monero-docs/docs/cryptography/asymmetric/ed25519.md

# Ed25519 curve

!!! note
    Author is nowhere close to being a cryptographer. Be sceptical on accuracy.

!!! note
    This article is only about the underlying curve. Public key derivation and signing algorithm will be treated separately. 

Monero employs Ed25519 elliptic curve as a basis for its key pair generation.
 
However, Monero does not exactly follow EdDSA reference signature scheme.

## Definition

This is the standard Ed25519 curve definition, no Monero specific stuff here,
except the naming convention. The convention comes from the CryptoNote
whitepaper and is widely used in Monero literature.

### Curve equation

    −x^2 + y^2 = 1 − (121665/121666) * x^2 * y^2

Note:

* curve is in two dimensions (nothing fancy, like all the curves is high school)
* curve is mirrored below y axis due to `y^2` part of the equation (not a polynomial)


### Base point `G`
 
The base point is a specific point on the curve. It is used
as a basis for further calculations. It is an arbitrary choice
by the curve authors, just to standardize the scheme.
 
Note that it is enough to specify the y value and the sign of the x value.
That's because the specific x can be calculated from the curve equation.
    
    G = (x, 4/5)  # take the point with the positive x
    
    # The hex representation of the base point
    5866666666666666666666666666666666666666666666666666666666666666    

### Prime order of the base point `l`

In layment terms, the "canvas" where the curve is drawn is assumed
to have a finite "resolution", so point coordinates must "wrap around"
at some point. This is achieved by modulo the `l` value (lowercase L).
In other words, the `l` defines the maximum scalar we can use.

    l = 2^252 + 27742317777372353535851937790883648493

The `l` is a prime number specified by the curve authors.

In practice this is the private key's strength.

### Total number of points on the curve

The total number of points on the curve is also a prime number:

    q = 2^255 - 19

In practice not all points are "useful" and so the private key strength is limited to `l` describe above.

## Implementation

Monero uses (apparently modified) Ref10 implementation by Daniel J. Bernstein.

## Reference

* [Understanding Monero Cryptography](https://steemit.com/monero/@luigi1111/understanding-monero-cryptography-privacy-introduction) - excellent writeup by Luigi
* [StackOverflow answer](https://monero.stackexchange.com/questions/2290/why-how-does-monero-generate-public-ed25519-keys-without-using-the-standard-publ)
* [Python implementation](https://github.com/monero-project/mininero/blob/master/ed25519.py) - not the reference one but easier to understand
* [Encoding point to hex](https://monero.stackexchange.com/questions/6050/what-is-the-base-point-g-from-the-whitepaper-and-how-is-it-represented-as-a)
* [Ed25519 on Wikipedia](https://en.wikipedia.org/wiki/EdDSA#Ed25519)
* [A (Relatively Easy To Understand) Primer on Elliptic Curve Cryptography](https://blog.cloudflare.com/a-relatively-easy-to-understand-primer-on-elliptic-curve-cryptography/)