Addressing the Challenges of e-voting Through Crypto Design

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1 Addressing the Challenges of e-voting Through Crypto Design Thomas Zacharias University of Edinburgh 29 November 2017 Scotland s Democratic Future: Exploring Electronic Voting Scottish Government and University of Edinburgh School of Informatics Workshop

2 When do we actually setup an electronic voting procedure? What is the extent of digitilisation of a traditional election protocol that qualifies as e-voting? 2

3 When do we actually setup an electronic voting procedure? E-voting means the use of electronic means in one of the following three processes: Identification of voters Casting the vote Counting the vote Source: 3

4 When do we actually setup an electronic voting procedure? What is the extent of digitilisation of a traditional election protocol that qualifies as e-voting? Registration Voting Counting Auditing 4

5 Replacing the human protocol with electronic procedures 5

6 Replacing the human protocol with electronic procedures 6

7 Replacing the human protocol with electronic procedures 7

8 Replacing the human protocol with electronic procedures 8

9 Replacing the human protocol with electronic procedures 9

10 The benefits of e-voting Increase the participation of social groups that face considerable physical barriers. Increase the efficiency of the preparation of the election and the calculation of the final results. Reduce the financial cost of the elections (in long term). 10

11 The risks of e-voting Unauthorised intervention of third parties may cause denial of service. Large-scale manipulation by a small group of insiders. Lack of widely agreed standards for the election procedure (infrastructure, software). 11

12 The challenges of e-voting At any point of replacing the traditional protocol with electronic means, extreme care in the algorithmic design, implementation, and execution oversight is required. 12

13 The challenges of e-voting Preserve the fundamental requirements of a voting system. Integrity Privacy Availability 13

14 Addressing e-voting challenges via cryptographic tools Integrity Integrity Privacy Privacy Accessibility Availability 14

15 Modern cryptographic design 1. Understanding and formal definition of the studied notion. 2. Rigorous specification of the threat model. 3. Realisation via a well-described construction. 4. Mathematical proof of the construction s security under the defined threat model. 15

16 Modern cryptography in the real-world Secure internet communication (HTTPS over TLS/SSL, end-to-end encryption). Authentication (Digital signatures, password management). Privacy preservation (aggregate statistics, anonymous web browsing). Blockchain technologies (Bitcoin, Ethereum, smart contracts). 16

17 Modern cryptography in the real-world Secure web browsing. E-banking. . Privacy-preserving profile management. Secure messaging. Decentralised publicly verifiable transactions. 17

18 The necessity of cryptography for realising secure e-voting Collecting, storing and/or communicating sensitive election data overtly, can easily jeopardise election security. Offline (on site): a vote collection device that keeps logs of unencrypted ballot casting can not protect voters privacy when the logs are audited. Online: whenever two devices should interact, secure and authenticated communication must be applied to protect the exchanged data. 18

19 Addressing e-voting challenges via cryptographic tools Integrity Integrity Privacy Privacy Accessibility Availability 19

20 Addressing e-voting challenges via cryptographic tools: Privacy Availability Integrity Privacy 20

21 Ballot secrecy Encryption realises the concept of a digital envelope. The voter s selection is cryptographically sealed. An attacker can not obtain information about the encryption s content (vote selection). 21

22 Voter-vote Unlinkability At some point of the election process, the votes must be anonymised, to prevent coercion and vote-selling. 22

23 Voter-vote Unlinkability Mix-nets: a powerful cryptographic primitive realising vote shuffling in a digital ballot box. Enc(V & ) Enc(V ( ) Enc(V ) ) Mix-net C & C ( C ) C * Enc(V * ) C &, C (, C ), C * = {Enc V &, Enc V (, Enc V ), Enc V * } 23

24 Voter-vote Unlinkability Additive homomorphic tally: a mathematical property of special encryption schemes (E.g. ElGamal, Paillier) that allow for the counting of votes in encrypted manner. Enc X 1 Enc Y = Enc(X + Y) YES NO NO YES 24

25 Voter-vote Unlinkability Additive homomorphic tally: a mathematical property of special encryption schemes (E.g. ElGamal, Paillier) that allow for the counting of votes in encrypted manner. Enc X 1 Enc Y = Enc(X + Y) YES : 2 votes NO : 2 votes 25

26 Voter-vote Unlinkability Anonymous authentication: the voter proves her eligibility via a one-show anonymous credential (e.g. blind signature of the voter s ballot). I am an eligible user and this is my vote I confirm your eligibility and record your vote 26

27 Coercion resistance Concerns about coercion in other types of remote voting (e.g. postal voting) also apply to internet voting. In cases where postal voting is allowed, internet voting could be used, supporting multiple voting as a coercion countermeasure (Estonian e-voting). 27

28 Addressing e-voting challenges via cryptographic tools: Privacy Availability Integrity Privacy 28

29 Addressing e-voting challenges via cryptographic tools: Availability Integrity Privacy Availability 29

30 Usability: making e-voting feasible for every citizen When ballot encoding is applied during vote casting, the voter interacts via a user-friendly interface in a designated voting device, or their browser. Basic familiarity with a standard operating system (e.g., Windows, ios, Android) or web browser (e.g., Edge/Explorer, Safari, Chrome, Firefox) is the only required background. 30

31 Usability: making e-voting feasible for every citizen 31

32 Usability: making e-voting feasible for every citizen 32

33 Usability: making e-voting feasible for every citizen 33

34 Usability: making e-voting feasible for every citizen When code-voting is applied, the voter obtains a ballot with a pre-encoding of the election options (vote-code). 34

35 Usability: making e-voting feasible for every citizen What is your stance on the potential of adopting e-voting for national elections? Serial number: VERY POSITIVE 4 POSITIVE 5 NEUTRAL 2 NEGATIVE 3 VERY NEGATIVE 35

36 Usability: making e-voting feasible for every citizen When code-voting is applied, the voter obtains a ballot with a list of random cryptographic preencoding of the election options (vote-code). The voter simply submits the vote-code that correspond to her option selection, which can be done in lightwight electronic devices. 36

37 Usability: making e-voting feasible for every citizen Serial number: 100 Vote-code: 5 Serial number: VERY POSITIVE 4 POSITIVE 5 NEUTRAL 2 NEGATIVE 3 VERY NEGATIVE 37

38 Usability: making e-voting feasible for every citizen When code-voting is applied, the voter obtains a ballot with a list of random cryptographic preencoding of the election options (vote-code). The voter simply submits the vote-code that correspond to her option selection, which can be done in any electronic device with internet access. In case the voting device is trusted for privacy, vote casting can run via a user-friendly interface. 38

39 Fault tolerance: protecting the election s liveness Threshold cryptography in combination with a distributed system architecture allows for a secure and uninterrupted election access and execution, even when a substantial amount of election servers is not available. 39

40 Addressing e-voting challenges via cryptographic tools: Availability Integrity Privacy Availability 40

41 Addressing e-voting challenges via cryptographic tools: Integrity Privacy Availability Integrity 41

42 Eligibility The authentication/registration authority is equipped with the algorithm that specifies the set of eligible voters. Digital signatures: each voter has an ID-card with an embedded signing key (Estonia). Password-based: the authentication server(s) maintains a cryptographic hash table with all the passwords that correspond to eligible voters. 42

43 End-to-end verifiability Universal level: Any party can verify the correctness of the election transcript. 43

44 End-to-end verifiability Individual level: The voters obtain some audit data in order to verify themselves that their votes were: Cast-as-intended Recorded-as-cast Tallied-as-recorded 44

45 Universal Verifiability Every step of the election procedure is associated with a cryptographic proof of its execution. This is a ballot that encrypts a valid election option π 45

46 Universal Verifiability Every step of the election procedure is associated with a cryptographic proof of its execution. All the cast ballots are encryptions of a valid election option π & π ( π ) π * π 5 46

47 Universal Verifiability Every step of the election procedure is associated with a cryptographic proof of its execution. Enc(V & ) The output ciphertexts are a permutation of exactly the input ciphertexts Enc(V ( ) Enc(V ) ) Mix-net π C & C ( C ) C * Enc(V * ) 47

48 Universal Verifiability Every step of the election procedure is associated with a cryptographic proof of its execution. The decrypted results are a permutation of exactly all eligible recorded votes π C & C ( C ) C * π YES NO YES NO 48

49 Universal Verifiability Each proof satisfies soundness (invalid statements will not be accepted) and prevents any malicious party from leaking information about the associated sensitive data (zeroknowledge proofs). The sequence of cryptographic proofs constitutes a verifiable election transcript that is posted in a publicly accessible web site (bulletin board) for auditing while preserving election privacy. 49

50 Individual Verifiability State-of-the-art e-voting systems support elaborate verification mechanism that allow the voter to challenge the correct encoding of their vote. 1. Ballot encryption: audit the validity of the voting device by randomly checking the local cryptographic operations. 2. Code-voting: audit the honest of the ballot preparation authorities by randomly checking the consistency of the encoding of the election options. 50

51 Accountability The e-voting system should provide a mechanism provides undeniable evidence of which entity is responsible for an incorrect execution. 51

52 Accountability example: authenticated communication 52

53 Accountability example: authenticated communication The voter signs her ballot. Sign π Voter ( ) 53

54 Accountability example: authenticated communication The voter signs her ballot. The vote collection server replies with a signature on the received data. Sign Server ( Sign Voter ( π )) 54

55 Accountability example: authenticated communication The voter signs her ballot. The vote collection server replies with a signature on the received data. If an authority is realized by a distributed subsystem, then malicious nodes can be blacklisted. 55

56 Accountability example: posting on the bulleting board The authorities commit to the validity of their data. The election public key is posted prior to the voting period. The ballot preparation authorities provide cryptographic proofs of proper encoding of the election options (Code-voting). The tallying authorities post cryptographic proofs of correct decryption and election tally. 56

57 Addressing e-voting challenges via cryptographic tools Integrity Integrity Privacy Privacy Accessibility Availability 57

58 Addressing e-voting challenges via cryptographic tools π & π ( Mix-net π π ) π * YES NO YES NO 58

59 Is crypto enough to guarantee secure e-voting? 59

60 The importance of the human factor Cryptographic tools provide a solid background, but do not suffice. Active participation of the stakeholders is crucial. Verification mechanisms are meaningless if not even a small ratio of voters are willing to verify (or outsource verification to a trusted party). The trustees should audit the correct publishing of the election public key to protect voters privacy. A formally analysed human protocol and e-voting cryptographic design should be side-by-side. 60

61 Thank you!!! 61

62 Addressing the Challenges of e-voting Through Crypto Design Thomas Zacharias University of Edinburgh 29 November 2017 Scotland s Democratic Future: Exploring Electronic Voting Scottish Government and University of Edinburgh School of Informatics Workshop

Swiss E-Voting Workshop 2010

Swiss E-Voting Workshop 2010 Verifiability in Remote Voting Systems September 2010 Jordi Puiggali VP Research & Development Jordi.Puiggali@scytl.com Index Auditability in e-voting Types of verifiability