| 1 | // Package seal is the app's only encryption surface. |
| 2 | // |
| 3 | // The one rule (spec §0.1): a meeting's title is sealed at rest, and |
| 4 | // the database never holds a plaintext title or an outsider's address. |
| 5 | // The DEK arrives as platform config and is never written to the |
| 6 | // database it decrypts. |
| 7 | // |
| 8 | // Lifted from calendar's seal package, itself lifted from sheets', with |
| 9 | // only the label changed. Calendar names the third app as the trigger |
| 10 | // to extract this into estilo or rastrillo; Calls is that app, and the |
| 11 | // extraction is a task on the plan, not a change here. |
| 12 | package seal |
| 13 | |
| 14 | import ( |
| 15 | "crypto/aes" |
| 16 | "crypto/cipher" |
| 17 | "crypto/rand" |
| 18 | "crypto/sha256" |
| 19 | "errors" |
| 20 | "hash" |
| 21 | "io" |
| 22 | |
| 23 | "golang.org/x/crypto/hkdf" |
| 24 | ) |
| 25 | |
| 26 | // label is mixed into every derived key. Changing it makes every |
| 27 | // existing sealed value unreadable, so it is a constant, not config. |
| 28 | const label = "oficina/calls:" |
| 29 | |
| 30 | const ( |
| 31 | PurposeMeetingTitle = "meeting.title" |
| 32 | PurposeMeetingLink = "meeting.link" |
| 33 | PurposePendingAddress = "pending.address" |
| 34 | ) |
| 35 | |
| 36 | func sha256New() hash.Hash { return sha256.New() } |
| 37 | |
| 38 | type Sealer struct{ dek []byte } |
| 39 | |
| 40 | // New refuses anything but a 32-byte key and keeps its own copy. |
| 41 | func New(dek []byte) (*Sealer, error) { |
| 42 | if len(dek) != 32 { |
| 43 | return nil, errors.New("seal: the DEK must be exactly 32 bytes") |
| 44 | } |
| 45 | k := make([]byte, 32) |
| 46 | copy(k, dek) |
| 47 | return &Sealer{dek: k}, nil |
| 48 | } |
| 49 | |
| 50 | func (s *Sealer) purposeKey(purpose string) ([]byte, error) { |
| 51 | key := make([]byte, 32) |
| 52 | r := hkdf.New(sha256New, s.dek, nil, []byte(label+purpose)) |
| 53 | if _, err := io.ReadFull(r, key); err != nil { |
| 54 | return nil, err |
| 55 | } |
| 56 | return key, nil |
| 57 | } |
| 58 | |
| 59 | func (s *Sealer) aeadFor(purpose string) (cipher.AEAD, error) { |
| 60 | key, err := s.purposeKey(purpose) |
| 61 | if err != nil { |
| 62 | return nil, err |
| 63 | } |
| 64 | block, err := aes.NewCipher(key) |
| 65 | if err != nil { |
| 66 | return nil, err |
| 67 | } |
| 68 | return cipher.NewGCM(block) |
| 69 | } |
| 70 | |
| 71 | // Seal returns nonce || ciphertext. |
| 72 | func (s *Sealer) Seal(purpose string, plaintext []byte) ([]byte, error) { |
| 73 | aead, err := s.aeadFor(purpose) |
| 74 | if err != nil { |
| 75 | return nil, err |
| 76 | } |
| 77 | nonce := make([]byte, aead.NonceSize()) |
| 78 | if _, err := rand.Read(nonce); err != nil { |
| 79 | return nil, err |
| 80 | } |
| 81 | return aead.Seal(nonce, nonce, plaintext, nil), nil |
| 82 | } |
| 83 | |
| 84 | func (s *Sealer) Open(purpose string, ct []byte) ([]byte, error) { |
| 85 | aead, err := s.aeadFor(purpose) |
| 86 | if err != nil { |
| 87 | return nil, err |
| 88 | } |
| 89 | if len(ct) < aead.NonceSize() { |
| 90 | return nil, errors.New("seal: ciphertext too short") |
| 91 | } |
| 92 | nonce, body := ct[:aead.NonceSize()], ct[aead.NonceSize():] |
| 93 | return aead.Open(nil, nonce, body, nil) |
| 94 | } |
| 95 | |
| 96 | // SealString seals a string; an empty string seals to nil so an |
| 97 | // optional column stays NULL. |
| 98 | func (s *Sealer) SealString(purpose, plaintext string) ([]byte, error) { |
| 99 | if plaintext == "" { |
| 100 | return nil, nil |
| 101 | } |
| 102 | return s.Seal(purpose, []byte(plaintext)) |
| 103 | } |
| 104 | |
| 105 | func (s *Sealer) OpenString(purpose string, ct []byte) (string, error) { |
| 106 | if len(ct) == 0 { |
| 107 | return "", nil |
| 108 | } |
| 109 | b, err := s.Open(purpose, ct) |
| 110 | if err != nil { |
| 111 | return "", err |
| 112 | } |
| 113 | return string(b), nil |
| 114 | } |
| 115 | |
amadan