What is Ristretto?
Ristretto is a construction of a primeorder group using a nonprimeorder Edwards curve.
The Decaf paper suggests using a nonprimeorder curve \(\mathcal E\) to implement a primeorder group by constructing a quotient group. Ristretto uses the same idea, but with different formulas, in order to allow the use of cofactor\(8\) curves such as Curve25519.
Internally, a Ristretto point is represented by an Edwards point. Two Edwards points \(P, Q\) may represent the same Ristretto point, in the same way that different projective \( X, Y, Z \) coordinates may represent the same Edwards point. Group operations on Ristretto points are carried out with no overhead by performing the operations on the representative Edwards points.
To do this, Ristretto defines:

a new type for Ristretto points which contains the representative Edwards point;

equality on Ristretto points so that all equivalent representatives are considered equal;

an encoding function on Ristretto points so that all equivalent representatives are encoded as identical bitstrings;

a decoding function on bitstrings with builtin validation, so that only the canonical encodings of valid points are accepted;

a map from bitstrings to Ristretto points suitable for hashtopoint operations.
In other words, an existing Edwards curve implementation can implement the correct abstraction for complex protocols just by adding a new type and three or four functions. Moreover, equality checking for the Ristretto group is actually less expensive than equality checking for the underlying curve!
The Ristretto construction is described and justified in detail in the next chapter, and explicit formulas aimed at implementors are in the Explicit Formulas chapter.