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# Framework for a DLT Based COVID-19 Passport

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## Abstract and Figures

Uniquely identifying individuals across the various networks they interact with on a daily basis remains a challenge for the digital world that we live in, and therefore the development of secure and efficient privacy preserving identity mechanisms has become an important field of research. In addition, the popularity of decentralised decision making networks such as Bitcoin has seen a huge interest in making use of distributed ledger technology to store and securely disseminate end user identity credentials. In this paper we describe a mechanism that allows one to store the COVID-19 vaccination details of individuals on a publicly readable, decentralised, immutable blockchain, and makes use of a two-factor authentication system that employs biometric cryptographic hashing techniques to generate a unique identifier for each user. Our main contribution is the employment of a provably secure input-hiding, locality-sensitive hashing algorithm over an iris extraction technique , that can be used to authenticate users and anonymously locate vaccination records on the blockchain, without leaking any personally identifiable information to the blockchain.
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Framework for a DLT Based COVID-19 Passport
Sarang Chaudhari, Michael Clear, Philip Bradish, and Hitesh Tewari
Indian Institute of Technology, Delhi, Trinity College Dublin, Ireland
Abstract. Uniquely identifying individuals across the various networks
they interact with on a daily basis remains a challenge for the digital
world that we live in, and therefore the development of secure and eﬃ-
cient privacy preserving identity mechanisms has become an important
ﬁeld of research. In addition, the popularity of decentralised decision
making networks such as Bitcoin has seen a huge interest in making use
of distributed ledger technology to store and securely disseminate end
user identity credentials. In this paper we describe a mechanism that
allows one to store the COVID-19 vaccination details of individuals on
a publicly readable, decentralised, immutable blockchain, and makes use
of a two-factor authentication system that employs biometric crypto-
graphic hashing techniques to generate a unique identiﬁer for each user.
Our main contribution is the employment of a provably secure input-
hiding, locality-sensitive hashing algorithm over an iris extraction tech-
nique, that can be used to authenticate users and anonymously locate
vaccination records on the blockchain, without leaking any personally
identiﬁable information to the blockchain.
1 Introduction
Immunization is one of modern medicine’s greatest success stories. It is one of the
most cost-eﬀective public health interventions to date, averting an estimated 2 to
3 million deaths every year. An additional 1.5 million deaths could be prevented
if global vaccination coverage improves [13]. The current COVID-19 pandemic
which has resulted in millions of infections worldwide [3] has brought into sharp
focus the urgent need for a “passport” like instrument, which can be used to
easily identify a user’s vaccination record, travel history etc., as they traverse the
globe. However, such instruments have the potential to discriminate or create
bias against citizens [10] if they are not designed with the aim of protecting
the user’s identity and/or any personal information stored about them on the
system.
Given the large number of potential users of such a system and the involve-
ment of many organizations in diﬀerent jurisdictions, we need to design a system
that is easy to sign up to for end users, and for it to be rolled out at a rapid rate.
The use of hardware devices such as smart cards or mobile phones for storing
such data is going to be ﬁnancially prohibitive for many users, especially those
This publication has emanated from research conducted with the ﬁnancial support
of Science Foundation Ireland under Grant Number 13/RC/2094 (Lero).
arXiv:2008.01120v7 [cs.CR] 18 Jan 2021
2 Sarang Chaudhari, Michael Clear, Philip Bradish, and Hitesh Tewari
in developing countries. Past experience has shown that such “hardware tokens”
are sometimes prone to design ﬂaws that only come to light once a large number
of them are in circulation. Such ﬂaws usually require remedial action in terms
of software or hardware updates, which can prove to be very disruptive.
An alternative to the above dilemma is an online passport mechanism. An
obvious choice for the implementation of such a system is a blockchain, that
provides a “decentralized immutable ledger” which can be conﬁgured in a man-
ner such that it can be only written to by authorised entities (i.e. there is no
requirement for a hard computation such as proof-of-work (PoW) to be carried
out for monetary reward), but can be queried by anyone. However, one of the
main concerns for such a system is based on: How does one preserve the privacy
of user’s data on a public blockchain while providing a robust mechanism to
link users to their data records securely? In other words, one of the key require-
ments is to avoid having any personally identiﬁable information (PII) belonging
to users stored on the blockchain.
In the subsequent sections we describe some of the key components of our
system and the motivation that led us to use them. The three major compo-
nents are - extraction of iris templates, a hashing mechanism to store them
securely and a blockchain technology. Finally, we present a formal description
of our framework which uses the aforementioned components as building blocks.
However, ﬁrst we will brieﬂy discuss some related work and then brieﬂy some
preliminaries with deﬁnitions and notation that are used in the paper.
1.1 Related Work
There has been considerable work on biometric cryptosystems and cancellable
biometrics, which aims to protect biometric data when stored for the purpose of
authentication cf. [11]. Biometric hashing is one such approach that can achieve
the desired property of irreversibility, albeit without salting it does not achieve
unlinkability. Research in biometric hashing for generating the same hash for
diﬀerent biometric templates from the same user is at an infant stage and existing
work does not provide strong security assurances. Locality-sensitive hashing is
the approach we explore in this paper, which has been applied to biometrics in
existing work; for example a recent paper by Dang et al. [4] applies a variant of
SimHash, a hash function we use in this paper, to face templates. However the
technique of applying locality-sensitive hashing to a biometric template has not
been employed, to the best of our knowledge, in a system such as ours.
2 Preliminaries
2.1 Notation
A quantity is said to be negligible with respect to some parameter λ, written
negl(λ), if it is asymptotically bounded from above by the reciprocal of all poly-
nomials in λ.
Framework for a DLT Based COVID-19 Passport 3
For a probability distribution D, we denote by x$Dthe fact that xis sampled according to D. We overload the notation for a set Si.e. y$Sdenotes
that yis sampled uniformly from S. Let D0and D1be distributions. We denote
by D0
CD1and the D0
SD1the facts that D0and D1are computationally
indistinguishable and statistically indistinguishable respectively.
We use the notation [k] for an integer kto denote the set {1, . . . , k}.
Vectors are written in lowercase boldface letters.
The abbreviation PPT stands for probabilistic polynomial time.
2.2 Entropy
The entropy of a random variable is the average “information” conveyed by the
variable’s possible outcomes. A formal deﬁnition is as follows.
Deﬁnition 1. The entropy H(X)of a discrete random variable Xwhich takes
on the values x1, . . . , xnwith respective probabilities PrhX=x1i,...,PrhX=
xniis deﬁned as
H(X) :=
n
X
i=1
PrhX=xiilog PrhX=xii
In this paper, the logarithm is taken to be base 2, and therefore we measure the
amount of entropy in bits.
3 Iris Template Extraction
Iris biometrics is considered one of the most reliable techniques for implementing
identiﬁcation systems. For the ID of our system (discussed in the overview of
our framework in Section 6), we needed an algorithm that can provide us with
consistent iris templates, which will have not only low intra-class variability,
but also show high inter-class variability. This requirement is essential because
we would expect the iris templates for the same subject to be similar, as this
would then be hashed using the technique described in the section 4. Iris based
biometric techniques have received some good attention in the last decade. One
of the most successful technique was put forward by John Daugman [5], but
most of the current best-in-class techniques are patented and hence unavailable
for an open-source use. For the purpose of writing this paper, we have used the
work of Libor Masek [7] which is an open-source implementation of a reasonably
reliable iris recognition technique. Users can always opt for other commercial
biometric solutions when trying to deploy our work independently.
Masek’s technique works on grey-scale eye images, which are processed in
order to extract the binary template. First the segmentation algorithm, based
on a Hough Transform is used to localise the iris and pupil regions and also isolate
the eyelid, eyelash and reﬂections as shown in Figures 1a and 1b. The segmented
4 Sarang Chaudhari, Michael Clear, Philip Bradish, and Hitesh Tewari
(a) (b)
(c)
(d)
Fig. 1: Masek’s Iris Template Extraction Algorithm
iris region is then normalised i.e, unwrapped into a rectangular block of constant
polar dimensions as shown in Figure 1c. The iris features are extracted from
the normalised image by one-dimensional Log-Gabor ﬁlters to produce a bit-
wise iris template and mask as shown in Figure 1d. We denote the complete
algorithm by Iris.ExtractFeatureVector which takes a scanned image as input and
outputs a binary feature vector fv ∈ {0,1}n(this algorithm is called upon by
our framework in Section 6). This data is then used for matching, where the
Hamming distance is used as the matching metric. We have used CASIA-Iris-
Interval database [2] in Figure 1 and for some preliminary testing.
Table 1 shows the performance of Masek’s technique as reported by him
in his original thesis [8]. This algorithm performs quite well for a threshold of
0.4 where the false acceptance rate (FAR) is 0.005 and the false rejection rate
(FRR) is 0. These values are used when we present our results to compare the
performance of our technique when a biometric template is ﬁrst hashed and then
the hamming distance is measured to calculate the FAR and FRR, as opposed to
directly measuring the hamming distances in the original biometric templates.
One would assume the performance of our work to get better with the increase
in eﬃciency of extracting consistent biometric templates by other methods.
As aforementioned, we rely on the open-source MATLAB code by Libor
Masek. For each input image, the algorithm produces a binary template which
Framework for a DLT Based COVID-19 Passport 5
Threshold FAR FRR
0.20 0.000 74.046
0.25 0.000 45.802
0.30 0.000 25.191
0.35 0.000 4.580
0.40 0.005 0.000
0.45 7.599 0.000
0.50 99.499 0.000
Table 1: FAR and FRR for the CASIA-a Data Set
contains the iris information, and a corresponding noise mask which corresponds
to corrupt areas within the iris pattern, and marks bits in the template as cor-
rupt. These extracted iris templates and their corresponding masks are 20 ×480
binary matrices each. In the original work, only those bits in the iris pattern
that correspond to ‘0’ bits in the noise masks of both iris patterns were used
in the calculation of Hamming distance. A combined mask is then calculated
and both the templates are masked with it. Finally, the algorithm calculates the
bitwise XOR i.e. distance between the masked templates. The steps given below
provides an overview for the algorithm used by Libor:
Extraction:
Matching:
There are two major issues that we need to deal with before being able to
use these templates in our system, speciﬁcally, template masking and conversion
to linear vector.
The above matching technique requires one to have 2 pairs of iris patterns and
their corresponding masks to calculate the Hamming distance. However for the
application we are targeting, at any time during veriﬁcation, the system would
have to match the extracted template of an individual (i.e. template and mask)
against a hashed template stored on the blockchain. This means that we cannot
6 Sarang Chaudhari, Michael Clear, Philip Bradish, and Hitesh Tewari
incorporate the above matching algorithm into our system. We have two choices
to mitigate this problem and obtain the masked template for the remaining
steps:
1. We can calculate the masked template independently for each sample i.e.
An underlying assumption for this method is that the masks for an individual
would be approximately the same in every sample. This assumption is not
too far-fetched as was clear from the preliminary analysis of our database.
We will refer to these as type1templates.
2. We can maintain a global mask which can be the deﬁned as
for all extracted maskiof all imageibelonging to the training database. And
at the time of veriﬁcation, we generate the masked template as
This method has some added diﬃculty in ﬁnding the global mask and it also
discards more data from the iris patterns as opposed to directly using the re-
spective maskifor each templatei. But it helps in maintaining a consistency
among the masked templates. We will refer to these as type2templates.
In a follow-up to this paper, we will use and provide results for templates of
both types based on experiments we are conducting at the time of writing.
3.2 Conversion to Linear Vector
For our use case, we need a one-dimensional input stream which can be fed into
the hashing algorithm discussed in the subsequent sections. For converting those
masked template matrices into linear feature vectors, we have two naive choices
of concatenating either the row vectors or the column vectors. Before deciding
the type of conversion, let us look at an important key factor, which is rotational
inconsistencies in the iris templates.
Rotational inconsistencies are introduced due to rotations of the camera, head
tilts and rotations of the eye within the eye socket. The normalisation process
does not compensate these. In order to account for rotational inconsistencies,
when the Hamming distance of two templates is calculated, one template is
shifted left and right bit-wise, and a number of Hamming distance values are
calculated from successive shifts. This bit-wise shifting in the horizontal direction
corresponds to the rotation of the original iris region. This method was suggested
by Daugman [5], and corrects misalignment in the normalised iris pattern caused
by rotational diﬀerences during imaging. From the calculated Hamming distance
values, only the lowest is taken, since this corresponds to the best match between
two templates. Due to this, column-wise conversion seems like the most logical
choice as this would allow us to easily rotate the binary linear feature vectors.
Shifting the linear vector by 20 bits will correspond to shifting the iris template
once (recall that the dimension of iris templates is 20 ×480).
Framework for a DLT Based COVID-19 Passport 7
3.3 Wrap-up
Putting it all together, we deﬁne the steps of our algorithm Iris.ExtractFeatureVector
that we call upon later.
– Iris.ExtractFeatureVector(image):
(template, mask) = createiristemplate(image) where createiristemplate
is Masek’s open source algorithm.
Obtain masked template (either type1or type2as deﬁned in Section 3.1).
Convert masked template to linear vector as in Section 3.2.
Output binary linear vector fv ∈ {0,1}n
Note that the parameter nis a global system parameter measuring the length
of the binary feature vectors outputted by Iris.ExtractFeatureVector.
4 Locality-sensitive Hashing
To preserve the privacy of individuals on the blockchain, the biometric data has
to be encrypted before being written to the ledger. Hashing is a good alter-
native to achieve this, but techniques such as SHA-256 and SHA-3 cannot be
used, since the biometric templates that we extracted above can show diﬀerences
across various scans for the same individual. Hence using those hash functions
would produce completely diﬀerent hashes. Therefore, we seek a hash function
that generates “similar” hashes for similar biometric templates. This prompts
us to explore Locality-Sensitive Hashing (LSH), which has exactly this property.
Various LSH techniques have been researched to identify whether ﬁles (i.e. byte
streams) are similar based on their hashes. TLSH is a well-known LSH function
that exhibits high performance and matching accuracy but, does not provide a
suﬃcient degree of security for our application. Below we assess another type of
LSH function, which does not have the same runtime performance as TLSH, but
as we shall see, exhibits provable security for our application and therefore is a
good choice for adoption in our framework.
4.1 Input Hiding
In the cryptographic deﬁnition of one-way functions, it is required that it is hard
to ﬁnd any preimage of the function. However, we can relax our requirements for
many applications because it does not matter if for example a random preimage
can be computed as long as it is hard to learn information about the speciﬁc
preimage that was used to compute the hash. In this section, we introduce a
property that captures this idea, a notion we call input hiding.
Input hiding means that if we choose some preimage xand give the hash
h=H(x) to an adversary, it is either computationally hard or information-
theoretically impossible for the adversary to learn xor any partial information
about x. This is captured in the following formal deﬁnition.
8 Sarang Chaudhari, Michael Clear, Philip Bradish, and Hitesh Tewari
Deﬁnition 2. A hash function family Hwith domain X:= {0,1}nand range
Y:= {0,1}mis said to be input hiding if for all randomly chosen hash functions
H$H, all i[n], all randomly chosen inputs x$Xand all PPT adversaries
Ait holds that
Prhxi= 0 ∧ A(i, H(x)) 1i
Prhxi= 1 ∧ A(i, H(x)) 1i
negl(λ)
where λis the security parameter.
4.2 Our Variant of SimHash
Random projection hashing, proposed by Charikar [1], preserves the cosine dis-
tance between two vectors in the output of the hash, such that two hashes are
probabilistically similar depending on the cosine distance between the two preim-
age vectors. This hash function is called SimHash. We describe a slight variant
of SimHash here, which we call S3Hash. In our variant, the random vectors that
are used are sampled from the ﬁnite ﬁeld of F3={−1,0,1}. Suppose we choose
a hash length of mbits. Now for our purposes, the input vectors to the hash
are binary vectors in {0,1}nfor some n. First we choose mrandom vectors
ri${−1,0,1}nfor i∈ {1, . . . , m}. Let R={ri}i∈{1,...,m}be the set of these random vectors. The hash function S3HashR:{0,1}n→ {0,1}mis thus deﬁned as: S3HashR(x) = (sgn(hx,r1i),...,sgn(hx,rmi)) (1) where sgn :Z→ {0,1}returns 0 if its integer argument is negative and returns 1 otherwise. Note that the notation ,·i denotes the inner product between the two speciﬁed vectors. Let x1,x2∈ {0,1}nbe two input vectors. It holds for all i∈ {1, . . . , m}that Pr[h(1) i=h(2) i] = 1 θ(x1,x2) πwhere h(1) =S3HashR(x1), h(2) =S3HashR(x2) and θ(x1,x2) is the angle between x1and x2. Therefore the similarity of the inputs is preserved in the similarity of the hashes. An important question is: Is this hash function suitable for our application? The answer is in the aﬃrmative because it can be proved that the function information-theoretically obeys a property we call input-hiding that we deﬁned in Section 4.1. We recall that this property means that if we choose some binary vector x∈ {0,1}nand give the hash h=S3HashR(x) to an adversary, it is either computationally hard or information-theoretically impossible for the adversary to learn xor any partial information about x. This property is suﬃcient in our application since we only have to ensure that no information is leaked about the user’s iris template. We now prove that our variant locality-sensitive hash function S3Hash is information-theoretically input hiding. Theorem 1. Let Xdenote the random variable corresponding to the domain of the hash function. If H(X)m+λthen S3Hash is information-theoretically input hiding where λis the security parameter and H(X)is the entropy of X. Framework for a DLT Based COVID-19 Passport 9 Proof. The random vectors in Rcan be thought of as vectors of coeﬃcients corresponding to a set of mlinear equation in nunknowns on the left hand side and on the right hand side we have the melements, one for each equation, which are components of the hash i.e. (h1, . . . , hm). Now the inner product is evaluated over the integers and the sgn function maps an integer to an element of {0,1} depending on its sign. The random vectors are chosen to be ternary. Suppose we choose a ﬁnite ﬁeld Fpwhere p2(m+ 1) is a prime. Since there will be no overﬂow when evaluating the inner product in this ﬁeld, a solution in this ﬁeld is also a solution over the integers. We are interested only in the binary solutions. Because m < n, the system is underdetermined. Since there are nmdegrees of freedom in a solution, it follows that there are 2nmbinary solutions and each one is equally likely. Now let rdenote the redundancy of the input space i.e. r=nH(X). The fraction of the 2nmsolutions that are valid inputs is 2nmr. If 2nmr>2λ, then the probability of an adversary choosing the “correct” preimage is negligible in the security parameter λ. For this condition to hold, it is required that nmr > λ (recall that r=nH(X)), which follows if H(X)m+λas hypothesized in the statement of the theorem. It follows that information-theoretically an unbounded adversary has a negligible advantage in the input hiding deﬁnition. Our initial estimates suggest that the entropy of the distribution of binary feature vectors outputted by Iris.ExtractFeatureVector is greater than m+λfor parameter choices such as m= 256 and λ= 128. A more thorough analysis however is deferred to future work. 4.3 Evaluation We have ran experiments with S3Hash applied to feature vectors obtained using our Iris.ExtractFeatureVector algorithm. The distance measure we use is the ham- ming distance. The results of these experiments are shown in Table 2. Results for a threshold of 0.3 in particular indicates that our approach shows promise. We hope to make further improvements in future work. 5 Blockchain A blockchain is used in the system for immutable storage of individuals’ vaccina- tion records. The blockchain we employ is a permissioned ledger to which blocks can only be added by authorized entities or persons such as hospitals, primary health care centers, clinicians etc. Such entities have to obtain a public-key cer- tiﬁcate from a trusted third party and store it on the blockchain as a transaction before they are allowed to add blocks to the ledger. The opportunity to add a new block is controlled in a round robin fashion, thereby eliminating the need to perform a computationally intensive PoW process. Any transactions that are broadcast to the P2P network are signed by the entity that created the transac- tion, and can be veriﬁed by all other nodes by downloading the public key of the 10 Sarang Chaudhari, Michael Clear, Philip Bradish, and Hitesh Tewari Threshold FAR FRR 0.25 3.99 64.26 0.26 6.35 57.35 0.27 9.49 49.63 0.28 13.92 43.79 0.29 19.60 36.47 0.3 26.23 30.79 0.31 34.01 25.10 0.32 42.30 19.33 0.33 50.91 16.00 0.34 59.04 11.94 0.35 67.05 8.53 Table 2: FAR & FRR for the CASIA-Iris-Interval Data Set signer from the ledger itself. An example of distributed ledger technology that fulﬁlls the above requirements is MultiChain [9]. 5.1 Interface We now describe an abstract interface for the permissioned blockchain that cap- tures the functionality we need. Consider a set of parties ˆ P. A subset of parties Pˆ Pare authorized to write to the blockchain. Each party P Phas a se- cret key skPwhich it uses to authenticate itself and gain permission to write to the blockchain. How a party acquires authorization is beyond the scope of this paper. For our purposes, the permissioned blockchain consists of the following algorithms: Blockchain.Broadcast(P,skP,tx): On input a party identiﬁer Pthat identiﬁes the sending party, a secret key skPfor party Pand a transaction tx (whose form is described below), then broadcast the transaction tx to the peer-to- peer network for inclusion in the next block. The transaction will be included iﬀ P P. Blockchain.AnonBroadcast(skP,tx) : On input a secret key skPfor a party P and a transaction tx, then anonymously broadcast the transaction tx to the peer-to-peer network for inclusion in the next block. The transaction will be included iﬀ P P. Blockchain.GetNumBlocks(): Return the total number of blocks currently in the blockchain. Blockchain.RetrieveBlock(blockNo): Retrieve and return the block at index blockNo, which is a non-negative integer between 0 and Blockchain.GetNumBlocks() 1. A transaction has the form (type,payload,party,signature). A transaction in an anonymous broadcast is of the form (type,payload,,). The payload of a trans- action is interpreted and parsed depending on its type. In our application, there Framework for a DLT Based COVID-19 Passport 11 are two permissible types: ’rec’ (a record transaction which consists of a pair (ID, record)) and ’hscan’ (biometric hash transaction which consists of a hash of an iris feature vector). This will become clear from context in our formal descrip- tion of our framework in the next section which makes use of the above interface as a building block. The ﬁnal point is that a block is a pair (hash,transactions) consisting of the hash of the block and a set of transactions {txi}i[`]. 6 Our Framework 6.1 Overview In this section we provide a formal description of our proposed framework which makes use of the building blocks presented in the previous sections. Our proposed system utilises a two-factor authentication mechanism to uniquely identify an individual on the blockchain. The parameters required to recreate an identiﬁer are based on information that “one knows” and biometric information that “one possess”. Fig. 2: Algorithm Workﬂow Figure 2 describes the overall algorithm that we employ in our proposed system. When a user presents themselves to an entity or organisation partic- 12 Sarang Chaudhari, Michael Clear, Philip Bradish, and Hitesh Tewari ipating in the system, they are asked for their DoB(dd/mm/yyyy) and Gen- der(male/female/other). In addition, the organization captures a number of scans of the user’s iris, and creates a hash H1(fv) from the feature vector ex- tracted from the “best” biometric scan data. Our system can combine the user’s DoB and Gender with H1(fv) to generate a unique 256-bit identiﬁer (ID) for the user: ID =H2(DoB || Gender || H1(fv)) (2) The algorithm tries to match the calculated hash H1(fv) with existing “anony- mous” hashes that are stored on the blockchain. It may get back a set of hashes that are somewhat “close” to the calculated hash. In that case the algorithm concatenates each returned hash (Matchi) with the user’s DoB and Gender to produce f ID. It then tries to match f ID with an ID in a vaccination record transaction on the blockchain. If a match is found then the user is already registered on the system and has at least one vaccination record. At this point we may just wish to retrieve the user’s records or add an additional record, e.g. when a booster dose has been administered to the user. However if we go through the set of returned matches and cannot match f ID to an existing ID in a vaccination record on the blockchain, i.e. this is the ﬁrst time the user is presenting to the service, then we store the iris scan hash data H1(fv) as an anonymous record on the blockchain, and subsequently the ID and COVID-19 vaccination details for the user as a separate transaction. In each case the transaction is broadcast at a random interval on the blockchain peer-to-peer (P2P) network for it to be veriﬁed by other nodes in the system, and eventually added to a block on the blockchain. Uploading the two transactions belonging to a user at random intervals ensures that the transactions are stored on separate blocks on the blockchain, and an attacker is not easily able to identify the relationship between the two. Figure 3 shows a blockchain in which there are three anonymous transactions (i.e. Hash of Scan Data) and three COVID-19 vaccination record transactions stored on the blockchain pertaining to diﬀerent users. The reader is referred to Section 4 for more details on how the hash is calculated in our system. Note that the storage of the anonymous hash data has to be carried out only once per registered user in the system. 6.2 Formal Description We present a formal description of our framework in Figure 4 and Figure 5. Note that the algorithms described in these ﬁgures are intended to formally describe the fundamental desired functionality of our framework and are so described for ease of exposition and clarity; in particular, they are naive and non-optimized, speciﬁcally not leveraging more eﬃcient data structures as would a real-world implementation. Let Hbe a family of collision-resistant hash functions. The algorithms in Figure 4 are stateful (local variables that contain retrieved information from Framework for a DLT Based COVID-19 Passport 13 Fig. 3: Blockchain Structure the blockchain are shared and accessible to all algorithms). Furthermore, the parameters tuple params generated in Setup is an implicit argument to all other algorithms. The algorithms invoked by the algorithms in Figure 4 can be found in Fig- ure 5. 7 Conclusions and Future Work In this paper we have detailed a framework to build a global vaccination passport using a distributed ledger. The main contribution of our work is to combine a Locality-sensitive hashing mechanism with a blockchain to store the vaccination records of users. A variant of the SimHash LSH function is used to derive an identiﬁer that leaks no personal information about an individual. The only way to extract a user’s record from the blockchain is by the user presenting themselves in person to an authorised entity, and providing an iris scan along with other personal data in order to derive the correct user identiﬁer. However, our research has raised many additional challenges and research questions whose resolution require further investigation and experimentation, intended for future work. First and foremost, we need to improve the accuracy of the Iris.ExtractFeatureVector algorithm (e.g: deciding whether to use type1or type2template masking) and to accurately compute the entropy of the feature vectors. Furthermore, our variant of the SimHash algorithm, referred to in this paper as S3Hash, requires further analysis and evaluation, especially with respect to the domain of the random vectors ri, which are restricted to be ternary in this paper. Additionally, we must choose a suitable blockchain. Finally, the overall protocol would beneﬁt from a thorough security analysis where not just privacy but other security properties are tested. The blockchain based mechanism that we have proposed can also be used as ageneralised healthcare management system [6, 12] with the actual data being stored oﬀ-chain for the purpose of eﬃciency. Once a user’s identiﬁer has been recreated it can be used to pull all records associated with the user, thereby re- 14 Sarang Chaudhari, Michael Clear, Philip Bradish, and Hitesh Tewari Algorithm Setup(1λ) ri${−1,0,1}nfor i∈ {1,...,m}for i[m].
R← {r1,...,rm}
H1$H H2S3HashR numBlocks 0 ids ← ∅ records ← ∅ hscans ← ∅ Sync() Return params := (H1, H2) Algorithm AddRecord(P,skP,dob,gender,scan,record) ID Authenticate(dob,gender,scan) If ID =: ID Enroll(P,skP,dob,gender,scan,record) Return payload (ID,record) σSign(skP,payload) tx (’rec’,payload,P, σ) Blockchain.Broadcast(P,skP,tx) records records ∪ {(ID,record)} Algorithm FetchRecords(dob,gender,scan) ID Authenticate(dob,gender,scan) If ID =: Return results ← {record : (ID,record)records} Return results Fig. 4: Our Framework for a COVID-19 Passport trieving their full medical history. We are in the middle of developing a prototype implementation of the system and hope to present the results of our evaluation in a follow-on paper. At some time in the future, we hope to trial the system in the ﬁeld, with the hope of rolling it out on a larger scale. Our implementation will be made open source. References 1. S. Charikar. Similarity estimation techniques from rounding algorithms. In Pro- ceedings of the 34th Annual ACM Symposium on Theory of Computing, pages 380–388, 2002. 2. Chinese Academy Sciences’ Institute of Automation (CASIA) - Iris Image Database. http://biometrics.idealtest.org/. Framework for a DLT Based COVID-19 Passport 15 3. COVID-19 Dashboard by the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University (JHU). https://coronavirus.jhu.edu/map.html. 4. T. M. Dang, L. Tran, T. D. Nguyen, and D. Choi. Fehash: Full entropy hash for face template protection. In Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition (CVPR) Workshops, June 2020. 5. J. Daugman. Statistical richness of visual phase information: Update on recognizing persons by iris patterns. International Journal of Computer Vision, 45:25–38, 2001. 6. M. Hanley and H. Tewari. Managing lifetime healthcare data on the blockchain. In 2018 IEEE SmartWorld, Ubiquitous Intelligence & Computing, Advanced & Trusted Computing, Scalable Computing & Communications, Cloud & Big Data Computing, Internet of People and Smart City Innovation, pages 246–251, Guangzhou, 2018. 7. L. Masek. Recognition of human iris patterns for biometric identiﬁcation. Final Year Project, The School of Computer Science and Software Engineering, The University of Western Australia, 2003. 8. L. Masek and P.Kovesi. Matlab source code for a biometric identiﬁcation system based on iris patterns. The School of Computer Science and Software Engineering, The University of Western Australia, 2003. 9. Multichain. https://www.multichain.com/. 10. A. L. Phelan. Covid-19 immunity passports and vaccination certiﬁcates: scientiﬁc, equitable, and legal challenges. The Lancet, 395(10237):1595 – 1598, 2020. 11. C. Rathgeb and A. Uhl. A survey on biometric cryptosystems and cancelable biometrics. EURASIP J. Information Security, 2011:3, 2011. 12. H. Tewari. Blockchain research beyond cryptocurrencies. IEEE Communications Standards Magazine, 3(4):21–25, Dec. 2019. 13. World Health Organization. https://www.who.int/news-room/ facts-in-pictures/detail/immunization. 16 Sarang Chaudhari, Michael Clear, Philip Bradish, and Hitesh Tewari Algorithm Authenticate(dob,gender,scan) Sync() fv Iris.ExtractFeatureVector(scan) hscan H2(fv) ID H1(dob kgender khscan) If ID ids: Return ID For each hhscans: dDist(hscan, h) If d < THRESHOLD: f ID H1(dob kgender kh) If f ID ids: Return f ID Return Algorithm Enroll(P,skP,dob,gender,scan,initRecord) Sync() fv Iris.ExtractFeatureVector(scan) hscan H2(fv) ID H1(dob kgender khscan) payload (ID,initRecord) σSign(skP,payload) tx (’rec’,payload,P, σ) Blockchain.Broadcast(P,skP,tx) t${1,...,100}
tx0(’hscan’,hscan)
after time t
ids ids ∪ {ID}
hscans hscans ∪ {hscan}
records records ∪ {(ID,initRecord)}
Return ID
Algorithm Sync()
newNumBlocks Blockchain.GetNumBlocks()
If newNumBlocks >numBlocks:
For numBlocks i < newNumBlocks:
block Blockchain.RetrieveBlock(i)
(hash,transactions)block
For each tx transactions:
If type = ’rec’:
ids ids ∪ {ID}
records records ∪ {(ID,record)}
Else if type = ’hscan’:
hscans hscans ∪ {hscan}
numBlocks newNumBlocks
Fig. 5: Additional Algorithms Used By Our Framework

## Supplementary resources (2)

... Others investigated the potential use of different technologies and approaches such as IoT (Ennafiri & Mazri, 2020); big data (Lv, Ma, Li, & Wu, 2021); Blockchain, AI, and IoMT ; collaborative city digital twins (Pang, Huang, Xie, Li, & Cai, 2021); and smart city networks and infrastructure (Allam & Jones, 2020). There are also some studies discussing methods for creating vaccine passports (Chaudhari, Clear, & Tewari, 2020) and improved vaccine delivery supply chain (Weintraub et al., 2020). ...
... In Alanazi (2020) the author discusses a wide range of applications for blockchain for smart health and Healthcare 4.0 and in the authors discuss the value of a general adoption to address the issues of trust and privacy in Healthcare 4.0 applications. Specific examples using blockchain for Healthcare 4.0 include: UAV path planning during the pandemic (Aggarwal, Kumar, Alhussein, & Muhammad, 2021); creating trusted medical records (Bodkhe, Tanwar, Bhattacharya, & Verma, 2021); securing remote patient monitoring Aguiar et al., 2020;Alafif et al., 2021;Alanazi, 2020;Allam & Jones, 2020;Bharadwaj et al., 2021;Bishop & Leigh, 2020;Bodkhe et al., 2021;Chakravarty et al., 2021;Chau et al., 2020;Chaudhari et al., 2020;Chettri et al., 2020;Defendi et al., 2021;Ennafiri & Mazri, 2020;Hathaliya, Sharma, Tanwar, & Gupta, 2019;He et al., 2021;Ho, 2021;Jamshidi et al., 2021;Johansson et al., 2018;Kiszewski et al., 2021;Kwok, 2021;Lv et al., 2021;Mbunge, 2020;Mbunge et al., 2021;Nasajpour et al., 2020;Pang et al., 2021;Pavli & Maltezou, 2021;Romette et al., 2018;Sosa et al., 2021;Strizova et al., 2021;Tarfaoui et al., 2020;Udugama et al., 2020;Weintraub et al., 2020;Zhang et al., 2020;Zimmerling & Chen, 2021); creating a telesurgery framework ; and protecting electronic health records . ...
... Examples include the lack of and/or difficulty of finding adequate data sources and clean data; the abundance of irrelevant, incorrect, and fragmented data; difficulties sharing data sources across organizations and/or countries; non-standardized formats, organization and sharing protocols; unclear regulations on data collection, storage, sharing and use; the need to achieve secure and privacy-supporting sharing and use of data; ownership and control of user data; and limited trust in the available data and its uses. Other issues appearing consistently include the privacy issues as in Bharadwaj et al. (2021), Chaudhari et al. (2020), Sosa et al. (2021), Weintraub et al. (2020; Collaboration capabilities and tools (Bharadwaj et al., 2021;He et al., 2021;Pang et al., 2021); experience in medical research coupled with technology aspects (Chakravarty et al., 2021;Lv et al., 2021); ethics, equity, and human factor issues were discussed in Chaudhari et al. (2020), He et al. (2021), Lv et al. (2021), Pavli and Maltezou (2021), Sosa et al. (2021); and high performance computing and communication capabilities were also highlighted in Alafif et al. (2021), Bharadwaj et al. (2021), Chakravarty et al. (2021), Defendi et al. (2021), Jamshidi et al. (2021), Nasajpour et al. (2020. ...
Chapter
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Seldom do we think that we will witness something such as COVID-19 in our lifetime. We have witnessed SARS before, and we have grown reading the history of Bubonic plague, Cholera, Chicken Pox, and Typhoid. We have also felt proud learning the victory of our forefathers over these diseases. However, when the First COVID patient was found in late 2019 in the Wuhan province of China, even in our worst nightmare, we did not think it would turn into the most devastating pandemic in human history so far. Being one of the closest neighbors of China, the first COVID patient in India was identified on January 27, 2020. Just 3days later, on January 30, 2020, the World Health Organization (WHO) declared the 2019–20 Novel Corona Virus Diseases (COVID-19) epidemic a Public Health Emergency of International Concern (PHEIC).
... In this system, the data owner grants access to other entities so that the user has control over his data. Based on blockchain, a framework was proposed in [21] to ensure users' privacy, which uses a locality-sensitive hash function to generate a secure identifier. The identifier can only be derived if the user provides his biometric and personal information, whereas, although the authors give details of the pseudoidentity generation, the description of the vaccination certificate is very brief. ...
... None of the above researches [20][21][22] addressed how the passport holder can verify the legality of inspectors, which is extremely important for users. Some existing authentication schemes are designed for scenarios such as the smart grid, the Internet of Things, and the smart medical [23][24][25][26]. ...
... In this section, we make a functional property comparison between the proposed scheme and the existing immunity passport schemes [20][21][22]. Then, the proposed scheme is compared with the existing authentication schemes [23,24] in terms of computational overheads, communicational overheads, and energy overheads. ...
Article
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The implementation of immunity passport has been hampered by the controversies over vaccines in various countries, the privacy of vaccinators, and the forgery of passports. While some existing schemes have been devoted to accelerating this effort, the problems above are not well solved in existing schemes. In this paper, we present an immunity passport scheme based on the dual-blockchain architecture, which frees people from the cumbersome epidemic prevention process while traveling abroad. Specially, the dual-blockchain architecture is established to fit with the scenarios of immunity passport. Searchable encryption and anonymous authentication are utilized to ensure users’ privacy. In addition, the performance and security evaluations show that our scheme achieves the proposed security goals and surpasses other authentication schemes in communicational and computational overheads.
... There are a number of proposals in the literature propose the use of blockchain for vaccine certification operations by utilizing smart contracts and/or other techniques such as hashing [15], [16], Verifiable Credentials (VC) [17] and iris scan [18]. Some other proposals in the literature [19]- [21] propose the use of blockchain to deal with 'immunity certificates' stored in shared blockchains with confidentiality ensured. ...
... if h(photo_stream) = (h certif icate (photo_stream)) then 6 if (V ax_inf o) pkI is verified Retrieve bc (tx certif icate −→ h(user_id), h(photo_stream), signed(vax_inf o), P m); 12 if (QR_code(P m) = tx(P m)) then 13 certif icate −→ correct 14 else 15 certif icate −→ incorrect16 return false 17 Retrieve bc (tx factory −→ (factory, lot#, BF factory , #doses, issuer_id)) 18 Retrieve bc (tx issuer −→ (issuer_id, lot#, certif icateList(Address certif ))) 19 if (lot# in BF factory ) then 20 lot# → legit 21 else 22 lot# −→ corrupt 23 return false 24 if (Address certif in certif icateList[lot#])&(#doses = size(certif icateList)) P m against the one on the bc and verify certificate information using issuer public key,(13)(14)(15)(16)(17)(18)(19). ...
Conference Paper
This paper proposes a global blockchain-based vaccination certification. To facilitate issuing and verifying certificates globally, the framework is lightweight on the user's side. The essential certification operations, issuing certificate, verifying certificate, and verifying vaccine, are implemented preserving user's privacy. Our framework's smart contracts design is implemented on the Ethereum blockchain test network to support indexing and querying the certification data. The experimental results show the effectiveness of the proposed approach in improving the performance of the primary functionalities. The results show the retrieving time of certificate information is efficient no matter how long the chain is, compared with scanning the blocks to find the target data.
... Our system takes a scanned image of an iris as the input and outputs a binary feature vector (fv). Masek's technique works on grey-scale eye images, which are processed in order to extract the binary template [11]. Figure 2 shows the overall process of the iris template extraction technique. ...
... This will ensure that information about the users' irises cannot be leaked from the stored hashes. In [11], the authors show that the S3Hash (a variant of Charikar's Simhash) algorithm is input hiding, provided the entropy of the input vector is sufficiently larger than the size of the hash. ...
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Vaccination passports are being issued by governments around the world in order to open up their travel and hospitality sectors. Civil liberty campaigners on the other hand argue that such mandatory instruments encroach upon our fundamental right to anonymity, freedom of movement, and are a backdoor to issuing “identity documents” to citizens by their governments. In this paper we present a privacy-preserving framework that uses two-factor authentication to create a unique identifier that can be used to locate a person’s vaccination record on a blockchain, but does not store any personal information about them. Our main contribution is the employment of a locality sensitive hashing algorithm over an iris extraction technique, that can be used to authenticate users and anonymously locate vaccination records on the blockchain, without leaking any personally identifiable information to the blockchain. Our proposed system allows for the safe reopening of society, while maintaining the privacy of citizens.
... Typically, this is done through a medical certificate proving that the holder is immune or is not currently infected. Based on the related literature, three basic types of digital health certificates can be identified: (a) vaccination certificates, referring to whether a person has received the vaccine or not [11]- [15]; (b) diagnostic test certificates, demonstrating that a person has undergone a test [14], [16], [17]; and (c) immunity certificates or immunity passports, attesting that a person was infected in the past and has developed antibodies [12]- [14], [17]- [19]. The underlying technologies for preserving the security and privacy of digital health certificates are blockchain and traditional public key cryptography. ...
... In [11], the focus is on privacy, with the authors proposing a hashing algorithm that enables users to store information on the blockchain anonymously using an ID that is created from their iris. In this case, the vaccination certificate data and the hash of the user ID are stored in the blockchain. ...
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Digital COVID-19 certificates serve as reliable proof that an individual was vaccinated, tested negative, or healed from COVID-19, facilitating health, occupational, educational, and travel activities during the pandemic. This paper contributes the first to our knowledge state-of-the-art and holistic review of this ecosystem, attempting to answer the following questions: (i) is there a harmonization among academia, organizations, and governments in terms of the certificate deployment technology?, (ii) what is the proliferation of such schemes worldwide and how similar are they?, and (iii) are smartphone applications that accompany such schemes privacy-preserving from an end-user’s perspective? To respond to these questions, a four-tier approach is followed: (a) we scrutinize the so far academic works suggesting some type of digital certificate, highlighting common characteristics and weaknesses; (b) we constructively report on the different initiatives proposed by organizations or alliances; (c) we briefly review 54 country initiatives around the globe; and (d) we analyze both statically and dynamically all official Android smartphone applications offered for such certificates to reveal possible hiccups affecting the security or privacy of the end-user. From a bird’s eye view, the great majority of the proposed or developed schemes follow either the blockchain model or the asymmetric cryptosystem, the spread of schemes especially in Europe and partly in Asia is high, some degree of distinctiveness among the relevant schemes developed by countries does exist, and there are substantial variations regarding the privacy level of the applications between Europe on the one hand and Asia and America on the other.
... On the other hand, the human factor plays a strong role in making, or breaking, the Healthcare 4.0 vision. Ethics, equity, and human factor issues were discussed in [56][63] [64][65] [69]. Politics, governments, private/public sector involvement, intellectual property protection, and public trust are also important. ...
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COVID‐19 pandemic undoubtedly lingers on and has brought unprecedented changes globally including travel arrangements. Blockchain‐based solutions have been proposed to aid travel amid the pandemic hap. Presently, extant solutions are country or regional‐based, downplay privacy, non‐responsive, often impractical, and come with blockchain‐related complexities presenting technological hurdle for travelers. We therefore propose a solution namely, Borderless to foster global travel allowing travelers and countries collaboratively engage in a secure adaptive proof protocol dubbed Proof‐of‐COVID‐19 status a number of arbitrary statements to ascertain the fact that the traveler poses no danger irrespective of the country located. As far as we know, this is first of its kind. Borderless is implemented as a decentralized application leveraging blockchain as a trust anchor and decentralized storage technology. Security analysis and evaluation are performed proving security, privacy‐preservation, and cost‐effectiveness along with implementation envisioning it as a blueprint to facilitate cross‐border travel during the present and future pandemics. Our experimental results show it takes less than 60 and 3 s to onboard users and perform proof verification respectively attesting to real usability scenarios along with the traits of arbitrary proofs to aid responsiveness to the dynamics of pandemics and blockchain abstraction from travelers.
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Recognition of human iris patterns for biometric identification. Final Year Project, The School of Computer Science and Software Engineering
• L Masek
L. Masek. Recognition of human iris patterns for biometric identification. Final Year Project, The School of Computer Science and Software Engineering, The University of Western Australia, 2003.
Matlab source code for a biometric identification system based on iris patterns. The School of
• L Masek
• P Kovesi
L. Masek and P.Kovesi. Matlab source code for a biometric identification system based on iris patterns. The School of Computer Science and Software Engineering, The University of Western Australia, 2003.