X25519(3) | Library Functions Manual | X25519(3)

# NAME

`X25519`, `X25519_keypair`, `ED25519_keypair`, `ED25519_sign`, `ED25519_verify` — Elliptic Curve Diffie-Hellman and signature primitives based on Curve25519

# SYNOPSIS

library “libcrypto”
`#include <openssl/curve25519.h>`

int
`X25519`(uint8_t out_shared_key[X25519_KEY_LENGTH], const uint8_t private_key[X25519_KEY_LENGTH], const uint8_t peer_public_value[X25519_KEY_LENGTH]);

void
`X25519_keypair`(uint8_t out_public_value[X25519_KEY_LENGTH], uint8_t out_private_key[X25519_KEY_LENGTH]);

void
`ED25519_keypair`(uint8_t out_public_key[ED25519_PUBLIC_KEY_LENGTH], uint8_t out_private_key[ED25519_PRIVATE_KEY_LENGTH]);

int
`ED25519_sign`(uint8_t *out_sig, const uint8_t *message, size_t message_len, const uint8_t public_key[ED25519_PUBLIC_KEY_LENGTH], const uint8_t private_key_seed[ED25519_PRIVATE_KEY_LENGTH]);

int
`ED25519_verify`(const uint8_t *message, size_t message_len, const uint8_t signature[ED25519_SIGNATURE_LENGTH], const uint8_t public_key[ED25519_PUBLIC_KEY_LENGTH]);

# DESCRIPTION

Curve25519 is an elliptic curve over a prime field specified in RFC 7748 section 4.1. The prime field is defined by the prime number 2^255 - 19.

X25519 is the Diffie-Hellman primitive built from Curve25519 as described in RFC 7748 section 5. Section 6.1 describes the intended use in an Elliptic Curve Diffie-Hellman (ECDH) protocol.

`X25519`() writes a shared key to out_shared_key that is calculated from the given private_key and the peer_public_value by scalar multiplication. Do not use the shared key directly, rather use a key derivation function and also include the two public values as inputs.

`X25519_keypair`() sets out_public_value and out_private_key to a freshly generated public/private key pair. First, the out_private_key is generated with arc4random_buf(3). Then, the opposite of the masking described in RFC 7748 section 5 is applied to it to make sure that the generated private key is never correctly masked. The purpose is to cause incorrect implementations on the peer side to consistently fail. Correct implementations will decode the key correctly even when it is not correctly masked. Finally, the out_public_value is calculated from the out_private_key by multiplying it with the Montgomery base point uint8_t u[32] = {9}.

The size of a public and private key is `X25519_KEY_LENGTH` = 32 bytes each.

Ed25519 is a signature scheme using a twisted Edwards curve that is birationally equivalent to Curve25519.

`ED25519_keypair`() sets out_public_key and out_private_key to a freshly generated public/private key pair. First, the out_private_key is generated with arc4random_buf(3). Then, the out_public_key is calculated from the private key.

`ED25519_sign`() signs the message of message_len bytes using the public_key and the private_key and writes the signature to out_sig.

`ED25519_verify`() checks that signing the message of message_len bytes using the public_key would indeed result in the given signature.

The sizes of a public and private keys are `ED25519_PUBLIC_KEY_LENGTH` and `ED25519_PRIVATE_KEY_LENGTH`, which are both 32 bytes, and the size of a signature is `ED25519_SIGNATURE_LENGTH` = 64 bytes.

# RETURN VALUES

`X25519`() and `ED25519_sign`() return 1 on success or 0 on error. `X25519`() can fail if the input is a point of small order. `ED25519_sign`() always succeeds in LibreSSL, but the API reserves the return value 0 for memory allocation failure.

`ED25519_verify`() returns 1 if the signature is valid or 0 otherwise.

# SEE ALSO

ECDH_compute_key(3), EVP_DigestSign(3), EVP_DigestVerify(3), EVP_PKEY_derive(3), EVP_PKEY_keygen(3)

Daniel J. Bernstein, A state-of-the-art Diffie-Hellman function: How do I use Curve25519 in my own software?, https://cr.yp.to/ecdh.html.

Daniel J. Bernstein, Niels Duif, Tanja Lange, Peter Schwabe, and Bo-Yin Yang, High-Speed High-Security Signatures, Cryptographic Hardware and Embedded Systems — CHES 2011, Springer, Lecture Notes in Computer Science, vol 6917, https://doi.org/10.1007/978-3-642-23951-9_9, Nara, Japan, September 29, 2011.

# STANDARDS

RFC 7748: Elliptic Curves for Security

RFC 8032: Edwards-Curve Digital Signature Algorithm (EdDSA)

June 8, 2025 | Debian