# Sortix nightly manual

This manual documents Sortix nightly, a development build that has not been officially released. You can instead view this document in the latest official manual.

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

# NAME

`X25519`

,
`X25519_keypair`

—
Elliptic Curve Diffie-Hellman primitive based on
Curve25519

# SYNOPSIS

```
#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]`);

# DESCRIPTION

Curve25519 is an elliptic curve over a prime field specified in RFC 7748. 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.
# RETURN VALUES

`X25519`

() returns 1 on success or 0 on error.
Failure can occur when the input is a point of small order.
# SEE ALSO

ECDH_compute_key(3) D. 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.# STANDARDS

RFC 7748: Elliptic Curves for SecurityFebruary 18, 2022 | Debian |