module Gem::Security - Documentation for Ruby 4.0
Description: module Gem::Security: Security implements cryptographic signatures for gems. The section below is a step-by-step guide to using signed gems and generating your own.
Keywords: ruby,module,Gem::Security

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# module 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.sig # metadata signature
    data.tar.gz
    data.tar.gz.sig # data signature
    checksums.yaml.gz
    checksums.yaml.gz.sig # checksums 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; they can’t modify the package contents without invalidating the signature, and they can’t modify or remove signature or the signing certificate chain, or RubyGems will simply refuse to install the package. Oh well, maybe they’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
      -A, --key-algorithm ALGORITHM    Select key algorithm for --build from RSA, DSA, or EC. Defaults to RSA.
      -s, --sign CERT                  Signs CERT with the key from -K
                                       and the certificate from -C
      -d, --days NUMBER_OF_DAYS        Days before the certificate expires
      -R, --re-sign                    Re-signs the certificate from -C with the key from -K

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 -rpsych -e 'puts Psych.load($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 `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.

## 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.

DEFAULT_KEY_ALGORITHM

  Default algorithm to use when building a key pair

EC_NAME

  Named curve used for Elliptic Curve

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_CIPHER

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

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_DAY

  One day in seconds

ONE_YEAR

  One year in seconds

Policies

  `Hash` of configured security policies

RSA_DSA_KEY_LENGTH

  Length of keys created by RSA and DSA keys

SigningPolicy

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

### Public Class Methods

alt_name_or_x509_entry (certificate, x509_entry)
Source

    # File lib/rubygems/security.rb, line 385
    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)
Source

    # File lib/rubygems/security.rb, line 401
    def self.create_cert(subject, key, age = ONE_YEAR, extensions = EXTENSIONS, serial = 1)
      cert = OpenSSL::X509::Certificate.new

      cert.public_key = get_public_key(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

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.

create_cert_email (email, key, age = ONE_YEAR, extensions = EXTENSIONS)
Source

    # File lib/rubygems/security.rb, line 440
    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

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`.

create_cert_self_signed (subject, key, age = ONE_YEAR, extensions = EXTENSIONS, serial = 1)
Source

    # File lib/rubygems/security.rb, line 452
    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

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

create_digest (algorithm = DIGEST_NAME)
Source

    # File lib/rubygems/security.rb, line 462
    def self.create_digest(algorithm = DIGEST_NAME)
      OpenSSL::Digest.new(algorithm)
    end

Creates a new digest instance using the specified `algorithm`. The default is SHA256.

create_key (algorithm)
Source

    # File lib/rubygems/security.rb, line 470
    def self.create_key(algorithm)
      if defined?(OpenSSL::PKey)
        case algorithm.downcase
        when "dsa"
          OpenSSL::PKey::DSA.new(RSA_DSA_KEY_LENGTH)
        when "rsa"
          OpenSSL::PKey::RSA.new(RSA_DSA_KEY_LENGTH)
        when "ec"
          OpenSSL::PKey::EC.generate(EC_NAME)
        else
          raise Gem::Security::Exception,
          "#{algorithm} algorithm not found. RSA, DSA, and EC algorithms are supported."
        end
      end
    end

Creates a new key pair of the specified `algorithm`. RSA, DSA, and EC are supported.

email_to_name (email_address)
Source

    # File lib/rubygems/security.rb, line 489
    def self.email_to_name(email_address)
      email_address = email_address.gsub(/[^\w@.-]+/i, "_")

      cn, dcs = email_address.split "@"

      dcs = dcs.split "."

      OpenSSL::X509::Name.new([
        ["CN", cn],
        *dcs.map {|dc| ["DC", dc] },
      ])
    end

Turns `email_address` into an `OpenSSL::X509::Name`

get_public_key (key)
Source

    # File lib/rubygems/security.rb, line 425
    def self.get_public_key(key)
      # Ruby 3.0 (Ruby/OpenSSL 2.2) or later
      return OpenSSL::PKey.read(key.public_to_der) if key.respond_to?(:public_to_der)
      return key.public_key unless key.is_a?(OpenSSL::PKey::EC)

      ec_key = OpenSSL::PKey::EC.new(key.group.curve_name)
      ec_key.public_key = key.public_key
      ec_key
    end

Gets the right public key from a PKey instance

re_sign (expired_certificate, private_key, age = ONE_YEAR, extensions = EXTENSIONS)
Source

    # File lib/rubygems/security.rb, line 508
    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.to_s unless
        expired_certificate.check_private_key(private_key)

      unless expired_certificate.subject.to_s ==
             expired_certificate.issuer.to_s
        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

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

reset ()
Source

    # File lib/rubygems/security.rb, line 533
    def self.reset
      @trust_dir = nil
    end

Resets the trust directory for verifying gems.

sign (certificate, signing_key, signing_cert, age = ONE_YEAR, extensions = EXTENSIONS, serial = 1)
Source

    # File lib/rubygems/security.rb, line 544
    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_NAME
    end

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

Returns the newly signed certificate.

trust_dir ()
Source

    # File lib/rubygems/security.rb, line 572
    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

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

trusted_certificates (&block)
Source

    # File lib/rubygems/security.rb, line 583
    def self.trusted_certificates(&block)
      trust_dir.each_certificate(&block)
    end

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

write (pemmable, path, permissions = 0o600, passphrase = nil, cipher = KEY_CIPHER)
Source

    # File lib/rubygems/security.rb, line 592
    def self.write(pemmable, path, permissions = 0o600, passphrase = nil, cipher = KEY_CIPHER)
      path = File.expand_path path

      File.open path, "wb", permissions do |io|
        if passphrase && cipher
          io.write pemmable.to_pem cipher, passphrase
        else
          io.write pemmable.to_pem
        end
      end

      path
    end

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`.