From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 30 Nov 2011 18:05:24 GMT
Subject: Re: [PATCH] Implement fast hash-collision detection
Message-ID: <7vzkfdqye3.fsf@alter.siamese.dyndns.org>
In-Reply-To: <CACsJy8A6kGmn0h0xdxfTC4krXgc8hzO1fHTdqfk0YnASGN5K0w@mail.gmail.com>

Nguyen Thai Ngoc Duy <pclouds@gmail.com> writes:

> What I'm thinking is whether it's possible to decouple two sha-1 roles
> in git, as object identifier and digest, separately.

Why it would be a good thing? If you have a collided identifier, somebody
has to choose which blob a particular tree wants to have at the path, and
because the tree would not record anything but the identifier, you cannot.

> ...
> The day sha-1 is broken, a project can generate new digests from its
> old good repo and enforce developers to use new digests for
> verification instead of sha-1. sha-1 is still used by git as
> identifier after that day.

And an old blob that is identified with a SHA-1 now has a new blob that
has different contents but happens to have the same SHA-1. How does Git
decide which blob to use when a particular object is named by the SHA-1?