| 1 | // Package seal is the app's only encryption surface. |
| 2 | // |
| 3 | // The one rule: a sheet's contents are sealed at rest, and the bucket never |
| 4 | // holds a plaintext cell. The DEK arrives as platform config and is never |
| 5 | // written to the database it decrypts. |
| 6 | // |
| 7 | // Purpose keys are derived with HKDF so that a value sealed as a cell input |
| 8 | // cannot be opened as a cached value. That is not paranoia: the two are |
| 9 | // adjacent columns of the same row, and a bug that swapped them would be |
| 10 | // invisible without this. |
| 11 | package seal |
| 12 | |
| 13 | import ( |
| 14 | "crypto/aes" |
| 15 | "crypto/cipher" |
| 16 | "crypto/rand" |
| 17 | "crypto/sha256" |
| 18 | "errors" |
| 19 | "hash" |
| 20 | "io" |
| 21 | |
| 22 | "golang.org/x/crypto/hkdf" |
| 23 | ) |
| 24 | |
| 25 | func sha256New() hash.Hash { return sha256.New() } |
| 26 | |
| 27 | // Sealer holds the data encryption key. One per process. |
| 28 | type Sealer struct{ dek []byte } |
| 29 | |
| 30 | // New returns a Sealer. The DEK must be exactly 32 bytes. |
| 31 | func New(dek []byte) (*Sealer, error) { |
| 32 | if len(dek) != 32 { |
| 33 | return nil, errors.New("seal: DEK must be 32 bytes") |
| 34 | } |
| 35 | c := make([]byte, 32) |
| 36 | copy(c, dek) |
| 37 | return &Sealer{dek: c}, nil |
| 38 | } |
| 39 | |
| 40 | // purposeKey derives the AEAD key for purpose via HKDF over the DEK. A |
| 41 | // fixed 32-byte read is always inside HKDF-SHA256's 8160-byte expand |
| 42 | // limit, so this cannot fail today — but the error is still checked and |
| 43 | // propagated: this is the app's only encryption surface, and a silently |
| 44 | // discarded read here would degrade into a short or zero-padded key if |
| 45 | // the key length ever changed, which is the kind of bug that leaves |
| 46 | // ciphertext looking perfectly fine while being trivially breakable. |
| 47 | func (s *Sealer) purposeKey(purpose string) ([]byte, error) { |
| 48 | key := make([]byte, 32) |
| 49 | r := hkdf.New(sha256New, s.dek, nil, []byte("oficina/sheets:"+purpose)) |
| 50 | if _, err := io.ReadFull(r, key); err != nil { |
| 51 | return nil, err |
| 52 | } |
| 53 | return key, nil |
| 54 | } |
| 55 | |
| 56 | // aeadFor derives the purpose key and returns its AEAD. |
| 57 | func (s *Sealer) aeadFor(purpose string) (cipher.AEAD, error) { |
| 58 | key, err := s.purposeKey(purpose) |
| 59 | if err != nil { |
| 60 | return nil, err |
| 61 | } |
| 62 | block, err := aes.NewCipher(key) |
| 63 | if err != nil { |
| 64 | return nil, err |
| 65 | } |
| 66 | return cipher.NewGCM(block) |
| 67 | } |
| 68 | |
| 69 | // Seal encrypts plaintext under purpose. The nonce is prepended. |
| 70 | func (s *Sealer) Seal(purpose string, plaintext []byte) ([]byte, error) { |
| 71 | aead, err := s.aeadFor(purpose) |
| 72 | if err != nil { |
| 73 | return nil, err |
| 74 | } |
| 75 | nonce := make([]byte, aead.NonceSize()) |
| 76 | if _, err := rand.Read(nonce); err != nil { |
| 77 | return nil, err |
| 78 | } |
| 79 | return aead.Seal(nonce, nonce, plaintext, nil), nil |
| 80 | } |
| 81 | |
| 82 | // Open decrypts a value sealed under the same purpose. |
| 83 | func (s *Sealer) Open(purpose string, ct []byte) ([]byte, error) { |
| 84 | aead, err := s.aeadFor(purpose) |
| 85 | if err != nil { |
| 86 | return nil, err |
| 87 | } |
| 88 | if len(ct) < aead.NonceSize() { |
| 89 | return nil, errors.New("seal: ciphertext too short") |
| 90 | } |
| 91 | nonce, body := ct[:aead.NonceSize()], ct[aead.NonceSize():] |
| 92 | return aead.Open(nil, nonce, body, nil) |
| 93 | } |
| 94 | |
amadan