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76-595: Covid Watch's TCN Protocol received significant news coverage and was followed by similar decentralized protocols in early April 2020 like DP-3T , PACT, and Google/Apple Exposure Notification framework . Covid Watch then helped other groups like the TCN Coalition and MIT SafePaths implement the TCN Protocol within their open source projects to further the development of decentralized technology and foster global interoperability of contact tracing and exposure alerting apps,

152-487: A k ) {\displaystyle tck_{0}=H\_tck(rak)} , where H _ t c k ( ) {\displaystyle H\_tck()} is the SHA-256 hash function as H _ t c k ( s ) = S H A 256 ( b'H _ TCK' | | s ) {\displaystyle H\_tck(s)=SHA256({\text{b'H}}\_{\text{TCK'}}||s)} . This TCK

228-473: A collision , requires on average only 2 evaluations using a birthday attack . Some of the applications that use cryptographic hashes, such as password storage, are only minimally affected by a collision attack . Constructing a password that works for a given account requires a preimage attack, as well as access to the hash of the original password (typically in the shadow file) which may or may not be trivial. Reversing password encryption (e.g., to obtain

304-521: A little endian unsigned 2 byte integer, and H _ t c n ( ) {\displaystyle H\_tcn()} is the SHA-256 hash function as H _ t c n ( s ) = S H A 256 ( b'H _ TCN' | | s ) [ 0 : 128 ] {\displaystyle H\_tcn(s)=SHA256({\text{b'H}}\_{\text{TCN'}}||s)[0:128]} . The following diagram demonstrates

380-443: A TCN contained in the report. If a matching TCN is found, then the user has come in close contact with an infected patient, and is warned by the client. Since each device locally verifies contact logs, and thus contact logs are never transmitted to third parties, the central reporting server cannot by itself ascertain the identity or contact log of any client in the network. This is in contrast to competing protocols like BlueTrace, where

456-446: A U.S. federal standard. The SHA-2 family of algorithms are patented in the U.S. The United States has released the patent under a royalty-free license. As of 2011, the best public attacks break preimage resistance for 52 out of 64 rounds of SHA-256 or 57 out of 80 rounds of SHA-512, and collision resistance for 46 out of 64 rounds of SHA-256. With the publication of FIPS PUB 180-2, NIST added three additional hash functions in

532-418: A bill of rights for users of such apps. Currently the protocol is used by TCN Coalition members CoEpi and Covid Watch , and was likely a source of inspiration for the similar Google / Apple contact tracing project . Decentralized Privacy-Preserving Proximity Tracing Decentralized Privacy-Preserving Proximity Tracing ( DP-3T , stylized as dp t ) is an open protocol developed in response to

608-413: A coarse timestamp and signal strength. The signal strength is later used as part of the infection reporting process to estimate the distance between an infected patient and the user. When reporting infection, there exists a central reporting server controlled by the local health authority. Before a user can submit a report, the health authority must first confirm infection and generate a code authorizing

684-494: A contact log. Then, once a user tests positive for infection, a report is sent to a central server. Each client on the network then collects the reports from the server and independently checks their local contact logs for an EphID contained in the report. If a matching EphID is found, then the user has come in close contact with an infected patient, and is warned by the client. Since each device locally verifies contact logs, and thus contact logs are never transmitted to third parties,

760-435: A data and data length pair. It is formatted as m e m o = t a g | | l e n ( d a t a ) | | d a t a {\displaystyle memo=tag||len(data)||data} . The data is standardized for different tags, and can be as follows: The protocol can be divided into two responsibilities: an encounter between two devices running TCN apps, and

836-457: A global fixed string, P R F ( ) {\displaystyle PRF()} is a pseudo-random function like HMAC-SHA256 , and P R G ( ) {\displaystyle PRG()} is a stream cipher producing n ∗ 16 {\displaystyle n*16} bytes. This stream is then split into 16-byte chunks and randomly sorted to obtain the EphIDs of

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912-457: A handshake containing TCNs. In order to achieve this the encounter handshake operates in two modes (both with two sub-modes), broadcast oriented and connection oriented. Broadcast oriented operates using the modes broadcaster and observer, while connection oriented operates using peripheral and central. The two modes are used to circumvent certain device limitations, particularly in regard to iOS restrictions in place before version 13.4. In both modes

988-546: A hash security lower than 112 bits after 2013. The previous revision from 2007 specified the cutoff to be the end of 2010. In August 2012, NIST revised SP800-107 in the same manner. The NIST hash function competition selected a new hash function, SHA-3 , in 2012. The SHA-3 algorithm is not derived from SHA-2. The SHA-2 hash function is implemented in some widely used security applications and protocols, including TLS and SSL , PGP , SSH , S/MIME , and IPsec . The inherent computational demand of SHA-2 algorithms has driven

1064-550: A key aspect of achieving widespread adoption. Covid Watch volunteers and nonprofit staff also built a fully open source mobile app for sending anonymous exposure alerts first using the TCN Protocol and later using the very similar Google/Apple Exposure Notification Framework (ENF). The protocol, like BlueTrace and the Google / Apple contact tracing project , use Bluetooth Low Energy to track and log encounters with other users. The major distinction between TCN and protocols like BlueTrace

1140-583: A large fleet of Bluetooth Low Energy devices. This attack leverages the linkability of a user during a day, and therefore is possible on within a day on all users of some centralized systems such as the system proposed in the United Kingdom, but does not function on 'unlinkable' versions of DP-3T where infected users' identifiers are not transmitted using a compact representation such as a key or seed. SHA-2 Pseudo-collision attack against up to 46 rounds of SHA-256. SHA-2 ( Secure Hash Algorithm 2 )

1216-492: A later date. By using the same algorithm used to generate the original EphIDs, clients can reproduce every EphID used for the period past and including t {\displaystyle t} , which they then check against their local contact log to determine whether the user has been in close proximity to an infected patient. In the entire protocol, the health authority never has access to contact logs, and only serve to test patients and authorize report submissions. When

1292-430: A length in bits not a multiple of eight while supporting both variants. Hash values of an empty string (i.e., a zero-length input text). Even a small change in the message will (with overwhelming probability) result in a different hash, due to the avalanche effect . For example, adding a period to the end of the following sentence changes approximately half (111 out of 224) of the bits in the hash, equivalent to picking

1368-426: A local list of size n = ( 24 ∗ 60 ) / l {\displaystyle n=(24*60)/l} new EphIDs to broadcast throughout the day, where l {\displaystyle l} is the lifetime of an EphID in minutes. To prevent malicious third parties from establishing patterns of movement by tracing static identifiers over a large area, EphIDs are rotated frequently. Given

1444-514: A method for generating initial values for truncated versions of SHA-512. Additionally, a restriction on padding the input data prior to hash calculation was removed, allowing hash data to be calculated simultaneously with content generation, such as a real-time video or audio feed. Padding the final data block must still occur prior to hash output. In July 2012, NIST revised SP800-57, which provides guidance for cryptographic key management. The publication disallowed creation of digital signatures with

1520-653: A national app is being built upon DP-3T by SAP SE and Deutsche Telekom alongside CISPA , one of the organisations that authored the protocol. As of September 30, 2020, contact tracing apps using DP-3T are available in Austria , Belgium , Croatia , Germany, Ireland , Italy , the Netherlands , Portugal and Switzerland . The DP-3T protocol works off the basis of Ephemeral IDs (EphID), semi-random rotating strings that uniquely identify clients. When two clients encounter each other, they exchange EphIDs and store them locally in

1596-440: A new hash at random: Pseudocode for the SHA-256 algorithm follows. Note the great increase in mixing between bits of the w[16..63] words compared to SHA-1. The computation of the ch and maj values can be optimized the same way as described for SHA-1 . SHA-224 is identical to SHA-256, except that: SHA-512 is identical in structure to SHA-256, but: SHA-384 is identical to SHA-512, except that: SHA-512/t

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1672-419: A password to try against a user's account elsewhere) is not made possible by the attacks. (However, even a secure password hash cannot prevent brute-force attacks on weak passwords .) In the case of document signing, an attacker could not simply fake a signature from an existing document—the attacker would have to produce a pair of documents, one innocuous and one damaging, and get the private key holder to sign

1748-426: A period from late 2014 and early 2015. Similarly, Microsoft announced that Internet Explorer and Edge would stop honoring public SHA-1-signed TLS certificates from February 2017. Mozilla disabled SHA-1 in early January 2016, but had to re-enable it temporarily via a Firefox update, after problems with web-based user interfaces of some router models and security appliances . For a hash function for which L

1824-1487: A report is received, clients individually recalculate TCKs and TCNs for a given period using the original algorithms: t c k s = H _ t c k ( r v k | | t c k s − 1 ) t c n s = H _ t c n ( l e _ u 16 ( s ) | | t c k s ) t c k s + 1 = H _ t c k ( r v k | | t c k s ) t c n s + 1 = H _ t c n ( l e _ u 16 ( s + 1 ) | | t c k s + 1 ) . . . t c k e = H _ t c k ( r v k | | t c k e − 1 ) t c n e = H _ t c n ( l e _ u 16 ( e ) | | t c k e ) {\displaystyle {\begin{array}{lcr}tck_{s}=H\_tck(rvk||tck_{s-1})\\tcn_{s}=H\_tcn(le\_u16(s)||tck_{s})\\tck_{s+1}=H\_tck(rvk||tck_{s})\\tcn_{s+1}=H\_tcn(le\_u16(s+1)||tck_{s+1})\\...\\tck_{e}=H\_tck(rvk||tck_{e-1})\\tcn_{e}=H\_tcn(le\_u16(e)||tck_{e})\end{array}}} This

1900-583: A secret held only by the client. To generate a TCN, first a report authorization key (RAK) and report verification key (RVK) are created as the signing and verification keys of a signature scheme (RAK-RVK pair). In the reference implementation this pair is created using the Ed25519 signature scheme. Then, using the RAK an initial temporary contact key (TCK) is generated using the algorithm t c k 0 = H _ t c k ( r

1976-525: A user installs a DP-3T app, they are asked if they want to opt in to sharing data with epidemiologists . If the user consents, when they are confirmed to have been within close contact of an infected patient the respective contact log entry containing the encounter is scheduled to be sent to a central statistics server. In order to prevent malicious third parties from discovering potential infections by detecting these uploads, reports are sent at regular intervals, with indistinguishable dummy reports sent when there

2052-430: A wide scale, and that the proposed countermeasures are, at best, able to mitigate attacks in a limited number of scenarios. Contrarily, centralized systems offer many countermeasures, by accounting and auditing. In the same work Vaudenay advocates that, since neither the centralized nor the decentralized approaches offer sufficient level of privacy protection, different solutions should be explored, in particular suggesting

2128-412: Is a cryptographic hash function such as SHA-256 . S K 0 {\displaystyle SK_{0}} is calculated by a standard secret key algorithm such as Ed25519 . The client will use S K t {\displaystyle SK_{t}} during day t {\displaystyle t} to generate a list of EphIDs. At the beginning of the day, a client generates

2204-985: Is a set of cryptographic hash functions designed by the United States National Security Agency (NSA) and first published in 2001. They are built using the Merkle–Damgård construction , from a one-way compression function itself built using the Davies–Meyer structure from a specialized block cipher. SHA-2 includes significant changes from its predecessor, SHA-1 . The SHA-2 family consists of six hash functions with digests (hash values) that are 224, 256, 384 or 512 bits: SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, SHA-512/256 . SHA-256 and SHA-512 are novel hash functions whose digests are eight 32-bit and 64-bit words, respectively. They use different shift amounts and additive constants, but their structures are otherwise virtually identical, differing only in

2280-659: Is based on similar principles as the DP-3T protocol, and supports a variant of it since May 2020. Huawei added a similar implementation of DP-3T to its Huawei Mobile Services APIs known as "Contact Shield" in June 2020. The DP-3T SDK and calibration apps intend to support the Apple/Google API as soon as it is released to iOS and Android devices. On the 21 April 2020, the Swiss Federal Office of Public Health announced that

2356-564: Is identical to SHA-512 except that: The SHA-512/t IV generation function evaluates a modified SHA-512 on the ASCII string "SHA-512/ t ", substituted with the decimal representation of t . The modified SHA-512 is the same as SHA-512 except its initial values h0 through h7 have each been XORed with the hexadecimal constant 0xa5a5a5a5a5a5a5a5 . Sample C implementation for SHA-2 family of hash functions can be found in RFC ; 6234 . In

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2432-546: Is no data to transmit. To facilitate compatibility between DP-3T apps administered by separate health authorities, apps maintain a local list of the regions a user has visited. Regions are large areas directly corresponding to health authority jurisdiction; the exact location is not recorded. The app will later connect these regions to their respective foreign central reporting server, and fetch reports from these servers in addition to its normal home reporting server. Apps will also submit reports to these foreign reporting servers if

2508-449: Is no upper limit to encounters calculated using the same RAK-RVK pair, which is why they are cycled often. To prevent clients calculating unused TCNs, e {\displaystyle e} indicates the last TCN index generated with the given RVK. Additionally, since the RVK is used to calculate a TCK, and t c k s − 1 {\displaystyle tck_{s-1}}

2584-400: Is not used to generate any TCNs, but is used in the next TCK; where all future TCKs are calculated using the algorithm t c k i = H _ t c k ( r v k | | t c k i − 1 ) {\displaystyle tck_{i}=H\_tck(rvk||tck_{i-1})} . A 128 bit TCN is then generated from a given TCK using

2660-449: Is provided, no valid TCNs in the reporting period can be derived from an illegitimate report. The only correct TCN calculable from a mismatched RVK and t c k s − 1 {\displaystyle tck_{s-1}} is t c n s − 1 {\displaystyle tcn_{s-1}} , the TCN before the start of the reporting period. Once

2736-468: Is the fact the central reporting server never has access to contact logs nor is it responsible for processing and informing clients of contact. Because contact logs are never transmitted to third parties, it has major privacy benefits over approaches like the one used in BlueTrace. This approach however, by its very nature, does not allow for human-in-the-loop reporting, potentially leading to false positives if

2812-402: Is the number of bits in the message digest , finding a message that corresponds to a given message digest can always be done using a brute force search in 2 evaluations. This is called a preimage attack and may or may not be practical depending on L and the particular computing environment. The second criterion, finding two different messages that produce the same message digest, known as

2888-595: Is then calculated using the RAK, and it is transmitted to the server as s _ r e p o r t = r e p o r t | | s i g {\displaystyle s\_report=report||sig} . Because any given TCK can only be used to derive an equal or higher indexed TCNs, by submitting t c k s − 1 {\displaystyle tck_{s-1}} no encounters prior to t c n s − 1 {\displaystyle tcn_{s-1}} can be calculated. However, there

2964-434: Is used by client devices to check their local contact logs for potential encounters with the infected patient, but has the dual benefit of verifying reports since false reports will never produce matching TCNs. In the report structure, the memo is a space for freeform messages that differ between TCN implementations. The section is between 2 and 257 bytes, and made up of a tag identifying the specific implementation, as well as

3040-814: The CMVP program , jointly run by the National Institute of Standards and Technology (NIST) and the Communications Security Establishment (CSE). For informal verification, a package to generate a high number of test vectors is made available for download on the NIST site; the resulting verification, however, does not replace the formal CMVP validation, which is required by law for certain applications. As of December 2013, there are over 1300 validated implementations of SHA-256 and over 900 of SHA-512, with only 5 of them being capable of handling messages with

3116-474: The COVID-19 pandemic to facilitate digital contact tracing of infected participants. The protocol, like competing protocol Pan-European Privacy-Preserving Proximity Tracing (PEPP-PT), uses Bluetooth Low Energy to track and log encounters with other users. The protocols differ in their reporting mechanism, with PEPP-PT requiring clients to upload contact logs to a central reporting server, whereas with DP-3T,

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3192-666: The private data they collect. Vaudenay's work presents several attacks against DP-3T and similar systems. In response, the DP-3T group claim that out of twelve risks Vaudenay presents, eight are also present in centralized systems, three do not work, and one, which involves physical access to the phone, works but can be mitigated. In a subsequent work Vaudenay reviews attacks against both centralized and decentralized tracing systems and referring to identification attacks of diagnosed people concludes that: By comparing centralized and decentralized architectures, we observe that attacks against decentralized systems are undetectable, can be done at

3268-427: The 'x86-64' numbers are native 64-bit code. While SHA-256 is designed for 32-bit calculations, it does benefit from code optimized for 64-bit processors on the x86 architecture. 32-bit implementations of SHA-512 are significantly slower than their 64-bit counterparts. Variants of both algorithms with different output sizes will perform similarly, since the message expansion and compression functions are identical, and only

3344-501: The ConTra Corona, Epione and Pronto-C2 systems as a "third way". Tang surveys the major digital contact tracing systems and shows that DP-3T is subject to what he calls "targeted identification attacks". Theoretical attacks on DP-3T have been simulated showing that persistent tracking of users of the first version of the DP-3T system who have voluntarily uploaded their identifiers can be made easy to any 3rd party who can install

3420-523: The SHA family. The algorithms are collectively known as SHA-2, named after their digest lengths (in bits): SHA-256, SHA-384, and SHA-512. The algorithms were first published in 2001 in the draft FIPS PUB 180-2, at which time public review and comments were accepted. In August 2002, FIPS PUB 180-2 became the new Secure Hash Standard , replacing FIPS PUB 180-1, which was released in April 1995. The updated standard included

3496-515: The Swiss national coronavirus contact tracing app will be based on DP-3T. On the 22 April 2020, the Austrian Red Cross , leading on the national digital contact tracing app, announced its migration to the approach of DP-3T. Estonia also confirmed that their app would be based on DP-3T. On April 28, 2020, it was announced that Finland was piloting a version of DP-3T called "Ketju". In Germany ,

3572-450: The U.S. National Institute of Standards and Technology says, "Federal agencies should stop using SHA-1 for...applications that require collision resistance as soon as practical, and must use the SHA-2 family of hash functions for these applications after 2010" (emphasis in original). NIST's directive that U.S. government agencies ought to, but not explicitly must, stop uses of SHA-1 after 2010

3648-407: The algorithm t c n i > 0 = H _ t c n ( l e _ u 16 ( i ) | | t c k i ) {\displaystyle tcn_{i>0}=H\_tcn(le\_u16(i)||tck_{i})} , where l e _ u 16 ( ) {\displaystyle le\_u16()} formats a supplied number as

3724-418: The basis of Temporary Contact Numbers (TCN), semi-random identifiers derived from a seed. When two clients encounter each other, a unique TCN is generated, exchanged, and then locally stored in a contact log. Then, once a user tests positive for infection, a report is sent to a central server. Each client on the network then collects the reports from the server and independently checks their local contact logs for

3800-467: The central reporting server cannot by itself ascertain the identity or contact log of any client in the network. This is in contrast to competing protocols like PEPP-PT, where the central reporting server receives and processes client contact logs. Similar to the TCN Protocol and its Temporary Contact Numbers, the DP-3T protocol makes use of 16 byte Ephemeral IDs (EphID) to uniquely identify devices in

3876-470: The central reporting server never has access to contact logs nor is it responsible for processing and informing clients of contact. Because contact logs are never transmitted to third parties, it has major privacy benefits over the PEPP-PT approach; however, this comes at the cost of requiring more computing power on the client side to process infection reports. The Apple/Google Exposure Notification project

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3952-467: The central reporting server receives and processes client contact logs. The entire protocol is based on the principle of temporary contact numbers (TCN), a unique and anonymous 128-bit identifier generated deterministically from a seed value on a client device. TCNs are used to identify people with which a user has come in contact, and the seed is used to compactly report infection to a central reporting server. TCN reports are authenticated to be genuine by

4028-419: The client to upload the report. The health authority additionally instructs the patient on which day their report should begin (denoted as t {\displaystyle t} ). The client then uploads the pair S K t {\displaystyle SK_{t}} and t {\displaystyle t} to the central reporting server, which other clients in the network download at

4104-456: The day. The DP-3T protocol is made up of two separate responsibilities, tracking and logging close range encounters with other users (device handshake), and the reporting of those encounters such that other clients can determine if they have been in contact with an infected patient (infection reporting). Like most digital contact tracing protocols, the device handshake uses Bluetooth Low Energy to find and exchange details with local clients, and

4180-503: The global TCN Coalition was founded by Covid Watch and other groups that had coalesced around what was essentially the same approach and largely overlapping protocols, with the goal to reduce fragmentation, and enable global interoperability of tracing and alerting apps, a key aspect of achieving widespread adoption. The TCN Coalition also helped establish the Data Rights for Digital Contact Tracing and Alerting framework, which functions as

4256-404: The infection reporting stage uses HTTPS to upload a report to a central reporting server. Additionally, like other decentralized reporting protocols , the central reporting server never has access to any client's contact logs; rather the report is structured such that clients can individually derive contact from the report. In order to find and communicate with clients in proximity of a device,

4332-509: The initial hash values and output sizes are different. The best implementations of MD5 and SHA-1 perform between 4.5 and 6 cycles per byte on modern processors. Testing was performed by the University of Illinois at Chicago on their hydra8 system running an Intel Xeon E3-1275 V2 at a clock speed of 3.5 GHz, and on their hydra9 system running an AMD A10-5800K APU at a clock speed of 3.8 GHz. The referenced cycles per byte speeds above are

4408-448: The innocuous document. There are practical circumstances in which this is possible; until the end of 2008, it was possible to create forged SSL certificates using an MD5 collision which would be accepted by widely used web browsers. Increased interest in cryptographic hash analysis during the SHA-3 competition produced several new attacks on the SHA-2 family, the best of which are given in

4484-776: The key derivation process: TCNs are unique to each device encounter, and RAK-RVK pairs are cycled at regular intervals to allow a client to report only specific periods of contact. When a client wishes to submit a report for the TCN indices s > 0 {\displaystyle s>0} to e {\displaystyle e} , it structures the report as r e p o r t = r v k | | t c k s − 1 | | l e _ u 16 ( s ) | | l e _ u 16 ( e ) | | m e m o {\displaystyle report=rvk||tck_{s-1}||le\_u16(s)||le\_u16(e)||memo} . A signature

4560-488: The notification of potential infection to users that came in contact with a patient. For the purposes of this specification, these areas are named the encounter handshake , and infection reporting . The encounter handshake runs on Bluetooth LE and defines how two devices acknowledge each other's presence. The infection reporting is built on HTTPS and defines how infection notices are distributed among clients. When two devices come within range of each other, they exchange

4636-513: The number of rounds. SHA-224 and SHA-384 are truncated versions of SHA-256 and SHA-512 respectively, computed with different initial values. SHA-512/224 and SHA-512/256 are also truncated versions of SHA-512, but the initial values are generated using the method described in Federal Information Processing Standards (FIPS) PUB 180-4. SHA-2 was first published by the National Institute of Standards and Technology (NIST) as

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4712-506: The original SHA-1 algorithm, with updated technical notation consistent with that describing the inner workings of the SHA-2 family. In February 2004, a change notice was published for FIPS PUB 180-2, specifying an additional variant, SHA-224, defined to match the key length of two-key Triple DES . In October 2008, the standard was updated in FIPS PUB 180-3, including SHA-224 from the change notice, but otherwise making no fundamental changes to

4788-404: The peripheral. When a user tests positive for infection, they upload a signed report, allowing the past 14 days of encounters to be calculated, to a central server. On a regular basis, client devices download reports from the server and check their local contact logs using the verification algorithm. If there is a matching record, the app notifies the user to potential infection. On 5 April 2020,

4864-941: The proposal of more efficient solutions, such as those based on application-specific integrated circuits (ASICs) hardware accelerators. SHA-256 is used for authenticating Debian software packages and in the DKIM message signing standard; SHA-512 is part of a system to authenticate archival video from the International Criminal Tribunal of the Rwandan genocide . SHA-256 and SHA-512 are proposed for use in DNSSEC . Unix and Linux vendors are moving to using 256- and 512-bit SHA-2 for secure password hashing. Several cryptocurrencies , including Bitcoin , use SHA-256 for verifying transactions and calculating proof of work or proof of stake . The rise of ASIC SHA-2 accelerator chips has led to

4940-455: The protocol is identified with the 16 bit UUID 0xC019 . In broadcast mode, a broadcaster advertises a 16-byte TCN using the service data field of the advertisement data. The observer reads the TCN from this field. In connection-oriented mode, the peripheral advertises using the UUID. The service exposes a read and writeable packet for sharing TCNs. After sharing a TCN, the central disconnects from

5016-468: The protocol makes use of both the server and client modes of Bluetooth LE, switching between the two frequently. In server mode the device advertises its EphID to be read by clients, with clients scanning for servers. When a client and server meet, the client reads the EphID and subsequently writes its own EphID to the server. The two devices then store the encounter in their respective contact logs in addition to

5092-498: The proximity of a client. These EphIDs are logged locally on a receiving client's device and are never transmitted to third parties. To generate an EphID, first a client generates a secret key that rotates daily ( S K t {\displaystyle SK_{t}} ) by computing S K t = H ( S K t − 1 ) {\displaystyle SK_{t}=H(SK_{t-1})} , where H ( ) {\displaystyle H()}

5168-656: The reports are not verified by public health agencies. The TCN protocol received notoriety as one of the first widely released digital contact tracing protocols alongside BlueTrace, the Exposure Notification framework, and the Pan-European Privacy-Preserving Proximity Tracing (PEPP-PT) project. It also stood out for its incorporation of blockchain technology, and its influence over the Google/Apple project. The TCN protocol works off

5244-399: The secret day key S K t {\displaystyle SK_{t}} , each device computes S _ E p h I D ( B K ) = P R G ( P R F ( S K t , B K ) ) {\displaystyle S\_EphID(BK)=PRG(PRF(SK_{t},BK))} , where B K {\displaystyle BK} is

5320-400: The standard. The primary motivation for updating the standard was relocating security information about the hash algorithms and recommendations for their use to Special Publications 800-107 and 800-57. Detailed test data and example message digests were also removed from the standard, and provided as separate documents. In January 2011, NIST published SP800-131A, which specified a move from

5396-509: The table below, internal state means the "internal hash sum" after each compression of a data block. In the bitwise operations column, "Rot" stands for rotate no carry , and "Shr" stands for right logical shift . All of these algorithms employ modular addition in some fashion except for SHA-3. More detailed performance measurements on modern processor architectures are given in the table below. The performance numbers labeled 'x86' were running using 32-bit code on 64-bit processors, whereas

5472-445: The table below. Only the collision attacks are of practical complexity; none of the attacks extend to the full round hash function. At FSE 2012, researchers at Sony gave a presentation suggesting pseudo-collision attacks could be extended to 52 rounds on SHA-256 and 57 rounds on SHA-512 by building upon the biclique pseudo-preimage attack. Implementations of all FIPS-approved security functions can be officially validated through

5548-432: The then-current minimum of 80-bit security (provided by SHA-1) allowable for federal government use until the end of 2013, to 112-bit security (provided by SHA-2) being both the minimum requirement (starting in 2014) and the recommended security level (starting from the publication date in 2011). In March 2012, the standard was updated in FIPS PUB 180-4, adding the hash functions SHA-512/224 and SHA-512/256, and describing

5624-502: The use of scrypt -based proof-of-work schemes. SHA-1 and SHA-2 are the Secure Hash Algorithms required by law for use in certain U.S. Government applications, including use within other cryptographic algorithms and protocols, for the protection of sensitive unclassified information. FIPS PUB 180-1 also encouraged adoption and use of SHA-1 by private and commercial organizations. SHA-1 is being retired for most government uses;

5700-399: The user tests positive for infection. Cryptography and security scholar Serge Vaudenay , analyzing the security of DP-3T argued that: some privacy protection measurements by DP3T may have the opposite affect [ sic ] of what they were intended to. Specifically, sick and reported people may be deanonymized, private encounters may be revealed, and people may be coerced to reveal

5776-445: Was hoped to accelerate migration away from SHA-1. The SHA-2 functions were not quickly adopted initially, despite better security than SHA-1. Reasons might include lack of support for SHA-2 on systems running Windows XP SP2 or older and a lack of perceived urgency since SHA-1 collisions had not yet been found. The Google Chrome team announced a plan to make their web browser gradually stop honoring SHA-1-dependent TLS certificates over

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