Class: Sigstore::Verifier

Inherits:
Object
  • Object
show all
Includes:
Loggable
Defined in:
lib/sigstore/verifier.rb

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Methods included from Loggable

included, #logger

Constructor Details

#initialize(rekor_client:, fulcio_cert_chain:, timestamp_authorities:, rekor_keyring:, ct_keyring:) ⇒ Verifier

Returns a new instance of Verifier.



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# File 'lib/sigstore/verifier.rb', line 31

def initialize(rekor_client:, fulcio_cert_chain:, timestamp_authorities:, rekor_keyring:, ct_keyring:)
  @rekor_client = rekor_client
  @fulcio_cert_chain = fulcio_cert_chain
  @timestamp_authorities = timestamp_authorities
  @rekor_keyring = rekor_keyring
  @ct_keyring = ct_keyring
end

Instance Attribute Details

#rekor_client ⇒ Object (readonly)

Returns the value of attribute rekor_client.



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# File 'lib/sigstore/verifier.rb', line 29

def rekor_client
  @rekor_client
end

Class Method Details

.for_trust_root(trust_root:) ⇒ Object 



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# File 'lib/sigstore/verifier.rb', line 39

def self.for_trust_root(trust_root:)
  new(
    rekor_client: Rekor::Client.new(url: trust_root.tlog_for_signing.base_url),
    fulcio_cert_chain: trust_root.fulcio_cert_chain,
    timestamp_authorities: trust_root.timestamp_authorities,
    rekor_keyring: Internal::Keyring.new(keys: trust_root.rekor_keys),
    ct_keyring: Internal::Keyring.new(keys: trust_root.ctfe_keys)
  )
end

.production(trust_root: TrustedRoot.production) ⇒ Object 



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# File 'lib/sigstore/verifier.rb', line 49

def self.production(trust_root: TrustedRoot.production)
  for_trust_root(trust_root:)
end

.staging(trust_root: TrustedRoot.staging) ⇒ Object 



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# File 'lib/sigstore/verifier.rb', line 53

def self.staging(trust_root: TrustedRoot.staging)
  for_trust_root(trust_root:)
end

Instance Method Details

#verify(input:, policy:, offline:) ⇒ Object 



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# File 'lib/sigstore/verifier.rb', line 57

def verify(input:, policy:, offline:)
  # First, establish a time for the signature. This timestamp is required to validate the certificate chain,
  # so this step comes first.

  bundle = input.sbundle
  materials = bundle.verification_material

  # 1)
  # If the verification policy uses the Timestamping Service, the Verifier MUST verify the timestamping response
  # using the Timestamping Service root key material, as described in Spec: Timestamping Service, with the raw bytes
  # of the signature as the timestamped data. The Verifier MUST then extract a timestamp from the timestamping
  # response. If verification or timestamp parsing fails, the Verifier MUST abort.

  timestamps = extract_timestamp_from_verification_data(materials.timestamp_verification_data) || []

  # 2)
  # If the verification policy uses timestamps from the Transparency Service, the Verifier MUST verify the signature
  # on the Transparency Service LogEntry as described in Spec: Transparency Service against the pre-distributed root
  # key material from the transparency service. The Verifier SHOULD NOT (yet) attempt to parse the body.
  # The Verifier MUST then parse the integratedTime as a Unix timestamp (seconds since January 1, 1970 UTC).
  # If verification or timestamp parsing fails, the Verifier MUST abort.

  begin
    # TODO: should this instead be an input to the verify method?
    # See https://docs.google.com/document/d/1kbhK2qyPPk8SLavHzYSDM8-Ueul9_oxIMVFuWMWKz0E/edit?disco=AAABQVV-gT0
    entry = find_rekor_entry(bundle, input.hashed_input, offline:)
  rescue Sigstore::Error::MissingRekorEntry
    return VerificationFailure.new("Rekor entry not found")
  else
    if entry.inclusion_proof&.checkpoint
      Internal::Merkle.verify_merkle_inclusion(entry)
      Rekor::Checkpoint.verify_checkpoint(@rekor_keyring, entry)
    elsif !offline
      return VerificationFailure.new("Missing Rekor inclusion proof")
    else
      logger.warn "inclusion proof not present in bundle: skipping due to offline verification"
    end
  end

  Internal::SET.verify_set(keyring: @rekor_keyring, entry:) if entry.inclusion_promise

  timestamps << Time.at(entry.integrated_time).utc

  # TODO: implement this step

  store = OpenSSL::X509::Store.new

  @fulcio_cert_chain.each do |cert|
    store.add_cert(cert.openssl)
  end

  # 3)
  # The Verifier MUST perform certification path validation (RFC 5280 §6) of the certificate chain with the
  # pre-distributed Fulcio root certificate(s) as a trust anchor, but with a fake “current time.”
  # If a timestamp from the timestamping service is available, the Verifier MUST perform path validation using the
  # timestamp from the Timestamping Service. If a timestamp from the Transparency Service is available, the Verifier
  # MUST perform path validation using the timestamp from the Transparency Service. If both are available, the
  # Verifier performs path validation twice. If either fails, verification fails.
  chains = timestamps.map do |ts|
    store_ctx = OpenSSL::X509::StoreContext.new(store, bundle.leaf_certificate.openssl)
    store_ctx.time = ts

    unless store_ctx.verify
      return VerificationFailure.new(
        "failed to validate certificate from fulcio cert chain: #{store_ctx.error_string}"
      )
    end

    chain = store_ctx.chain || raise(Error::InvalidCertificate, "no valid cert chain found")
    chain.shift # remove the cert itself
    chain.map! { Internal::X509::Certificate.new(_1) }
  end

  chains.uniq! { |chain| chain.map(&:to_der) }
  unless chains.size == 1
    raise "expected exactly one certificate chain, got #{chains.size} chains:\n" +
          chains.map do |chain|
            chain.map(&:to_text).join("\n")
          end.join("\n\n")
  end

  # 4)
  # Unless performing online verification (see §Alternative Workflows), the Verifier MUST extract the
  # SignedCertificateTimestamp embedded in the leaf certificate, and verify it as in RFC 9162 §8.1.3,
  # using the verification key from the Certificate Transparency Log.
  chain = chains.first
  if (result = verify_scts(bundle.leaf_certificate, chain)) && !result.verified?
    return result
  end

  # 5)
  # The Verifier MUST then check the certificate against the verification policy.

  usage_ext = bundle.leaf_certificate.extension(Internal::X509::Extension::KeyUsage)
  return VerificationFailure.new("Key usage is not of type `digital signature`") unless usage_ext.digital_signature

  extended_key_usage = bundle.leaf_certificate.extension(Internal::X509::Extension::ExtendedKeyUsage)
  unless extended_key_usage.code_signing?
    return VerificationFailure.new("Extended key usage is not of type `code signing`")
  end

  policy_check = policy.verify(bundle.leaf_certificate)
  return policy_check unless policy_check.verified?

  # 6)
  # By this point, the Verifier has already verified the signature by the Transparency Service (§Establishing a Time
  #  for the Signature). The Verifier MUST parse body: body is a base64-encoded JSON document with keys apiVersion
  #  and kind. The Verifier implementation contains a list of known Transparency Service formats (by apiVersion and
  #  kind); if no type is found, abort. The Verifier MUST parse body as the given type.
  #
  # Then, the Verifier MUST check the following; exactly how to do this will be specified by each type in Spec:
  # Sigstore Registries (§Signature Metadata Formats):
  #
  #  * The signature from the parsed body is the same as the provided signature.
  #  * The key or certificate from the parsed body is the same as in the input certificate.
  #  * The “subject” of the parsed body matches the artifact.

  signing_key = bundle.leaf_certificate.public_key

  case bundle.content
  when :message_signature
    verified = verify_raw(signing_key, bundle.message_signature.signature, input.hashed_input.digest)
    return VerificationFailure.new("Signature verification failed") unless verified
  when :dsse_envelope
    verify_dsse(bundle.dsse_envelope, signing_key) or
      return VerificationFailure.new("DSSE envelope verification failed")

    case bundle.dsse_envelope.payloadType
    when "application/vnd.in-toto+json"
      in_toto = begin
        JSON.parse(bundle.dsse_envelope.payload)
      rescue JSON::ParserError
        raise Error::InvalidBundle, "invalid JSON for in-toto statement in DSSE payload"
      end
      verify_in_toto(input, in_toto)
    else
      raise Sigstore::Error::Unimplemented,
            "unsupported DSSE payload type: #{bundle.dsse_envelope.payloadType.inspect}"
    end
  else
    raise Error::InvalidBundle, "unknown content type: #{bundle.content}"
  end

  VerificationSuccess.new
end

#verify_scts(leaf_certificate, chain) ⇒ Object

Raises:



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# File 'lib/sigstore/verifier.rb', line 257

def verify_scts(leaf_certificate, chain)
  sct_list = leaf_certificate
             .extension(Internal::X509::Extension::PrecertificateSignedCertificateTimestamps)
             .signed_certificate_timestamps
  raise Error::InvalidCertificate, "no SCTs found" if sct_list.empty?

  sct_list.each do |sct|
    verified = verify_sct(
      sct,
      leaf_certificate,
      chain,
      @ct_keyring
    )
    return VerificationFailure.new("SCT verification failed") unless verified
  end

  nil
end