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// Package kdf provides a key derivation function to calculate key output
// and negotiate shared secrets for curve X25519 keys.
package kdf
import (
"crypto/sha256"
"io"
"golang.org/x/crypto/curve25519"
"golang.org/x/crypto/hkdf"
)
// HKDF is a hashed key derivation function type that can be used to derive keys.
type HKDF func(inputKeyMaterial, salt, info []byte, outputLength int) ([]byte, error)
// DeriveSecrets derives the requested number of bytes using HKDF with the given
// input, salt, and info.
func DeriveSecrets(inputKeyMaterial, salt, info []byte, outputLength int) ([]byte, error) {
kdf := hkdf.New(sha256.New, inputKeyMaterial, salt, info)
secrets := make([]byte, outputLength)
length, err := io.ReadFull(kdf, secrets)
if err != nil {
return nil, err
}
if length != outputLength {
return nil, err
}
return secrets, nil
}
// CalculateSharedSecret uses DH Curve25519 to find a shared secret. The result of this function
// should be used in `DeriveSecrets` to output the Root and Chain keys.
func CalculateSharedSecret(theirKey, ourKey [32]byte) [32]byte {
var sharedSecret [32]byte
curve25519.ScalarMult(&sharedSecret, &ourKey, &theirKey)
return sharedSecret
}
// KeyMaterial is a structure for representing a cipherkey, mac, and iv
type KeyMaterial struct {
CipherKey []byte
MacKey []byte
IV []byte
}
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