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If an attacker can upload a compromised ISO I assume they can also upload a compromised checksum? In the age of https downloads — where the payload cannot be mo

If an attacker can upload a compromised ISO I assume they can also upload a compromised checksum? In the age of https downloads — where the payload cannot be modified in transit — it never made sense to me why ISO checksums are a thing. For checksums to actually do anything there needs to be a chain of trust back to a trusted entity.

normie3000 1y ago
> In the age of https downloads — where the payload cannot be modified in transit — it never made sense to me why ISO checksums are a thing. Is there no way a download over HTTPS can be corrupted non-maliciously, or can fail to complete?
- basilikum 1y ago
  It can fail mid-way of course, but it really shouldn't corrupt in any other way. HTTPS is authenticated after all and malicious manipulation is harder to defend against than accidental corruption. But this is reality and you can have bugs and errors outside the transport of course. Your file system could corrupt data, your drive could be bad etc. For security you want signatures with a known, trusted key.
- __turbobrew__ 1y ago
  TLS uses message authentication codes which should detect tampering or bit errors. In theory, a cosmic ray could hit the RAM of the device on the receiving end and bit flip after the ISO has been decrypted but still in RAM. Checksumming does not rule bit flips out though as you could checksum the ISO and the bitflip happens between then and when the ISO is actually used to install the system. Maybe in theory you could checksum the post installed filesystem, but Im not sure if any distros actually do that or not and it would require deterministic install layouts.
kbolino 1y ago
Mirrors. Lots of small, volunteer-run, low/zero-budget open-source projects cannot afford to pay for the server/CDN bandwidth they would need to host all their binary artifacts (ISOs, packages, etc.). They end up relying on mirrors provided for free by third parties instead. By publishing the checksums, they allow you to verify that the ISO image you downloaded from some mirror is the same one that they originally published.

normie3000 1y ago

> In the age of https downloads — where the payload cannot be modified in transit — it never made sense to me why ISO checksums are a thing. Is there no way a download over HTTPS can be corrupted non-maliciously, or can fail to complete?

basilikum 1y ago
It can fail mid-way of course, but it really shouldn't corrupt in any other way. HTTPS is authenticated after all and malicious manipulation is harder to defend against than accidental corruption. But this is reality and you can have bugs and errors outside the transport of course. Your file system could corrupt data, your drive could be bad etc. For security you want signatures with a known, trusted key.
__turbobrew__ 1y ago
TLS uses message authentication codes which should detect tampering or bit errors. In theory, a cosmic ray could hit the RAM of the device on the receiving end and bit flip after the ISO has been decrypted but still in RAM. Checksumming does not rule bit flips out though as you could checksum the ISO and the bitflip happens between then and when the ISO is actually used to install the system. Maybe in theory you could checksum the post installed filesystem, but Im not sure if any distros actually do that or not and it would require deterministic install layouts.

kbolino 1y ago

Mirrors. Lots of small, volunteer-run, low/zero-budget open-source projects cannot afford to pay for the server/CDN bandwidth they would need to host all their binary artifacts (ISOs, packages, etc.). They end up relying on mirrors provided for free by third parties instead. By publishing the checksums, they allow you to verify that the ISO image you downloaded from some mirror is the same one that they originally published.