To show that a message existed at a particular date and time, you could use a time-stamping service operated by a trusted third party, in which they digitally s

To show that a message existed at a particular date and time, you could use a time-stamping service operated by a trusted third party, in which they digitally sign a timestamp and the cryptographic digest of your message. Several public certification authorities offer these services, described in RFC 3161, at no charge, because they’re used by code signing schemes, to prove that a digital signature on an object file was made during the validity period of the signer’s identity certificate. Here’s Microsoft’s explanation: https://docs.microsoft.com/en-us/windows/win32/seccrypto/time-stamping-authenticode-signatures https://docs.microsoft.com/en-us/windows/win32/seccrypto/tim... You don’t have to use an identity certificate, however; OpenSSL, for example, can work work with timestamps directly: https://www.openssl.org/docs/manmaster/man1/tsget.html https://www.openssl.org/docs/manmaster/man1/tsget.html https://www.openssl.org/docs/manmaster/man1/openssl-ts.html https://www.openssl.org/docs/manmaster/man1/openssl-ts.html Some examples of time-stamping servers: https://knowledge.digicert.com/generalinformation/INFO4231.html https://knowledge.digicert.com/generalinformation/INFO4231.h... https://www.entrust.com/knowledgebase/ssl/time-stamp-url https://www.entrust.com/knowledgebase/ssl/time-stamp-url https://sectigo.com/resource-library/time-stamping-server https://sectigo.com/resource-library/time-stamping-server Whether such a timestamp would be acceptable proof, on its own or accompanied by an explanation from an independent expert, would depend on the audience and the situation, of course.