package khepri import ( "crypto/aes" "crypto/cipher" "crypto/hmac" "crypto/rand" "crypto/sha256" "encoding/json" "errors" "fmt" "io" "os" "os/user" "time" "github.com/fd0/khepri/backend" "github.com/fd0/khepri/chunker" "golang.org/x/crypto/scrypt" ) // max size is 8MiB, defined in chunker const ivSize = aes.BlockSize const hmacSize = sha256.Size const maxCiphertextSize = ivSize + chunker.MaxSize + hmacSize const CiphertextExtension = ivSize + hmacSize var ( // ErrUnauthenticated is returned when ciphertext verification has failed. ErrUnauthenticated = errors.New("ciphertext verification failed") // ErrNoKeyFound is returned when no key for the repository could be decrypted. ErrNoKeyFound = errors.New("no key could be found") // ErrBufferTooSmall is returned when the destination slice is too small // for the ciphertext. ErrBufferTooSmall = errors.New("destination buffer too small") ) // TODO: figure out scrypt values on the fly depending on the current // hardware. const ( scryptN = 65536 scryptR = 8 scryptP = 1 scryptSaltsize = 64 aesKeysize = 32 // for AES256 hmacKeysize = 32 // for HMAC with SHA256 ) // Key represents an encrypted master key for a repository. type Key struct { Created time.Time `json:"created"` Username string `json:"username"` Hostname string `json:"hostname"` Comment string `json:"comment,omitempty"` KDF string `json:"kdf"` N int `json:"N"` R int `json:"r"` P int `json:"p"` Salt []byte `json:"salt"` Data []byte `json:"data"` user *keys master *keys } // keys is a JSON structure that holds signing and encryption keys. type keys struct { Sign []byte Encrypt []byte } // CreateKey initializes a master key in the given backend and encrypts it with // the password. func CreateKey(be backend.Server, password string) (*Key, error) { // fill meta data about key k := &Key{ Created: time.Now(), KDF: "scrypt", N: scryptN, R: scryptR, P: scryptP, } hn, err := os.Hostname() if err == nil { k.Hostname = hn } usr, err := user.Current() if err == nil { k.Username = usr.Username } // generate random salt k.Salt = make([]byte, scryptSaltsize) n, err := rand.Read(k.Salt) if n != scryptSaltsize || err != nil { panic("unable to read enough random bytes for salt") } // call scrypt() to derive user key k.user, err = k.scrypt(password) if err != nil { return nil, err } // generate new random master keys k.master, err = k.newKeys() if err != nil { return nil, err } // encrypt master keys (as json) with user key buf, err := json.Marshal(k.master) if err != nil { return nil, err } k.Data = GetChunkBuf("key") n, err = k.EncryptUser(k.Data, buf) k.Data = k.Data[:n] // dump as json buf, err = json.Marshal(k) if err != nil { return nil, err } // store in repository and return _, err = be.Create(backend.Key, buf) if err != nil { return nil, err } FreeChunkBuf("key", k.Data) return k, nil } // OpenKey tries do decrypt the key specified by id with the given password. func OpenKey(be backend.Server, id backend.ID, password string) (*Key, error) { // extract data from repo data, err := be.Get(backend.Key, id) if err != nil { return nil, err } // restore json k := &Key{} err = json.Unmarshal(data, k) if err != nil { return nil, err } // check KDF if k.KDF != "scrypt" { return nil, errors.New("only supported KDF is scrypt()") } // derive user key k.user, err = k.scrypt(password) if err != nil { return nil, err } // decrypt master keys buf, err := k.DecryptUser(k.Data) if err != nil { return nil, err } // restore json k.master = &keys{} err = json.Unmarshal(buf, k.master) if err != nil { return nil, err } return k, nil } // SearchKey tries to decrypt all keys in the backend with the given password. // If none could be found, ErrNoKeyFound is returned. func SearchKey(be backend.Server, password string) (*Key, error) { // list all keys ids, err := be.List(backend.Key) if err != nil { panic(err) } // try all keys in repo var key *Key for _, id := range ids { key, err = OpenKey(be, id, password) if err != nil { continue } return key, nil } return nil, ErrNoKeyFound } func (k *Key) scrypt(password string) (*keys, error) { if len(k.Salt) == 0 { return nil, fmt.Errorf("scrypt() called with empty salt") } keybytes := hmacKeysize + aesKeysize scryptKeys, err := scrypt.Key([]byte(password), k.Salt, k.N, k.R, k.P, keybytes) if err != nil { return nil, fmt.Errorf("error deriving keys from password: %v", err) } if len(scryptKeys) != keybytes { return nil, fmt.Errorf("invalid numbers of bytes expanded from scrypt(): %d", len(scryptKeys)) } ks := &keys{ Encrypt: scryptKeys[:aesKeysize], Sign: scryptKeys[aesKeysize:], } return ks, nil } func (k *Key) newKeys() (*keys, error) { ks := &keys{ Encrypt: make([]byte, aesKeysize), Sign: make([]byte, hmacKeysize), } n, err := rand.Read(ks.Encrypt) if n != aesKeysize || err != nil { panic("unable to read enough random bytes for encryption key") } n, err = rand.Read(ks.Sign) if n != hmacKeysize || err != nil { panic("unable to read enough random bytes for signing key") } return ks, nil } func (k *Key) newIV(buf []byte) error { _, err := io.ReadFull(rand.Reader, buf[:ivSize]) buf = buf[:ivSize] if err != nil { return err } return nil } // Encrypt encrypts and signs data. Stored in ciphertext is IV || Ciphertext || // HMAC. Encrypt returns the ciphertext's length. For the hash function, SHA256 // is used, so the overhead is 16+32=48 byte. func (k *Key) encrypt(ks *keys, ciphertext, plaintext []byte) (int, error) { if cap(ciphertext) < len(plaintext)+ivSize+hmacSize { return 0, ErrBufferTooSmall } _, err := io.ReadFull(rand.Reader, ciphertext[:ivSize]) if err != nil { panic(fmt.Sprintf("unable to generate new random iv: %v", err)) } c, err := aes.NewCipher(ks.Encrypt) if err != nil { panic(fmt.Sprintf("unable to create cipher: %v", err)) } e := cipher.NewCTR(c, ciphertext[:ivSize]) e.XORKeyStream(ciphertext[ivSize:cap(ciphertext)], plaintext) ciphertext = ciphertext[:ivSize+len(plaintext)] hm := hmac.New(sha256.New, ks.Sign) n, err := hm.Write(ciphertext) if err != nil || n != len(ciphertext) { panic(fmt.Sprintf("unable to calculate hmac of ciphertext: %v", err)) } ciphertext = hm.Sum(ciphertext) return len(ciphertext), nil } // EncryptUser encrypts and signs data with the user key. Stored in ciphertext // is IV || Ciphertext || HMAC. Returns the ciphertext length. For the hash // function, SHA256 is used, so the overhead is 16+32=48 byte. func (k *Key) EncryptUser(ciphertext, plaintext []byte) (int, error) { return k.encrypt(k.user, ciphertext, plaintext) } // Encrypt encrypts and signs data with the master key. Stored in ciphertext is // IV || Ciphertext || HMAC. Returns the ciphertext length. For the hash // function, SHA256 is used, so the overhead is 16+32=48 byte. func (k *Key) Encrypt(ciphertext, plaintext []byte) (int, error) { return k.encrypt(k.master, ciphertext, plaintext) } // Decrypt verifes and decrypts the ciphertext. Ciphertext must be in the form // IV || Ciphertext || HMAC. func (k *Key) decrypt(ks *keys, ciphertext []byte) ([]byte, error) { // check for plausible length if len(ciphertext) <= ivSize+hmacSize { panic("trying to decryipt invalid data: ciphertext too small") } hm := hmac.New(sha256.New, ks.Sign) // extract hmac l := len(ciphertext) - hm.Size() ciphertext, mac := ciphertext[:l], ciphertext[l:] // calculate new hmac n, err := hm.Write(ciphertext) if err != nil || n != len(ciphertext) { panic(fmt.Sprintf("unable to calculate hmac of ciphertext, err %v", err)) } // verify hmac mac2 := hm.Sum(nil) if !hmac.Equal(mac, mac2) { return nil, ErrUnauthenticated } // extract iv iv, ciphertext := ciphertext[:aes.BlockSize], ciphertext[aes.BlockSize:] // decrypt data c, err := aes.NewCipher(ks.Encrypt) if err != nil { panic(fmt.Sprintf("unable to create cipher: %v", err)) } // decrypt e := cipher.NewCTR(c, iv) plaintext := make([]byte, len(ciphertext)) e.XORKeyStream(plaintext, ciphertext) return plaintext, nil } // Decrypt verifes and decrypts the ciphertext with the master key. Ciphertext // must be in the form IV || Ciphertext || HMAC. func (k *Key) Decrypt(ciphertext []byte) ([]byte, error) { return k.decrypt(k.master, ciphertext) } // DecryptUser verifes and decrypts the ciphertext with the master key. Ciphertext // must be in the form IV || Ciphertext || HMAC. func (k *Key) DecryptUser(ciphertext []byte) ([]byte, error) { return k.decrypt(k.user, ciphertext) } // Each calls backend.Each() with the given parameters, Decrypt() on the // ciphertext and, on successful decryption, f with the plaintext. func (k *Key) Each(be backend.Server, t backend.Type, f func(backend.ID, []byte, error)) error { return backend.Each(be, t, func(id backend.ID, data []byte, e error) { if e != nil { f(id, nil, e) return } buf, err := k.Decrypt(data) if err != nil { f(id, nil, err) return } f(id, buf, nil) }) } func (k *Key) String() string { if k == nil { return "" } return fmt.Sprintf("", k.Username, k.Hostname, k.Created) }