# EdDSA key generation

**Ed25519** and **Ed448** use small **private keys** (32 or 57 bytes respectively), small **public keys** (32 or 57 bytes) and **small signatures** (64 or 114 bytes) with **high security level** at the same time (128-bit or 224-bit respectively).

Assume the elliptic curve for the EdDSA algorithm comes with a generator point **G** and a subgroup order ***q*** for the EC points, generated from **G**.

The **EdDSA key-pair** consists of:

* **private key** (integer): ***privKey***
* **public key** (EC point): ***pubKey*** = ***privKey*** \* **G**

The **private key** is generated from a **random integer**, known as ***seed*** (which should have similar bit length, like the curve order). The ***seed*** is first hashed, then the last few bits, corresponding to the curve **cofactor** (8 for Ed25519 and 4 for X448) are cleared, then the highest bit is cleared and the second highest bit is set. These transformations guarantee that the private key will always belong to the same subgroup of EC points on the curve and that the private keys will always have similar bit length (to protect from timing-based side-channel attacks). For **Ed25519** the private key is 32 bytes. For **Ed448** the private key is 57 bytes.

The public key ***pubKey*** is a point on the elliptic curve, calculated by the EC point multiplication: ***pubKey*** = ***privKey*** \* **G** (the private key, multiplied by the generator point **G** for the curve). The public key is encoded as **compressed** EC point: the **y**-coordinate, combined with the lowest bit (the parity) of the **x**-coordinate. For **Ed25519** the public key is 32 bytes. For **Ed448** the public key is 57 bytes.

[Source](https://cryptobook.nakov.com/digital-signatures/eddsa-and-ed25519)


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