| 1 | package store |
| 2 | |
| 3 | import ( |
| 4 | "context" |
| 5 | "crypto/rand" |
| 6 | "crypto/sha256" |
| 7 | "errors" |
| 8 | "time" |
| 9 | |
| 10 | "gorm.io/gorm" |
| 11 | |
| 12 | "github.com/carlosframework/contra/internal/contra/crypto" |
| 13 | "github.com/carlosframework/contra/internal/contra/model" |
| 14 | ) |
| 15 | |
| 16 | // LinkTTL is how long a handoff code lives. Ten minutes is PROTOCOL.md's, and |
| 17 | // it is the number doing most of the work here: the code is short enough to |
| 18 | // read aloud, so its safety comes from the window and from both ends being |
| 19 | // signed in, not from its length. |
| 20 | const LinkTTL = 10 * time.Minute |
| 21 | |
| 22 | // LinkCodeSymbols is the normalised length of a handoff code: eight Crockford |
| 23 | // symbols, drawn from the 32-character alphabet, so exactly 40 bits with no |
| 24 | // modulo bias to reason about. slopbox's 31-character alphabet needed a |
| 25 | // rejection loop it did not have. |
| 26 | const LinkCodeSymbols = 8 |
| 27 | |
| 28 | // MaxLiveLinks caps how many handoffs one account can have open at once. |
| 29 | // Without it, an attacker holding a session could open codes until one of |
| 30 | // them collided with something a person was about to approve. |
| 31 | const MaxLiveLinks = 5 |
| 32 | |
| 33 | var ( |
| 34 | ErrNoLink = errors.New("store: no such link code, or it expired") |
| 35 | ErrLinkUsed = errors.New("store: that link has already been approved") |
| 36 | ErrTooManyLinks = errors.New("store: too many device links open for this account") |
| 37 | ) |
| 38 | |
| 39 | // LinkCodeHash is the only form of the code the server ever holds. The |
| 40 | // caller normalises first — the same Crockford normalisation the printed |
| 41 | // codes use, so O/I/L confusions are resolved identically on both screens. |
| 42 | func LinkCodeHash(normalized string) []byte { |
| 43 | sum := sha256.Sum256([]byte(normalized)) |
| 44 | return sum[:] |
| 45 | } |
| 46 | |
| 47 | // StartLink records a fresh device's ephemeral public key against the hash of |
| 48 | // a code that device minted. The code itself is never sent here. |
| 49 | // |
| 50 | // It is the client that mints the code, not the server. A server-chosen code |
| 51 | // would be a value the server knows in full for every live handoff; this way |
| 52 | // the only copies are on the two screens. |
| 53 | func (s *Store) StartLink(ctx context.Context, id, codeHash, accountID, |
| 54 | devicePublic []byte, now time.Time) (*model.DeviceLink, error) { |
| 55 | if len(id) != 16 || len(codeHash) != sha256.Size || len(accountID) != 16 { |
| 56 | return nil, ErrShortField |
| 57 | } |
| 58 | if len(devicePublic) != crypto.PublicKeyLen || devicePublic[0] != 0x04 { |
| 59 | return nil, ErrShortField |
| 60 | } |
| 61 | |
| 62 | var live int64 |
| 63 | if err := s.db.WithContext(ctx).Model(&model.DeviceLink{}). |
| 64 | Where("account_id = ? AND expires_at > ?", accountID, now). |
| 65 | Count(&live).Error; err != nil { |
| 66 | return nil, err |
| 67 | } |
| 68 | if live >= MaxLiveLinks { |
| 69 | return nil, ErrTooManyLinks |
| 70 | } |
| 71 | |
| 72 | link := &model.DeviceLink{ |
| 73 | ID: id, |
| 74 | CodeHash: codeHash, |
| 75 | AccountID: accountID, |
| 76 | DevicePublic: devicePublic, |
| 77 | ExpiresAt: now.Add(LinkTTL), |
| 78 | } |
| 79 | if err := s.db.WithContext(ctx).Create(link).Error; err != nil { |
| 80 | return nil, err |
| 81 | } |
| 82 | return link, nil |
| 83 | } |
| 84 | |
| 85 | // PendingLink resolves a code for the account that owns it. Scoping the |
| 86 | // lookup to the session's account is what makes a short code safe: a stranger |
| 87 | // cannot reach the row at all, so the code is confirming which device rather |
| 88 | // than proving who is asking. |
| 89 | func (s *Store) PendingLink(ctx context.Context, accountID, codeHash []byte, |
| 90 | now time.Time) (*model.DeviceLink, error) { |
| 91 | var link model.DeviceLink |
| 92 | err := s.db.WithContext(ctx). |
| 93 | Where("code_hash = ? AND account_id = ? AND expires_at > ?", |
| 94 | codeHash, accountID, now). |
| 95 | Take(&link).Error |
| 96 | if errors.Is(err, gorm.ErrRecordNotFound) { |
| 97 | return nil, ErrNoLink |
| 98 | } |
| 99 | if err != nil { |
| 100 | return nil, err |
| 101 | } |
| 102 | return &link, nil |
| 103 | } |
| 104 | |
| 105 | // ApproveLink stores the sealed envelope. The `sealed IS NULL OR sealed = x”` |
| 106 | // precondition is what makes approval single-use: a second approval of the |
| 107 | // same code cannot overwrite the first, so a live handoff cannot be spent on |
| 108 | // a payload the waiting device can never open. |
| 109 | func (s *Store) ApproveLink(ctx context.Context, accountID, codeHash, |
| 110 | sealed []byte, now time.Time) error { |
| 111 | if len(sealed) < crypto.PublicKeyLen+crypto.NonceLen { |
| 112 | return ErrShortField |
| 113 | } |
| 114 | res := s.db.WithContext(ctx).Model(&model.DeviceLink{}). |
| 115 | Where("code_hash = ? AND account_id = ? AND expires_at > ?"+ |
| 116 | " AND (sealed IS NULL OR sealed = x'')", codeHash, accountID, now). |
| 117 | Update("sealed", sealed) |
| 118 | if res.Error != nil { |
| 119 | return res.Error |
| 120 | } |
| 121 | if res.RowsAffected == 0 { |
| 122 | // Either there is no such live link, or it is already approved. Both |
| 123 | // are refusals; telling them apart would say whether a code exists. |
| 124 | return ErrNoLink |
| 125 | } |
| 126 | return nil |
| 127 | } |
| 128 | |
| 129 | // ClaimLink hands the envelope to the waiting device and deletes the row in |
| 130 | // the same transaction. Single use: the envelope leaves exactly once, so a |
| 131 | // second tab — or a second visit to a URL somebody pasted — gets nothing. |
| 132 | // |
| 133 | // A link that exists but is not approved yet returns (nil, nil), because |
| 134 | // "keep waiting" is not an error. |
| 135 | func (s *Store) ClaimLink(ctx context.Context, accountID, codeHash []byte, |
| 136 | now time.Time) ([]byte, error) { |
| 137 | var sealed []byte |
| 138 | err := s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error { |
| 139 | var link model.DeviceLink |
| 140 | err := tx.Where("code_hash = ? AND account_id = ? AND expires_at > ?", |
| 141 | codeHash, accountID, now).Take(&link).Error |
| 142 | if errors.Is(err, gorm.ErrRecordNotFound) { |
| 143 | return ErrNoLink |
| 144 | } |
| 145 | if err != nil { |
| 146 | return err |
| 147 | } |
| 148 | if len(link.Sealed) == 0 { |
| 149 | return nil |
| 150 | } |
| 151 | if err := tx.Delete(&model.DeviceLink{}, "id = ?", link.ID).Error; err != nil { |
| 152 | return err |
| 153 | } |
| 154 | sealed = link.Sealed |
| 155 | return nil |
| 156 | }) |
| 157 | if err != nil { |
| 158 | return nil, err |
| 159 | } |
| 160 | return sealed, nil |
| 161 | } |
| 162 | |
| 163 | // CancelLink drops a link the waiting device gave up on, so a person who |
| 164 | // closed the tab does not leave a live code behind them. |
| 165 | func (s *Store) CancelLink(ctx context.Context, accountID, codeHash []byte) error { |
| 166 | return s.db.WithContext(ctx). |
| 167 | Where("code_hash = ? AND account_id = ?", codeHash, accountID). |
| 168 | Delete(&model.DeviceLink{}).Error |
| 169 | } |
| 170 | |
| 171 | // SweepLinks deletes expired links. slopbox has no sweeper and its rows |
| 172 | // accumulate forever; they are inert, because every query carries the expiry, |
| 173 | // but a table of them is a log of when each account enrolled a device and |
| 174 | // there is no reason to keep one. |
| 175 | func (s *Store) SweepLinks(ctx context.Context, now time.Time) (int64, error) { |
| 176 | res := s.db.WithContext(ctx). |
| 177 | Where("expires_at <= ?", now).Delete(&model.DeviceLink{}) |
| 178 | return res.RowsAffected, res.Error |
| 179 | } |
| 180 | |
| 181 | // NewLinkID mints the row's primary key. It is random and carries no meaning, |
| 182 | // like every other id in this schema. |
| 183 | func NewLinkID() ([]byte, error) { |
| 184 | id := make([]byte, 16) |
| 185 | if _, err := rand.Read(id); err != nil { |
| 186 | return nil, err |
| 187 | } |
| 188 | return id, nil |
| 189 | } |
| 190 | |
amadan