Downloading diagnosis keys

A regularly scheduled background task is executed to download files with batch of Temporary Exposure Keys (TEK) of positively diagnosed, called Diagnosis Keys files. Each of the batch file has unique timestamp. The timestamp that identifying when a certain Diagnosis Keys file was created is used to select only not yet analyzed files for download.

Steps:

• The app gets a list of available Diagnosis Keys (index.txt) files from CDN:
  • Moya Target endpoint: case get
  • Service function: DiagnosisKeysDownloadServiceProtocol.download() -> Promise<[URL]>
• Only files with the timestamp older than the latest successfully provided to analyze batch are selected for download
• Files are downloaded over HTTPS protocol to internal device storage
  • Moya Target endpoint: case download(fileName: String, destination: DownloadDestination)
  • Service function: DiagnosisKeysDownloadService.downloadFiles(withNames names: [String], keysDirectoryURL: URL) -> Promise<[URL]>
• Downloaded zip archives are decompressed to provide flat array of URL pairs:
  • export.bin - the binary containing Diagnosis Keys
  • export.sig - the raw signature and information needed for verification