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Implementations of a fast Elliptic-curve Diffie-Hellman primitive
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The referenced project is about an ESP8266 chip, not an Arduino itself.
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curve25519-donna-c64.c Ignore the top bit in curve points. Jun 7, 2014
curve25519-donna.c
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test-sc-curve25519.c x86-64: Add a test for constant timing Aug 20, 2008
test-sc-curve25519.s

README.md

curve25519-donna

Note: this code is from 2008. Since that time, many more, great implementations of curve25519 have been written, including several amd64 assembly versions by djb. You are probably better served now by NaCl or libsodium.

curve25519 is an elliptic curve, developed by Dan Bernstein, for fast Diffie-Hellman key agreement. DJB's original implementation was written in a language of his own devising called qhasm. The original qhasm source isn't available, only the x86 32-bit assembly output.

Since many x86 systems are now 64-bit, and portability is important, this project provides alternative implementations for other platforms.

Implementation Platform Author 32-bit speed 64-bit speed Constant Time
curve25519 x86 32-bit djb 265µs N/A yes
curve25519-donna-c64 64-bit C agl N/A 215µs yes
curve25591-donna Portable C agl 2179µs 610µs

(All tests run on a 2.33GHz Intel Core2)

Usage

The usage is exactly the same as djb's code (as described at http://cr.yp.to/ecdh.html) except that the function is called curve25519\_donna.

To generate a private key, generate 32 random bytes and:

mysecret[0] &= 248;
mysecret[31] &= 127;
mysecret[31] |= 64;

To generate the public key, just do:

static const uint8_t basepoint[32] = {9};
curve25519_donna(mypublic, mysecret, basepoint);

To generate a shared key do:

uint8_t shared_key[32];
curve25519_donna(shared_key, mysecret, theirpublic);

And hash the shared\_key with a cryptographic hash function before using.

For more information, see djb's page.

Building

If you run make, two .a archives will be built, similar to djb's curve25519 code. Alternatively, read on:

ESP8266

If you're interested in running curve25519 on an ESP8266, see this project.

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