peridot/vendor/github.com/ProtonMail/go-crypto/openpgp/internal/ecc/curve25519.go

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// Package ecc implements a generic interface for ECDH, ECDSA, and EdDSA.
package ecc
import (
"crypto/subtle"
"io"
"github.com/ProtonMail/go-crypto/openpgp/errors"
x25519lib "github.com/cloudflare/circl/dh/x25519"
)
type curve25519 struct{}
func NewCurve25519() *curve25519 {
return &curve25519{}
}
func (c *curve25519) GetCurveName() string {
return "curve25519"
}
// MarshalBytePoint encodes the public point from native format, adding the prefix.
// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.5.5.6
func (c *curve25519) MarshalBytePoint(point []byte) []byte {
return append([]byte{0x40}, point...)
}
// UnmarshalBytePoint decodes the public point to native format, removing the prefix.
// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.5.5.6
func (c *curve25519) UnmarshalBytePoint(point []byte) []byte {
if len(point) != x25519lib.Size+1 {
return nil
}
// Remove prefix
return point[1:]
}
// MarshalByteSecret encodes the secret scalar from native format.
// Note that the EC secret scalar differs from the definition of public keys in
// [Curve25519] in two ways: (1) the byte-ordering is big-endian, which is
// more uniform with how big integers are represented in OpenPGP, and (2) the
// leading zeros are truncated.
// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.5.5.6.1.1
// Note that leading zero bytes are stripped later when encoding as an MPI.
func (c *curve25519) MarshalByteSecret(secret []byte) []byte {
d := make([]byte, x25519lib.Size)
copyReversed(d, secret)
// The following ensures that the private key is a number of the form
// 2^{254} + 8 * [0, 2^{251}), in order to avoid the small subgroup of
// the curve.
//
// This masking is done internally in the underlying lib and so is unnecessary
// for security, but OpenPGP implementations require that private keys be
// pre-masked.
d[0] &= 127
d[0] |= 64
d[31] &= 248
return d
}
// UnmarshalByteSecret decodes the secret scalar from native format.
// Note that the EC secret scalar differs from the definition of public keys in
// [Curve25519] in two ways: (1) the byte-ordering is big-endian, which is
// more uniform with how big integers are represented in OpenPGP, and (2) the
// leading zeros are truncated.
// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh-06#section-5.5.5.6.1.1
func (c *curve25519) UnmarshalByteSecret(d []byte) []byte {
if len(d) > x25519lib.Size {
return nil
}
// Ensure truncated leading bytes are re-added
secret := make([]byte, x25519lib.Size)
copyReversed(secret, d)
return secret
}
// generateKeyPairBytes Generates a private-public key-pair.
// 'priv' is a private key; a little-endian scalar belonging to the set
// 2^{254} + 8 * [0, 2^{251}), in order to avoid the small subgroup of the
// curve. 'pub' is simply 'priv' * G where G is the base point.
// See https://cr.yp.to/ecdh.html and RFC7748, sec 5.
func (c *curve25519) generateKeyPairBytes(rand io.Reader) (priv, pub x25519lib.Key, err error) {
_, err = io.ReadFull(rand, priv[:])
if err != nil {
return
}
x25519lib.KeyGen(&pub, &priv)
return
}
func (c *curve25519) GenerateECDH(rand io.Reader) (point []byte, secret []byte, err error) {
priv, pub, err := c.generateKeyPairBytes(rand)
if err != nil {
return
}
return pub[:], priv[:], nil
}
func (c *genericCurve) MaskSecret(secret []byte) []byte {
return secret
}
func (c *curve25519) Encaps(rand io.Reader, point []byte) (ephemeral, sharedSecret []byte, err error) {
// RFC6637 §8: "Generate an ephemeral key pair {v, V=vG}"
// ephemeralPrivate corresponds to `v`.
// ephemeralPublic corresponds to `V`.
ephemeralPrivate, ephemeralPublic, err := c.generateKeyPairBytes(rand)
if err != nil {
return nil, nil, err
}
// RFC6637 §8: "Obtain the authenticated recipient public key R"
// pubKey corresponds to `R`.
var pubKey x25519lib.Key
copy(pubKey[:], point)
// RFC6637 §8: "Compute the shared point S = vR"
// "VB = convert point V to the octet string"
// sharedPoint corresponds to `VB`.
var sharedPoint x25519lib.Key
x25519lib.Shared(&sharedPoint, &ephemeralPrivate, &pubKey)
return ephemeralPublic[:], sharedPoint[:], nil
}
func (c *curve25519) Decaps(vsG, secret []byte) (sharedSecret []byte, err error) {
var ephemeralPublic, decodedPrivate, sharedPoint x25519lib.Key
// RFC6637 §8: "The decryption is the inverse of the method given."
// All quoted descriptions in comments below describe encryption, and
// the reverse is performed.
// vsG corresponds to `VB` in RFC6637 §8 .
// RFC6637 §8: "VB = convert point V to the octet string"
copy(ephemeralPublic[:], vsG)
// decodedPrivate corresponds to `r` in RFC6637 §8 .
copy(decodedPrivate[:], secret)
// RFC6637 §8: "Note that the recipient obtains the shared secret by calculating
// S = rV = rvG, where (r,R) is the recipient's key pair."
// sharedPoint corresponds to `S`.
x25519lib.Shared(&sharedPoint, &decodedPrivate, &ephemeralPublic)
return sharedPoint[:], nil
}
func (c *curve25519) ValidateECDH(point []byte, secret []byte) (err error) {
var pk, sk x25519lib.Key
copy(sk[:], secret)
x25519lib.KeyGen(&pk, &sk)
if subtle.ConstantTimeCompare(point, pk[:]) == 0 {
return errors.KeyInvalidError("ecc: invalid curve25519 public point")
}
return nil
}
func copyReversed(out []byte, in []byte) {
l := len(in)
for i := 0; i < l; i++ {
out[i] = in[l-i-1]
}
}