the same thing with the private key file as well:

# convert a PEM format RSA key into DER format:
$ openssl rsa -in input_key.pem -outform der -out output_key.der

# print out the key in a human readable format:
$ openssl rsa -in input_key.pem -noout -text

Bugs/TODO

  • There’s no way to define a system-wide trust list.

  • custom security policies (from a YAML file, etc)

  • Simple method to generate a signed certificate request

  • Support for OCSP, SCVP, CRLs, or some other form of cert status check (list is in order of preference)

  • Support for encrypted private keys

  • Some sort of semi-formal trust hierarchy (see long-winded explanation above)

  • Path discovery (for gem certificate chains that don’t have a self-signed root) – by the way, since we don’t have this, THE ROOT OF THE CERTIFICATE CHAIN MUST BE SELF SIGNED if Gem::Security::Policy#verify_root is true (and it is for the MediumSecurity and HighSecurity policies)

  • Better explanation of X509 naming (ie, we don’t have to use email addresses)

  • Honor AIA field (see note about OCSP above)

  • Honor extension restrictions

  • Might be better to store the certificate chain as a PKCS#7 or PKCS#12 file, instead of an array embedded in the metadata.

  • Flexible signature and key algorithms, not hard-coded to RSA and SHA1.

Original author

Paul Duncan <pabs@pablotron.org> pablotron.org/

Constants

AlmostNoSecurity

AlmostNo security policy: only verify that the signing certificate is the one that actually signed the data. Make no attempt to verify the signing certificate chain.

This policy is basically useless. better than nothing, but can still be easily spoofed, and is not recommended.

DIGEST_ALGORITHM

Digest algorithm used to sign gems

EXTENSIONS

The default set of extensions are:

  • The certificate is not a certificate authority

  • The key for the certificate may be used for key and data encipherment and digital signatures

  • The certificate contains a subject key identifier

HighSecurity

High security policy: only allow signed gems to be installed, verify the signing certificate, verify the signing certificate chain all the way to the root certificate, and only trust root certificates that we have explicitly allowed trust for.

This security policy is significantly more difficult to bypass, and offers a reasonable guarantee that the contents of the gem have not been altered.

KEY_ALGORITHM

Algorithm for creating the key pair used to sign gems

KEY_CIPHER

Cipher used to encrypt the key pair used to sign gems. Must be in the list returned by OpenSSL::Cipher.ciphers

KEY_LENGTH

Length of keys created by KEY_ALGORITHM

LowSecurity

Low security policy: only verify that the signing certificate is actually the gem signer, and that the signing certificate is valid.

This policy is better than nothing, but can still be easily spoofed, and is not recommended.

MediumSecurity

Medium security policy: verify the signing certificate, verify the signing certificate chain all the way to the root certificate, and only trust root certificates that we have explicitly allowed trust for.

This security policy is reasonable, but it allows unsigned packages, so a malicious person could simply delete the package signature and pass the gem off as unsigned.

NoSecurity

No security policy: all package signature checks are disabled.

ONE_YEAR

One year in seconds

Policies

Hash of configured security policies

SigningPolicy

Policy used to verify a certificate and key when signing a gem

Public Class Methods

alt_name_or_x509_entry(certificate, x509_entry) click to toggle source
 
               # File rubygems/security.rb, line 393
def self.alt_name_or_x509_entry certificate, x509_entry
  alt_name = certificate.extensions.find do |extension|
    extension.oid == "#{x509_entry}AltName"
  end

  return alt_name.value if alt_name

  certificate.send x509_entry
end
            
create_cert(subject, key, age = ONE_YEAR, extensions = EXTENSIONS, serial = 1) click to toggle source

Creates an unsigned certificate for subject and key. The lifetime of the key is from the current time to age which defaults to one year.

The extensions restrict the key to the indicated uses.

 
               # File rubygems/security.rb, line 409
def self.create_cert subject, key, age = ONE_YEAR, extensions = EXTENSIONS,
                     serial = 1
  cert = OpenSSL::X509::Certificate.new

  cert.public_key = key.public_key
  cert.version    = 2
  cert.serial     = serial

  cert.not_before = Time.now
  cert.not_after  = Time.now + age

  cert.subject    = subject

  ef = OpenSSL::X509::ExtensionFactory.new nil, cert

  cert.extensions = extensions.map do |ext_name, value|
    ef.create_extension ext_name, value
  end

  cert
end
            
create_cert_email(email, key, age = ONE_YEAR, extensions = EXTENSIONS) click to toggle source

Creates a self-signed certificate with an issuer and subject from email, a subject alternative name of email and the given extensions for the key.

 
               # File rubygems/security.rb, line 436
def self.create_cert_email email, key, age = ONE_YEAR, extensions = EXTENSIONS
  subject = email_to_name email

  extensions = extensions.merge "subjectAltName" => "email:#{email}"

  create_cert_self_signed subject, key, age, extensions
end
            
create_cert_self_signed(subject, key, age = ONE_YEAR, extensions = EXTENSIONS, serial = 1) click to toggle source

Creates a self-signed certificate with an issuer and subject of subject and the given extensions for the key.

 
               # File rubygems/security.rb, line 448
def self.create_cert_self_signed subject, key, age = ONE_YEAR,
                                 extensions = EXTENSIONS, serial = 1
  certificate = create_cert subject, key, age, extensions

  sign certificate, key, certificate, age, extensions, serial
end
            
create_key(length = KEY_LENGTH, algorithm = KEY_ALGORITHM) click to toggle source

Creates a new key pair of the specified length and algorithm. The default is a 2048 bit RSA key.

 
               # File rubygems/security.rb, line 459
def self.create_key length = KEY_LENGTH, algorithm = KEY_ALGORITHM
  algorithm.new length
end
            
email_to_name(email_address) click to toggle source

Turns email_address into an OpenSSL::X509::Name

 
               # File rubygems/security.rb, line 466
def self.email_to_name email_address
  email_address = email_address.gsub(/[^\w@.-]+/i, '_')

  cn, dcs = email_address.split '@'

  dcs = dcs.split '.'

  name = "CN=#{cn}/#{dcs.map { |dc| "DC=#{dc}" }.join '/'}"

  OpenSSL::X509::Name.parse name
end
            
re_sign(expired_certificate, private_key, age = ONE_YEAR, extensions = EXTENSIONS) click to toggle source

Signs expired_certificate with private_key if the keys match and the expired certificate was self-signed.

 
               # File rubygems/security.rb, line 484
def self.re_sign expired_certificate, private_key, age = ONE_YEAR,
                 extensions = EXTENSIONS
  raise Gem::Security::Exception,
        "incorrect signing key for re-signing " +
        "#{expired_certificate.subject}" unless
    expired_certificate.public_key.to_pem == private_key.public_key.to_pem

  unless expired_certificate.subject.to_s ==
         expired_certificate.issuer.to_s then
    subject = alt_name_or_x509_entry expired_certificate, :subject
    issuer  = alt_name_or_x509_entry expired_certificate, :issuer

    raise Gem::Security::Exception,
          "#{subject} is not self-signed, contact #{issuer} " +
          "to obtain a valid certificate"
  end

  serial = expired_certificate.serial + 1

  create_cert_self_signed(expired_certificate.subject, private_key, age,
                          extensions, serial)
end
            
reset() click to toggle source

Resets the trust directory for verifying gems.

 
               # File rubygems/security.rb, line 510
def self.reset
  @trust_dir = nil
end
            
sign(certificate, signing_key, signing_cert, age = ONE_YEAR, extensions = EXTENSIONS, serial = 1) click to toggle source

Sign the public key from certificate with the signing_key and signing_cert, using the Gem::Security::DIGEST_ALGORITHM. Uses the default certificate validity range and extensions.

Returns the newly signed certificate.

 
               # File rubygems/security.rb, line 521
def self.sign certificate, signing_key, signing_cert,
              age = ONE_YEAR, extensions = EXTENSIONS, serial = 1
  signee_subject = certificate.subject
  signee_key     = certificate.public_key

  alt_name = certificate.extensions.find do |extension|
    extension.oid == 'subjectAltName'
  end

  extensions = extensions.merge 'subjectAltName' => alt_name.value if
    alt_name

  issuer_alt_name = signing_cert.extensions.find do |extension|
    extension.oid == 'subjectAltName'
  end

  extensions = extensions.merge 'issuerAltName' => issuer_alt_name.value if
    issuer_alt_name

  signed = create_cert signee_subject, signee_key, age, extensions, serial
  signed.issuer = signing_cert.subject

  signed.sign signing_key, Gem::Security::DIGEST_ALGORITHM.new
end
            
trust_dir() click to toggle source

Returns a Gem::Security::TrustDir which wraps the directory where trusted certificates live.

 
               # File rubygems/security.rb, line 550
def self.trust_dir
  return @trust_dir if @trust_dir

  dir = File.join Gem.user_home, '.gem', 'trust'

  @trust_dir ||= Gem::Security::TrustDir.new dir
end
            
trusted_certificates(&block) click to toggle source

Enumerates the trusted certificates via Gem::Security::TrustDir.

 
               # File rubygems/security.rb, line 561
def self.trusted_certificates &block
  trust_dir.each_certificate(&block)
end
            
write(pemmable, path, permissions = 0600, passphrase = nil, cipher = KEY_CIPHER) click to toggle source

Writes pemmable, which must respond to to_pem to path with the given permissions. If passed cipher and passphrase those arguments will be passed to to_pem.

 
               # File rubygems/security.rb, line 570
def self.write pemmable, path, permissions = 0600, passphrase = nil, cipher = KEY_CIPHER
  path = File.expand_path path

  open path, 'wb', permissions do |io|
    if passphrase and cipher
      io.write pemmable.to_pem cipher, passphrase
    else
      io.write pemmable.to_pem
    end
  end

  path
end
            

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