# What Is Ed25519 (EdDSA over Curve25519)? Definition and Examples

> Ed25519 is the EdDSA signature scheme on the Edwards form of Curve25519, defined in RFC 8032, with 32-byte public keys and 64-byte deterministic signatures.

Source: https://ssl.sh/glossary/ed25519/
Last updated: 2026-11-10

Ed25519 is an instance of EdDSA, the Edwards-curve Digital Signature Algorithm, specified in [RFC 8032](https://www.rfc-editor.org/rfc/rfc8032). It offers about 128-bit security, is designed for fast constant-time implementation, and derives its per-signature nonce deterministically from the key and message, avoiding the random-nonce failures that have leaked [ECDSA](https://ssl.sh/glossary/ecdsa/) keys. RFC 8410 defines its use in X.509 certificates, TLS 1.3 includes an ed25519 signature scheme, and NIST added EdDSA to FIPS 186-5.

## Why it matters
Ed25519 is the default choice for SSH keys and is common in software signing and other protocols. For public websites the situation differs: as of November 2026, WebPKI CAs issue TLS certificates with RSA and ECDSA (P-256/P-384) keys, so Ed25519 server certificates are found in [private PKI](https://ssl.sh/glossary/private-ca/), where you control both the CA and the clients. Like other elliptic-curve schemes it is not quantum-resistant; [ML-DSA](https://ssl.sh/glossary/ml-dsa/) is the standardized post-quantum signature counterpart.

## Example
Create an Ed25519 key, a self-signed certificate for an internal service, and an SSH key:

```bash
openssl genpkey -algorithm ED25519 -out ed25519.key
openssl req -new -x509 -key ed25519.key -days 30 -subj "/CN=internal.example.com" -out ed25519.crt
ssh-keygen -t ed25519 -C "deploy@example.com"
```

## Common misconceptions
- **"Ed25519 and X25519 are the same thing."** They are built on the same underlying curve but serve different purposes: Ed25519 signs, X25519 performs key exchange.
- **"I can get a publicly trusted Ed25519 certificate for my website."** Public CAs issue RSA and ECDSA certificates for TLS; use Ed25519 within private PKI.
- **"Ed25519 is post-quantum."** It is elliptic-curve cryptography and would fall to Shor's algorithm like ECDSA and RSA.

## Related terms

- [ECC](https://ssl.sh/zh/glossary/ecc/): ECC 即椭圆曲线密码学，是基于有限域上椭圆曲线代数结构的公钥密码技术，能用较短的密钥提供很高的安全强度。
- [ECDSA](https://ssl.sh/zh/glossary/ecdsa/): ECDSA 是基于椭圆曲线的数字签名算法，在 TLS 证书中作为 RSA 的替代方案，密钥和签名更小、签名运算更快。
- [数字签名](https://ssl.sh/zh/glossary/digital-signature/): 数字签名是用私钥对消息计算出的一个值，任何持有对应公钥的人都能验证它，从而确认消息来源和内容未被篡改。
- [P-256](https://ssl.sh/zh/glossary/p-256/): P-256 是 NIST 标准化的 256 位素数域椭圆曲线，也叫 secp256r1 或 prime256v1，广泛用于 ECDSA 证书和 ECDHE 密钥交换。
- [ML-DSA](https://ssl.sh/zh/glossary/ml-dsa/): ML-DSA 是 NIST 于 2024 年 8 月在 FIPS 204 中标准化的基于格的后量子数字签名算法，源自 CRYSTALS-Dilithium。
- [私有 CA](https://ssl.sh/zh/glossary/private-ca/): 私有 CA 是企业为自己的系统和设备搭建的证书颁发机构，其根证书只在企业主动安装的设备上受信任。