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I want to create some software that performs diffie hellman group key agreement but I don't want to reinvent the wheel even if I know how it's done. So I came accross the NaCl library, especially the scalar_mult functions.

As far I know you can perform Diffie Hellman key agreement on elliptic curves therefore, my thought is if I combine it with crypto_box ones for key generation, I can do a Diffie Hellman Key agreement.

My Idea is (described in some sort of pseudocode) shown in the following image:

Scalar multiplication DH example

Would you recommend it as shown above?

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    $\begingroup$ Well, the function specifies that it can be used for the (Computational) Diffie-Hellman problem, which is the underlying problem that allows DH key agreement calculations. So I presume it is yes. I'm however not a NaCl expert so I'll allow somebody else to answer (I think the function is mainly used in NaCl for ECIES, which would make for an easy answer, but I'm not 100% sure about that particular bit). $\endgroup$ – Maarten Bodewes Nov 12 '18 at 15:42
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Yes.

The DH function is $$\operatorname{X25519}(n, p) := x\bigl([\operatorname{clamp}(n)] x^{-1}(\operatorname{decode}(p))\bigr)$$ with scalar multiplication $[\alpha]P$ on the curve $y^2 = x^3 + 486662 x^2 + x$ over the prime field $\operatorname F_{2^{255} - 19}$, where $n$ is 32-byte string representing a private key, and $p$ is a 32-byte string representing a public key.

Here $\operatorname{decode}(p)$ maps a 32-byte string into an element of $\mathbb F_{2^{255} - 19}$, and $\operatorname{clamp}(n)$ maps a uniform random 32-byte string into a uniform random integer in $\{2^{254}, 2^{254} + 8, 2^{254} + 16, \dots, 2^{255} - 8\}$—that is, an integer multiple of 8 between $2^{254}$ and $2^{255} - 1$.

As a DH function, this has the property that $\operatorname{X25519}(n, \operatorname{X25519}(m, \underline 9)) = \operatorname{X25519}(m, \operatorname{X25519}(n, \underline 9))$, where $\underline 9$ is the encoding of the standard base point, whose $x$ coordinate is 9, which means that both parties will arrive at the same shared secret.

The security contract requires that that you must hash the shared secret: use $H(\operatorname{X25519}(n, p))$. Specifically, the security conjecture is that even if you reveal $\operatorname{X25519}(n, \underline 9)$ and $p \mapsto H(\operatorname{X25519}(n, p))$ for any $p$ of an attacker's choice, then the attacker must spend ${\sim}2^{128}$ bit operations to find any shared secret. The security contract also requires that you never use $n$ for any purpose except the two crypto_scalarmult functions:

  • crypto_scalarmult_base(p, n) sets $p$ to the public key $\operatorname{X25519}(n, \underline 9)$. You may publish $p$.
  • crypto_scalarmult(q, n, p) sets $q$ to the unhashed shared secret $\operatorname{X25519}(n, p)$, the secret shared between the private key $n$ and the public key $p$. You must hash $q$ before using it.
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