Hi. I'm using Jimmy Song's book and there is something I do not understand. I am working through the SHA-1 collision in the end of chapter 6 on Script:
Here is the ScriptPubKey:
6E - 2DUP
87 - EQUAL
91 - NOT
69 - VERIFY
A7 - SHA1
7C - SWAP
A7 - SHA1
87 - EQUAL
I'll assume that there are two values there that we can call x and y. The top of the stack is at the end of the list.
x, y
2DUP(x,y) = (x, y, y, y)
EQUAL(x, y, y, y) = (x, y, 1)
NOT(x, y, 1) = (x, y, 0)
VERIFY(x, y, 0) -> FAIL!
SHA1(x, y) = (x, SHA1[y])
SWAP(x, SHA1[y]) = (SHA1[y], x)
SHA1(SHA1[y], x) = (SHA1[y], SHA1[x])
EQUAL(SHA1[y], SHA1[x]) -> Will be true if you find the collision.
I do not understand what the four commands are doing at the beginning.
EDIT: I made a mistake on 2DUP.
x, y
2DUP(x,y) = (x, y, x, y)
EQUAL(x, y, y, y) = (x, y, 0) #Check that the two values are unequal
NOT(x, y, 1) = (x, y, 1)
VERIFY(x, y, 1) -> PASS!, will fail if they are equal.
SHA1(x, y) = (x, SHA1[y])
SWAP(x, SHA1[y]) = (SHA1[y], x)
SHA1(SHA1[y], x) = (SHA1[y], SHA1[x])
EQUAL(SHA1[y], SHA1[x]) -> Will be true if you find the collision.
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