// how to find the certificate was not easy. finally I found it using Mozilla Firefox.
// opened one of the bitmaps, e.g. https://raw.githubusercontent.com/ZinggJM/GxEPD2/master/extras/bitmaps/logo200x200.bmp
// clicked the lock symbol, Connection secure clicked >, show connection details, clicked More Information, clicked View Certificate, clicked Download PEM (cert),
// Opened the .pem file and copied the pem into R"CERT(...)CERT"; strings.
// for https://raw.githubusercontent.com the pem works on ESP8266 and ESP32, if time is set from ntp.
// but using a root certificate would preferred, as its validity may be longer. But this failed now.

const char github_io_pem [] PROGMEM = R"CERT(
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
)CERT";

// 12:04.2024: the following no longer works, needs further investigation
// **********************************************************************

// http://cacerts.digicert.com/DigiCertTLSRSASHA2562020CA1-1.crt
// CN: DigiCert TLS RSA SHA256 2020 CA1 => name: DigiCert_TLS_RSA_SHA256_2020_CA1
// not valid before: 2021-04-14 00:00:00
// not valid after:  2031-04-13 23:59:59
const char cert_DigiCert_TLS_RSA_SHA256_2020_CA1 [] PROGMEM = R"CERT(
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
  )CERT";

// how to find the certificate was not easy. finally I found it using Mozilla Firefox.
// opened one of the bitmaps, e.g. https://raw.githubusercontent.com/ZinggJM/GxEPD2/master/extras/bitmaps/logo200x200.bmp
// clicked the lock symbol, Connection secure clicked >, show connection details, clicked More Information, clicked View Certificate, clicked Download PEM (chain),
// Opened the .pem file and copied the pems into R"CERT(...)CERT"; strings.
// for https://raw.githubusercontent.com all three pems work on ESP8266 and ESP32, if time is set from ntp.
// but using a root certificate is preferred, as its validity may be longer.

// https://raw.githubusercontent.com
// issued by DigiCert Inc : DigiCert TLS RSA SHA256 2020 CA1
// not valid before: Fri, 15 Mar 2024 00:00:00 GMT
// not valid after:  Fri, 14 Mar 2025 23:59:59 GMT
const char github_io_chain_pem_first [] PROGMEM = R"CERT(
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
)CERT";
const char github_io_chain_pem_second [] PROGMEM = R"CERT(
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
)CERT";
const char github_io_chain_pem_third [] PROGMEM = R"CERT(
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
)CERT";