1 files changed, 80 insertions, 0 deletions
diff --git a/lib/atlassian/jira_connect/jwt/asymmetric.rb b/lib/atlassian/jira_connect/jwt/asymmetric.rb
new file mode 100644
index 00000000000..0611a17c005
--- /dev/null
+++ b/lib/atlassian/jira_connect/jwt/asymmetric.rb
@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+module Atlassian
+  module JiraConnect
+    module Jwt
+      # See documentation about Atlassian asymmetric JWT verification:
+      # https://developer.atlassian.com/cloud/jira/platform/understanding-jwt-for-connect-apps/#verifying-a-asymmetric-jwt-token-for-install-callbacks
+
+      class Asymmetric
+        include Gitlab::Utils::StrongMemoize
+
+        KeyFetchError = Class.new(StandardError)
+
+        ALGORITHM = 'RS256'
+        PUBLIC_KEY_CDN_URL = 'https://connect-install-keys.atlassian.com/'
+        UUID4_REGEX = /\A[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}\z/.freeze
+
+        def initialize(token, verification_claims)
+          @token = token
+          @verification_claims = verification_claims
+        end
+
+        def valid?
+          claims.present? && claims['qsh'] == verification_qsh
+        end
+
+        def iss_claim
+          return unless claims
+
+          claims['iss']
+        end
+
+        private
+
+        def claims
+          strong_memoize(:claims) do
+            _, jwt_headers    = decode_token
+            public_key        = retrieve_public_key(jwt_headers['kid'])
+
+            decoded_claims(public_key)
+          rescue JWT::DecodeError, OpenSSL::PKey::PKeyError, KeyFetchError
+          end
+        end
+
+        def decoded_claims(public_key)
+          decode_token(
+            public_key,
+            true,
+            **relevant_claims,
+            verify_aud: true,
+            verify_iss: true,
+            algorithm: ALGORITHM
+          ).first
+        end
+
+        def decode_token(key = nil, verify = false, **claims)
+          Atlassian::Jwt.decode(@token, key, verify, **claims)
+        end
+
+        def retrieve_public_key(key_id)
+          raise KeyFetchError unless UUID4_REGEX.match?(key_id)
+
+          public_key = Gitlab::HTTP.try_get(PUBLIC_KEY_CDN_URL + key_id).try(:body)
+
+          raise KeyFetchError if public_key.blank?
+
+          OpenSSL::PKey.read(public_key)
+        end
+
+        def relevant_claims
+          @verification_claims.slice(:aud, :iss)
+        end
+
+        def verification_qsh
+          @verification_claims[:qsh]
+        end
+      end
+    end
+  end
+end