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If so, what is that number and why can't you divide the public key by G to obtain the private key?

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G's not publicized. – Nick ODell Mar 4 '13 at 17:22
i thought the generator point G was publicly known? – joe Mar 4 '13 at 23:08

Uncompressed "G" is 0479BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8

The "04" at the beginning is simply added (for what reason I don't know).

The first 64 hex characters after the "04" supposedly represent the "x" value and the last 64 hex characters supposedly represent the "y" value.

I don't know the specifics of why it cannot be reversed (I am also seeking the answer to that question).

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Note that the "multiplication" in question is not ordinary integer multiplication, but elliptic curve point multiplication.

The whole point of working over an elliptic curve in this way is that multiplication is easy, but division is hard.

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No sufficiently efficient algorithm for division is known. Multiplication over a closed group is a lossy process.

For an imperfect analogy, consider multiplication over the group of 100-digit numbers where you just keep the last 100 digits of the result. Since you don't know what digits were discarded, you can't naively reverse a multiplication.

The corresponding algorithm in DSA is modular exponentiation -- (G^X) mod n, which is also, for practical purposes with large numbers, irreversible even if G and n are known.

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Multiplication in a closed group is not lossy. It is hard to go back, but with a*G = P, if you know P and G, there will be exactly one a (modulo the order of the group) that satisfies it. It's just very hard to compute. – Pieter Wuille May 15 '14 at 8:27

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