In cryptography, Curve25519 is an elliptic curve offering 128 bits of security and designed for use with the elliptic curve Diffie–Hellman (ECDH) key agreement scheme. It is one of the fastest ECC curves; it is not covered by any known patents, and it is less susceptible to weak random-number generators. The reference implementation is public domain software.
Contents
The original Curve25519 paper defined it as a Diffie–Hellman (DH) function. Daniel J. Bernstein has since proposed that the name Curve25519 be used for the underlying curve, and the name X25519 for the DH function.
Mathematical properties
The curve used is y2 = x3 + 486662x2 + x, a Montgomery curve, over the prime field defined by the prime number 2255 − 19, and it uses the base point x = 9. The protocol uses compressed elliptic point (only X coordinates), so it allows efficient use of the Montgomery ladder for ECDH, using only XZ coordinates.
Curve25519 is constructed such that it avoids many potential implementation pitfalls. By design, it avoids many side-channel attacks and issues with poor-quality random-number generators.
The curve is birationally equivalent to a twisted Edwards curve used in Ed25519 signature scheme.
Popularity
Curve25519 was first released by Daniel J. Bernstein in 2005, but interest increased considerably after 2013 when it was discovered that the NSA had implemented a backdoor into Dual_EC_DRBG. While not directly related, suspicious aspects of the NIST's P curve constants led to concerns that the NSA had chosen values that gave them an advantage in factoring public keys.
I no longer trust the constants. I believe the NSA has manipulated them through their relationships with industry
Since then, Curve25519 has become the de facto alternative to P-256, and is used in a wide variety of applications. In 2014 OpenSSH defaults to Curve25519-based ECDH.