NAME
X25519,
X25519_keypair,
ED25519_keypair,
ED25519_sign, ED25519_verify
— Elliptic Curve Diffie-Hellman
and signature primitives 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]);
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)