Actually, you can reverse a SHA1 into plaintext. That's exactly what we're talking about.
Can you know it was the same plaintext that went in? No, but as far as the SHA1 output is concerned, it's a suitable input and a suitable reverse.
For the application we're discussing, the fact that collisions can be engineered is sufficient to generate a reverse of the hash. At no point do you need to generate the original input in order to reverse it for this application.
Can you know it was the same plaintext that went in? No, but as far as the SHA1 output is concerned, it's a suitable input and a suitable reverse.
For the application we're discussing, the fact that collisions can be engineered is sufficient to generate a reverse of the hash. At no point do you need to generate the original input in order to reverse it for this application.