Add poor man’s ASN1 parsing

.editorconfig

@@ -10,5 +10,5 @@ indent_size = 2
 indent_style = space
 trim_trailing_whitespace = true
 
-[*.{ico,keep,pdf,svg}]
+[*.{ico,keep,pdf,svg,der}]
 insert_final_newline = false

app/lib/asn1/timestamp.rb

@@ -0,0 +1,25 @@
+class ASN1::Timestamp
+  ## Poor man’s rfc3161 timestamp decoding
+  # This works, as of 2019-05, for timestamps delivered by the universign POST api.
+  # We should properly access the ASN1 contents using the sequence and tags structure.
+  # However:
+  # * It’s hard to do right.
+  # * We currently don’t require it for proper operation; timestamps are never exposed to users.
+  # * There’s an ongoing PR https://github.com/ruby/openssl/pull/204 for proper timestamp decoding in the ruby openssl library; let’s use OpenSSL::TS once it exists.
+
+  def self.timestampInfo(asn1timestamp)
+    asn1 = OpenSSL::ASN1.decode(asn1timestamp)
+    tstInfo = OpenSSL::ASN1.decode(asn1.value[1].value[0].value[2].value[1].value[0].value)
+    tstInfo
+  end
+
+  def self.signature_time(asn1timestamp)
+    tstInfo = timestampInfo(asn1timestamp)
+    tstInfo.value[4].value
+  end
+
+  def self.signed_digest(asn1timestamp)
+    tstInfo = timestampInfo(asn1timestamp)
+    tstInfo.value[2].value[1].value.unpack1('H*')
+  end
+end

spec/lib/asn1/timestamp_spec.rb

@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe ASN1::Timestamp do
+  let(:asn1timestamp) { File.read('spec/fixtures/files/bill_signature/signature.der') }
+
+  describe '.timestamp_time' do
+    subject { described_class.signature_time(asn1timestamp) }
+
+    it { is_expected.to eq Time.zone.parse('2019-04-30 15:30:20 UTC') }
+  end
+
+  describe '.timestamp_signed_data' do
+    subject { described_class.signed_digest(asn1timestamp) }
+
+    let(:data) { Digest::SHA256.hexdigest('CECI EST UN BLOB') }
+
+    it { is_expected.to eq data }
+  end
+end