If you get the sha256 from the same place you got the main file, then anyone tampering with the main file could also recalculate the sha256 to match the tampered file.
A signature signed with a certificate uses complex math (public-key asymmetric cryptography) to give some certainty that the signed content (the sha256) is the same sha256 that the original file author created. It’s not mathematically feasible to recalculate the certificate signature.
Why don’t we just sign the whole original file with the public-key crypto and skip the sha256? Because asymmetric crypto is much, much slower than plain symmetric crypto or hash functions. It’s faster and easier to generate the valid hash or key, then sign or encrypt just the smaller key.
In other words, if the sha matches, then it wasn’t corrupted during downloading. If the signature matches, then it wasn’t tampered with before you downloaded it.
There’s also a third check. Even if the certificate signature is valid, you have to have confidence that the certificate is authentic and trusted to be from the original author. This is usually done by having a trusted third party sign the certificate with another, more trusted, certificate.
really its just different trust root authorities presuming we are still talking pk distribution infra involved here. alice and bob can of course always trade keys in other ways. if its distributed you have to root trust with a ledger (trust area: key ceremony, consensus protocol) or a CA (trust area: the CA chain, every step is another element of trust)
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If you get the sha256 from the same place you got the main file, then anyone tampering with the main file could also recalculate the sha256 to match the tampered file. A signature signed with a certificate uses complex math (public-key asymmetric cryptography) to give some certainty that the signed content (the sha256) is the same sha256 that the original file author created. It’s not mathematically feasible to recalculate the certificate signature. Why don’t we just sign the whole original file with the public-key crypto and skip the sha256? Because asymmetric crypto is much, much slower than plain symmetric crypto or hash functions. It’s faster and easier to generate the valid hash or key, then sign or encrypt just the smaller key.
In other words, if the sha matches, then it wasn’t corrupted during downloading. If the signature matches, then it wasn’t tampered with before you downloaded it.
There’s also a third check. Even if the certificate signature is valid, you have to have confidence that the certificate is authentic and trusted to be from the original author. This is usually done by having a trusted third party sign the certificate with another, more trusted, certificate.
really its just different trust root authorities presuming we are still talking pk distribution infra involved here. alice and bob can of course always trade keys in other ways. if its distributed you have to root trust with a ledger (trust area: key ceremony, consensus protocol) or a CA (trust area: the CA chain, every step is another element of trust)