Secure Document Publication in the Cloud Bhavani Thuraisingham

Secure Document Publication in the Cloud Bhavani Thuraisingham July 18, 2015

Motivation n Documents represented in XML (e. Xtensivle Markup Language) and stored in the Cloud n XML documents have to be secured n XML has become the standard document interchange language for the web n XML is a critical technology for the semantic web n RDF and other specifications are built on XML documents must satisfy security and privacy policies n Challenges: Access Control, Secure publishing, Secure Web Services Applications, Securing RDF, Secure semantic web, Temporal models, Privacy, Handling evolving XML specifications n Based on paper published in IEEE Transactions on Knowledge and Data Engineering, October 2004 (Bertino, Ferrari, Carminati, Thuraisingham)

Example XML Document Year: 2003 Annual report Assets Asset Dept Patents Asset Patent Expenses CS Name: UTD Patent Author ID Tot Date Funds Date Fund Dept Amout UTD Type 6/1/03 1 m$ NSF Other Equipment Tech-details Short-desc Cash Books

XML document: Example <? xml version="1. 0" encoding="UTF-8"? > <Annual Report Year = 2003 Name = CS“> <Assets> <Asset Dept = “CS"> <Expense Tot=". . “/> <Funds> <Funding Date=“ 6/1/03" Type=“NSF" Amount=". . . " /> </Funds> </Asset> <Asset Dept=“IST"> <Expenses Tot=". . " PC_ATTR="02"/> <Funds> <Funding Date=“ 10/1/03" Type=“Do. D" Amount=". . . " /> <Funding Date=“ 4/1/03" Type=“CIA" Amount=". . . " / > </Funds> </Assets> - - - - /Annual. Report> 11/10/2020 4

Publishing service: how it works A new class of information-centered applications based on Data dissemination Possible scenarios: Information commerce (Digital libraries, Electronic news, etc. ) Intra-company information systems Publishing Service n Push/Pull modes n Security requirements: n n WEB n n Confidentiality Integrity Authenticity Completeness Users 11/10/2020 5

Subject Credentials, Protection Objects and Policy Base n Subjects are given access to XML documents or portions of documents depending on user ID and/or Credentials n Credential specification is based on credentials a subject has n Professor is a credential; Secretary is a credential n Protection objects are objects to which access is controlled n Entire XML documents or portions of XML documents n Policy base stores security policies for protecting the XML source contents

Subject Credential Base Example <Professor cred. ID=“ 9” sub. ID = “ 16: CIssuer = “ 2”> <name> Alice Brown </name> <university> UTD <university/> <department> CS </department> <research-group> Security </research-group> </Professor> <Secretary cred. ID=“ 12” sub. ID = “ 4: CIssuer = “ 2”> <name> John James </name> <university> UTD <university/> <department>CS </department> <level> Senior </level> </Secretary>

Policy Base Example 11/10/2020 <? xml version="1. 0" encoding="UTF-8"? > <policy_base>. . . <policy_spec ID=‘P 1' cred_expr="//Professor[department='CS']" target="annual_report. xml" path="//Patent[@Dept='CS']//node()" priv="VIEW"/> <policy_spec ID=‘P 2' cred_expr="//Professor[department='CS']" target="annual_report. xml" path="//Patent[@Dept='IST']/Short-descr/node() and //Patent[@Dept='IST']/authors" priv="VIEW"/> <policy_spec ID=‘P 3' cred_expr="//Professor[department='IST'] " target="annual_report. xml" path="//Patent[@Dept='IST']//node()" priv="VIEW"/> <policy_spec ID=‘P 4' cred_expr="//Professor[department='IST']" target="annual_report. xml" path="//Patent[@Dept='CS']/Short-descr/node() and //Patent[@Dept='CS']/authors" priv="VIEW"/> <policy_spec ID=‘P 5' cred_expr="//secretary[department='CS' and level='junior']" target="annual_report. xml" path="//Asset[@Dept='CS']/node()" priv="VIEW "/> <policy_spec ID=‘P 6' cred_expr="//secretary[department='CS' and level='senior']" target="annual_report. xml" path="//Asset[@Dept='IST']/Funds/@Type and //Asset[@Dept='IST']/Funds/@Funding-Date" priv="VIEW "/> <policy_spec ID=‘P 7' cred_expr="//secretary[department='IST' and level='junior']" target="annual_report. xml" path="//Asset[@Dept='IST']/node()" priv="VIEW "/>. . . 8 </policy_base>

Access Control Strategy n Subjects request access to XML documents under two modes: Browsing and authoring n With browsing access subject can read/navigate documents n Authoring access is needed to modify, delete, append documents n Access control module checks the policy based and applies policy specs n Views of the document are created based on credentials and policy specs n In case of conflict, least access privilege rule is enforced n Works for Push/Pull modes

System Architecture for Access Control Pull/Query User X-Access Push/result X-Admin Tools Policy base Credential base XML Documents

Cloud-based Third-Party Architecture n The Owner is the producer of information It specifies access control policies n The Publisher is responsible for managing (a portion of) the Owner information and answering subject queries n Goal: Untrusted Publisher with respect to Authenticity and Completeness checking n 11/10/2020 Publisher is the Cloud; Owner is the organization or person who owns/creates the document; User/Subject queries for the documents from the cloud XML Source Credential base policy base SE-XML Owner credentials Publisher Reply document Query User/Subject 11

Subject Owner Interaction n Subjects register with Owner during subscription phase; during this phase subject is assigned by owner credentials stored at the owner site n Owner returns to the subject the Subject Policy Configuration (policy identifiers) that apply to the subject signed with the private key of the owner n Example: If polices P 1 and P 2 apply to John (e. g. CS prof) and policy P 6 applies to Jane (IST secretary), owner Joe sends John P 1 and P 2 and to Jane P 6 signed with Joe’s private key

Subject Policy Configuration <? xml version="1. 0" encoding="UTF-8" ? > <Subject. Policy. Configuration ID=“Professor. CS" created="08 -05 -2002"> <owner> <name>owner 1</name> <organization>CS</organization> <state>Texas</state> <uri>www. owner 1. com</uri> <owner> <policy>Vta. UBIxli. HS 1 hzrqk. Kh. YVTt. Yraf. VSm. Co. JPk. UVKYXCA 7 y. Vdc 7 a/ne 5 sg. Ig 0 t. GGRe 3 /D 2 Xg 6 Fbwp 3 SAKK/Ref 1 te. ZCp. D 0 nlkx 89 GOIIcw 8 o 9 R 3 Mb 2 YY/slk 5+Fu 0 xx. WXl. B Yu. WKWWNs. XENKTkgi. XL 4 m. B 1 SUt 4 bm. F 6 YG 4 l. Txfxdu. VAw=</policy> </Subject. Policy. Configuration> P 1, P 2 11/10/2020 13

Owner Publisher Interaction n For each document the owner sends the publisher the following information n n Information on which subjects can access which portions of the document according to the policy base (I. e. access control policies) Policy element which describes the policies for the document is also inserted Also for each element e based on the policies applied to e, the owner inserts policy configuration (binary string) converted to hexadecimal representation Merkle Signature of each document n The document together with the security information is called “Security Enhanced Document” (SE-XML) and will enable the subject to verify the authenticity of the document n Additional information encoded in the document called Secure Structure is used by the subject to verify completeness of the result

Policy Configuration/Policy Element Where m is the number of policies applied on document d n Consider the element e of an XML document d n The policy configuration of e consists of a binary string, of m bits where the i-th bit is equal to: n 1 if the policies, whose identifier is i, is applied on e n 0 otherwise n SE-XML document has information about the set of the m policies applied to the document: 11/10/2020 n <Policy> 1, 2, 3, 4, 5, 6, 7 </Policy> n For each element, policy information in hexadecimal representation is also inserted 15

Policy Configuration: example n Consider the short-descr element of the CS dept: n The only policies applied to it are: 1 st and 4 th policy (i. e. P 1, P 4) n According to the Policy element, these policies are located in the first and fourth position of the policy configuration The policy configuration associated with short-descr element is 1001000 short-descr’s PC value is 90 (hexadecimal representation) 11/10/2020 16

Publisher Policy evaluation: example 1. Decrypt with the Public key of the Owner the policies applied to the subject (i. e. , the Subject Policy Configuration) P 1, P 2 2. Match the obtained policy identifiers with the identifiers contained in the Policy element i. e. match P 1, P 2 with P 1, P 2, P 3, P 4, P 5, P 6, P 7 3. Find the policies applicable to the element specified in the query (e. g. , short description element) P 1, P 4 4. Find the matching policies from steps 2 and 3 P 1 The only policies applied on Short-descr element is “P 1” 11/10/2020 17

Security Enhanced XML document XML Document • Policy Information • Merkle Signature SE-XML Document 11/10/2020 18

Merkle Signature Example title Newspaper Frontpage Leading Author title paragraph Author Literary_page Politic_page Paragraphs title paragraph Author Politic topic Author date Sport_page Article title topic Author title news topic Author title Mh. X(Author)=h(h(Author)||h(Author. value)) Mh. X(title)=h(h(title)||h(title. value)) Mh. X(paragraph)=h(h(paragraph)||h(paragraph. content)|| Mh. X(Author)||Mh. X(title)) 11/10/2020 19

Merkle Signature Example Newspaper Frontpage Leading Author title paragraph Author title Politic paragraph Author topic Author date Literary_page Politic_page Paragraphs title Article title topic Author title Sport_page news topic Author title Mh. X(Newspaper)=h(h(Newspaper)||h(Newspaper. content)|| Mh. X ()……||Mh. X()||…||Mh. X()) 11/10/2020 Mh. X(Newspaper) Merkle Signature of Newspaper XML file 20

Security Enhanced XML document: Example <? xml version="1. 0" encoding="UTF-8"? > <Annual Report Year = 2003 Name = CS“ CG 4 g 3 D 8/, m. PVV/t+T 2 O 1 k. ZRFhdio=" > <Policy>1, 2, 3, 4, 5, 6, 7 </Policy> <Assets> <Asset Dept = “CS"> <Expense Tot=". . " PC_ATTR="08"/> <Funds> <Funding Date=“ 6/1/03" Type=“NSF" Amount=". . . " PC_ATTR="080808"/> </Funds> </Asset> <Asset Dept=“IST"> <Expenses Tot=". . " PC_ATTR="02"/> <Funds> <Funding Date=“ 10/1/03" Type=“Do. D" Amount=". . . " PC_ATTR="060602"/> <Funding Date=“ 4/1/03" Type=“CIA" Amount=". . . " PC_ATTR="060602"/> </Funds> </Assets> - - - - 11/10/2020 /Annual. Report> 21

Secure Structure Example <? xml version="1. 0" encoding="UTF-8"? > <x-82592553 x-4639474="r. VR 5 DQ" x-67303774="QTQXS“ Sign="OD 2 mc 9 a. VV/t. P 4 g 3 TG+1 kr 4 s. Fhdio=" > <Policy>1, 2, 3, 4, 5, 6, 7 </Policy> <x-1915689488> <x-785490824 x-40276037="Plc. ZUo"> <x-590292021 x-57205665=". . " PC_ATTR="08"/> <x-13947931> <x-1037159472 x-122584813="f. Nht. L" x-0260379="hg. KID" x-93640287=". . . " PC_ATTR="080808"/> </x-13947931> </x-785490824> <x-785490824 x-40276037="p. KGEs"> <x-590292021 x-57205665=". . " PC_ATTR="02"/> <x-13947931> <x-1037159472 x-122584813="g. Pd 39" x-0260379="hg. KID" x-93640287=". . . " PC_ATTR="060602"/> <x-1037159472 x-122584813="o 4 Gp. M" x-0260379="yr 0 Qj. J" x-93640287=". . . " PC_ATTR="060602"/> </x-13947931> </x-785490824> </x-1915689488> /x-82592553> 11/10/2020 22

Subject Publisher Interaction n The subject submits queries to publisher; it also sends its subject policy configuration n Publisher computes a view of the requested documents based on access control policies for the subject set by the owner (we assume this action is trusted) n To verify the authenticity of the answer, subject must recompute the same bottom up hash value signed by owner (i. e. Merkle signature) and compare it with the Merkle signature generated by the owner and inserted by the publisher n Subject may not get the entire document; therefore publisher sends to the subject additional hash values that refer to the missing portions of the document n Subject verifies the authenticity of the document

Merkle Hash Paths v v 1 1 2 3 2 2 3 w w 4 5 8 5 w 4 7 6 8 9 5 11 12 13 10 11 12 13 14 15 16 17 Mh. Path(7, 1) 7 6 8 9 10 Mh. Path(4, 1) 11/10/2020 4 7 6 3 9 10 11 12 13 14 15 16 17 Mh. Path(5, 1) 24

Completeness Verification <? xml version="1. 0" encoding="UTF-8"? > <x-82592553 x-4639474="r. VR 5 DQ" x-67303774="QTQXS“ Sign="OD 2 mc 9 a. VV/t. P 4 g 3 TG+1 kr 4 s. Fhdio="> Verify authenticity and integrity of ST by using the Merkle Signature <x-1915689488> <x-785490824 x-40276037="Plc. ZUo"> Verify the completness <x-590292021 x-57205665=". . " PC_ATTR="08"/> 1. Translate the submitted query <x-13947931> <Policy>1, 2, 3, 4, 5, 6, 7 </Policy> <x-1037159472 /Asset[@Dept=‘IST’]/Funds/* x-122584813="f. Nht. L" x-0260379="hg. KID" x-93640287=". . . " PC_ATTR="080808"/> </x-13947931> //x-785490824[@x-40276037=`p. KGEs']/ x-13947931/* 2. </x-785490824> <x-785490824 x-40276037="p. KGEs"> 3. 4. <x-590292021 x-57205665=". . " PC_ATTR="02"/> <x-13947931> <x-1037159472 x-122584813="g. Pd 39" x-0260379="hg. KID" x-93640287=". . . " PC_ATTR="060602"/> 5. Evaluate the obtained query on the secure structure Check the policy configuration Hash the result answer received by the Publisher Match it with the obtained node-set. <x-1037159472 x-122584813="o 4 Gp. M" x-0260379="yr 0 Qj. J" x-93640287=". . . " PC_ATTR="060602"/> </x-13947931> </x-785490824> 11/10/2020 </x-1915689488> /x-82592553> 25

APPENDIX Bhavani Thuraisingham

Reply Document Generation Algorithm n Reply Generator Algorithm n n Input: SE-XML version of XML document d; A query q on d submitted by s; the policy configuration of s Output: Reply document r n Evaluate q on d and return a well-formed XML document containing n n n all and only the nodes satisfying q Determine which access control policies apply to each node satisfying q Remove from node-set the nodes that s is not authorized to see based on information in SE-XML for d All the attributes in the resulting document are replaced with an Atteibute. Element element An additional attribute called Mh. Path is inserted in each node to be returned to s Insert Merkle signature of d Rebuild document by taking the set of nodes returned from the above steps and transform into a well-formed document

Example Reply Document n Reply document generated by Publisher to an IST Professor who requests all the patents of CS department n Mh. Path attribute associated with Short-descr contains all the hash values needed to computer the Merkle has value of Patent starting from Short-descr. Patent Sign Authors Mh. Path Short-descr Mh. Path

Authentication: Authenticable Element n Document d = (Vd, vd, Ed, Fed) is defined as follows: Vd is the set of all element nodes and attribute nodes in d, vd is the node representing the document element called the document root, Ed is the set of edges in d, and FEd is the edge labeling function n Definition 1: Let d = (Vd, vd, Ed, FEd) be an XML document, let g = (Vg, vg, Eg, FEg) be the SE-XML version of d, and r = (Vr, vr, Er, FEr) be the reply document to a query on d submitted by s. Let VT be the set of terminal nodes of r. Let Vr, e be the set of element nodes in the reply document r. For each v in Vr, e, v is authenticable by s iff. there exists vt in VT with v in path(vt) such that it is possible through a recursive bottom up computation to compute the Merkle hash value of vd using only the values in {w. Mh. Path: w is in path(vt)} n Theorem 1: Let g = (Vg, vg, Eg, FEg) be the SE-XML version of an XML document d and r = (Vr, vr, Er, FEr) be the reply document corresponding to a query submitted on d by subject s. Each node in Vr, e is authenticable by s n Proof of Theorem is by Induction on the length of the relative path of w with respect to v where v is in VT, w is in Vd and v is in path(w)

Authentication Subject Verification Algorithm: Input: Reply document r = (Vr, vr, Er, FEr) Output = True if all nodes in r are authentic. False otherwise n Starting from each terminal node in the reply document, the algorithm recomputes the Merkle hash value of the root of document d through a bottom-up computation that uses the values of attributes Mh. Path of each node belonging to the path connecting the terminal node to the root of the reply document n The obtained value is compared with the decryption of the Merkle Signature of d using the Owner’s public key n If the two values coincide then all the nodes belonging to the path are authentic, otherwise the algorithm terminates and returns false

Authentication: Authentic Element n Definition 2: Let g = (Vg, vg, Eg, FEg) be the SE-XML version of d, and r = (Vr, vr, Er, FEr) be the reply document to a query on d submitted by s. Each node v in Vr, e is authentic iff. V is authenticable by s and the computed Merkle has value of vd is equal to the decryption of Sign. val using the Owner public key n Theorem 2: n Let s be a subject, q be a query submitted by s, and r be the reply document received by s as an answer to q. n Subject verification algorithm returns True iff. Each v in Vr, e is authentic where Vr, e is the set of element nodes in the reply document r n Proof: n n By theorem 1 all nodes of a reply document correspond to elements are authenticable by s. Therefore bt definitions 1 and 2, there exists for each vt in VT a recursive bottom-up computation able to compute Merkle hash value of the root of the document d using only the values of Mh. Path attributes of all nodes belonging to path (vt), and this value coincides with the decryption of the value of the Sign attribute with the Owner public key The proof is to show that this recursive bottom-up computation is implemented by the Subject Verification Algorithms

Potential Attacks and Performance Issues n Attacks Subject attacks n Subject Si eavesdropping during subscription phase of subject Sm n Subject attacking the secure structure and deducing sensitive attributes n Publisher attacks n Publisher changes the Sign element in SE-XML n Performance Issues n Update management n Modification of document implies changes to Merkle hash and SE-XML document n Storage complexity of security information n E. g, SE-XML and Secure Structure n

Challenge: Integrating Confidentiality and Authentication n Currently the portion of the Publisher that actually enforces the access control policy must be trusted n I. e. Trust with respect to Confidentiality n Challenge: How can we come up with a unified approach that ensures Confidentiality, Authenticity and Completeness n Directions: Apply access control policies to portions of the document and encrypt the views computers; only those authorized to see the views have the keys for decryption n Querying encrypted databases

Application: Secure Web Services n How authenticity, confidentiality and integrity can be ensured in the presence of an untrusted UDDI? n Traditional techniques are not enough! n Possible solutions: n n Integrity, confidentiality: selective encryption of the data managed by the UDDI according to the specified access control policies Authenticity: Merkle hash trees n Additional security properties: n n Completeness Consistency

Authenticity …. traditional digital signatures do not fit well in third-party architectures!! Business. Entity <dsig: Signature> Query UDDI t. Model Business. Service Publisher. Assertion Binding. Template Business. Service requestor 11/10/2020 Service provider 35

Merkle Signature • An alternative way to sign an XML doc • By applying a unique digital signature on an XML doc it is possible to ensure the authenticity of: Business. Entity <dsig: Signature> t. Model Business. Service Publisher. Assertion Binding. Template • the whole document • any portion of it • It uses a different way to compute the digest of XML docs, based on the Merkle tree authentication mechanisms 11/10/2020 36