DoubleOdd Elliptic Curves
Doubleodd elliptic curves are a large subset of elliptic curves: these are curves whose order is equal to 2 modulo 4 (i.e. the order is the double of an odd integer). On top of doubleodd elliptic curves, we can define a prime order group suitable for building cryptographic protocols. This is similar to what Ristretto achieves for twisted Edwards curves, albeit with a different method. Some highlights of doubleodd elliptic curves are:

There is no cofactor to deal with. A clean prime order group abstraction is provided, with a canonical encoding for elements which is automatically verified when decoding.

The encoding is economical: for \(n\)bit security, group elements are encoded over \(2n+1\) bits (compared to \(2n+3\) for Ristretto).

All group operations are supported by unified and complete formulas amenable to efficient and secure (constanttime) implementations.

No drastically new cryptographic hypothesis is introduced. Doubleodd elliptic curves are fairly mundane; about 1/4th of all elliptic curves are doubleodd curves. Such curves are considered secure for cryptography, just like any other random elliptic curve with a large enough subgroup of prime order.

The formulas are fast and provide performance at least on par with twisted Edwards curves. In some respects they are faster; e.g. point doublings can be performed, for half of doubleodd curves, with six multiplications (down to 1M+5S for some specific curves).
In order to leverage these techniques, two specific doubleodd elliptic curves are defined: do255e and do255s. Both provide "128bit security" (technically, 127 bits); do255e is a GLV curve that can leverage an internal endomorphism for further improved performance, while do255s is a perfectly ordinary curve.
Resources
The whitepaper contains all the formulas and demonstrations; it also specifies the use of do255e and do255s in higherlevel cryptographic functionalities (key pair generation, key exchange, signature, and hashtocurve). Most of the present site consists in extracts from this whitepaper. Note: the whitepaper is now also published on ePrint.
Several implementations of do255e and do255s exist and are listed in the implementations section.
Outline
Subsequent pages in this site include the following:

A geometrical introduction to doubleodd elliptic curves: a selfcontained explanatory text (with pictures) to explain the motivation and the core ideas of this work.

The formulas section lists useful formulas for computing over points of doubleodd elliptic curves with several convenient systems of coordinates.

Explicit curve choices are listed: these are curves do255e and do255s. Selection criteria and curve parameters are provided.

Available opensource implementations are listed.
About
Doubleodd elliptic curves were designed by Thomas Pornin, who also wrote this Web site, with contributions from Paul Bottinelli, GĂ©rald Doussot and Eric Schorn.