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From the OpenSSL Wiki:

Also note that the derived shared secret is not suitable for use directly as a shared key. Typically the shared secret is passed through some hash function first in order to generate a key.

My understanding of ECDH is that it generates a very secure shared secret. Why does it have to be hashed too?

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There are two reasons:

Reason one is that the ECDH shared secret is not equidistributed; not all values are possible. In particular, and $x$ that is not a possible solution to the elliptic curve equation cannot happen at all.

Things that use the shared secret (such as AES) are generally assumed to have uniform keys; that is, all keys are possible (and are equiprobable); this isn't possible when using the ECDH shared secret directly.

In practice, it probably doesn't matter; it's hard to conceive of an attack that becomes practical because half of the possible $x$ coordinates are impossible.

Reason two is that not all bits of the ECDH shared secret are actually independent. If the part of the shared secret bits start going into the $y$ coordinate, then if the attacker somehow gets the $x$ coordinate bits, he gets the $y$ coordinate bits for free (well, one of two possibilities).

We try to avoid revealing some of the bits of the key; however having some bits imply others isn't what we want. A proper KDF (key derivation function) avoids this possibility.

Note: reason two doesn't apply to all elliptic curves; for example, with Curve25519, we don't explicitly compute the $y$ value, hence that's not available as part of the shared secret. However, it is present for other curves.

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  • $\begingroup$ There are implementations of ECDH that actually use the y coordinate as part of the shared secret? $\endgroup$ – CodesInChaos Aug 23 '17 at 21:44
  • $\begingroup$ @CodesInChaos: I thought I remembered seeing one; however a quick search doesn't bring it up... $\endgroup$ – poncho Aug 23 '17 at 21:58

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