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New version of Apple & Google Contact Tracing Protocol

Apple & Google

On April 24, 2020, Apple and Google announced an update to the jointly developed Contact & Tracing Protocol (Apple & Google Contact Tracing Protocol), which they now call the Exposure Notification Technology, as the name describes the protocol better.


On April 29, iOS 13.5 beta was released with the first implementation of this protocol. Beta aims to test developers a new API and get feedback. Access to the API is planned to be issued only to applications officially associated with state medical institutions.


This article will talk about what caused the changes in the protocol, whether the vulnerabilities remained after the update, and whether Apple and Google were able to respond to criticism (often unreasonable).


Name change


As companies note, contact tracing is, although necessary, but just part of the protocol.

The objective of the solution being developed is to notify people in advance that could potentially be infected with COVID-19 in order to reduce the spread of the disease.




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The working scheme of the old Apple / Google protocol

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The working scheme of the new Apple / Google protocol

  1. 16- Exposure Key.
    Rolling Proximity Key (RPIKey) Associated Encrypted Metadata Key (AEMKey):


    RPIKey = HKDF(ExpKey, NULL, UTF8("EN-RPIK"),16)

    AEMKey = HKDF(ExpKey, NULL, UTF8("EN-AEMK"),16)

    :


    • HKDF – - HKDF(Key, Salt, Info, OutputLength) RFC 5869, SHA-256
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    MAC Rolling Proximity ID (RPID)


    RPID = AES128(RPIKey, UTF8("EN-RPI") || 0x000000000000 || Ti)

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    • AES128(Key, Data) – - AES 128- . 128-
    • RPIKey – Rolling Proximity Key
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      • UTF8("EN-RPI") – 6 , EN-RPI UTF8
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    AEM = AES128−CTR(AEMKey, RPID,  Metadata)

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    AES128−CTR(Key, IV, Data) – - AES-CTR, 128- AEMKey. , .

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    Rolling Proximity ID Associated Encrypted Metadata , BLE Payload:





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The key schedule is fixed and defined by operating system components, preventing applications from including static or predictable information that could be used for tracking.

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Associated Metadata


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