| 1 | package docs |
| 2 | |
| 3 | import ( |
| 4 | "context" |
| 5 | "crypto/rand" |
| 6 | "crypto/sha256" |
| 7 | "encoding/base64" |
| 8 | "encoding/hex" |
| 9 | "time" |
| 10 | |
| 11 | "gorm.io/gorm" |
| 12 | ) |
| 13 | |
| 14 | // Link is one document's read-only public capability: whoever holds the |
| 15 | // token may read the document, signed in or not. |
| 16 | // |
| 17 | // The third rung of the suite doctrine's ruling 15, and the reason it has |
| 18 | // no Role field is the ruling's other half — every write has a named |
| 19 | // actor, and a link holds no name. See 0006_links.sql. |
| 20 | type Link struct { |
| 21 | ID int64 `gorm:"column:id;primaryKey"` |
| 22 | DocID int64 `gorm:"column:doc_id;uniqueIndex"` |
| 23 | // TokenHash, never the token (suite doctrine ruling 16). There is no |
| 24 | // field on this struct that can yield a working URL, which is the |
| 25 | // point: nothing that reads a Link out of the database — a query, a |
| 26 | // log line, a debugger — is holding a capability. |
| 27 | TokenHash string `gorm:"column:token_hash;uniqueIndex"` |
| 28 | CreatedAt time.Time `gorm:"column:created_at"` |
| 29 | } |
| 30 | |
| 31 | // TableName pins the table rather than leaving it to GORM's pluraliser, |
| 32 | // the same reason Grant does. |
| 33 | func (Link) TableName() string { return "links" } |
| 34 | |
| 35 | // tokenBytes is 128 bits of entropy, which base64url renders as 22 |
| 36 | // characters. The token is the entire authorization for a published |
| 37 | // document, so this is the one number in this file worth arguing about: |
| 38 | // 128 bits is the floor at which guessing stops being a strategy, and a |
| 39 | // URL a person pastes into a message is not improved by being longer. |
| 40 | const tokenBytes = 16 |
| 41 | |
| 42 | // newToken mints a capability. crypto/rand, never math/rand, and never |
| 43 | // anything derived from the document — a token computed from doc_id, sqid, |
| 44 | // title or a clock is guessable by whoever reads this file. |
| 45 | // |
| 46 | // rand.Read is documented never to return a short read without an error, |
| 47 | // so an error here means the system has no entropy and the honest response |
| 48 | // is to fail the request rather than publish under a weak token. |
| 49 | func newToken() (string, error) { |
| 50 | buf := make([]byte, tokenBytes) |
| 51 | if _, err := rand.Read(buf); err != nil { |
| 52 | return "", err |
| 53 | } |
| 54 | return base64.RawURLEncoding.EncodeToString(buf), nil |
| 55 | } |
| 56 | |
| 57 | // hashToken is the one-way form stored in the database. |
| 58 | // |
| 59 | // SHA-256, unsalted, and both halves of that are deliberate. A salt |
| 60 | // defends against precomputation over a small input space; there is no |
| 61 | // small input space here, because the token is 128 bits from crypto/rand |
| 62 | // and is its own salt. A password hash (bcrypt, argon2) would buy the same |
| 63 | // nothing while making every load of a published page do key-stretching |
| 64 | // work. The property that matters — the stored form does not yield the |
| 65 | // token — a fast hash gives just as completely. |
| 66 | // |
| 67 | // Hex rather than base64 for the stored form: it is fixed-width, it is |
| 68 | // what a person comparing rows by eye expects of a SHA-256, and the column |
| 69 | // is never parsed by anything but this function. |
| 70 | func hashToken(token string) string { |
| 71 | sum := sha256.Sum256([]byte(token)) |
| 72 | return hex.EncodeToString(sum[:]) |
| 73 | } |
| 74 | |
| 75 | // Publish turns a document's public link on and returns it, minting a |
| 76 | // fresh token every time. |
| 77 | // |
| 78 | // Fresh every time, deliberately: with Unpublish deleting the row, that |
| 79 | // makes "off" a revocation rather than a pause. Somebody who kept a URL |
| 80 | // from an earlier audience does not get back in when the owner publishes |
| 81 | // again for a different one. |
| 82 | // |
| 83 | // Owner-only through Get, so a writer grantee is answered exactly as a |
| 84 | // stranger is — no grant carries re-sharing, and publishing to everyone is |
| 85 | // the largest re-share there is. |
| 86 | func (s *Store) Publish(ctx context.Context, owner int64, sqid string) (string, error) { |
| 87 | doc, err := s.Get(ctx, owner, sqid) |
| 88 | if err != nil { |
| 89 | return "", err |
| 90 | } |
| 91 | token, err := newToken() |
| 92 | if err != nil { |
| 93 | return "", err |
| 94 | } |
| 95 | |
| 96 | link := Link{DocID: doc.ID, TokenHash: hashToken(token), CreatedAt: time.Now().UTC()} |
| 97 | // Delete-then-insert rather than an upsert on doc_id: the old token |
| 98 | // must stop working, and a row whose token changed and a row that was |
| 99 | // replaced are the same outcome only if nothing else ever holds a |
| 100 | // reference to the row. Nothing does, and this way the intent reads |
| 101 | // off the code. |
| 102 | err = s.ownerMutation(ctx, owner, doc.ID, func(tx *gorm.DB) error { |
| 103 | if err := tx.Where("doc_id = ?", doc.ID).Delete(&Link{}).Error; err != nil { |
| 104 | return err |
| 105 | } |
| 106 | return tx.Create(&link).Error |
| 107 | }) |
| 108 | if err != nil { |
| 109 | return "", err |
| 110 | } |
| 111 | // The plaintext token is returned here and never again: this is the |
| 112 | // only moment it exists outside the caller's hand (ruling 16). |
| 113 | return token, nil |
| 114 | } |
| 115 | |
| 116 | // Unpublish turns it off. Like Revoke, unpublishing something that is not |
| 117 | // published succeeds: the owner asked for an outcome — that this document |
| 118 | // is not readable by strangers — and that outcome holds either way. |
| 119 | func (s *Store) Unpublish(ctx context.Context, owner int64, sqid string) error { |
| 120 | doc, err := s.Get(ctx, owner, sqid) |
| 121 | if err != nil { |
| 122 | return err |
| 123 | } |
| 124 | return s.ownerMutation(ctx, owner, doc.ID, func(tx *gorm.DB) error { return tx.Where("doc_id = ?", doc.ID).Delete(&Link{}).Error }) |
| 125 | } |
| 126 | |
| 127 | // IsPublished reports whether a document currently has a public link. |
| 128 | // |
| 129 | // It answers a bool and not a link, and that is ruling 16 showing up in a |
| 130 | // signature: there is no way to ask this store for a published document's |
| 131 | // URL, because the store does not have it. The share page can say "this is |
| 132 | // published" and offer to replace or stop it; it cannot show the URL |
| 133 | // again, and no future caller can be tempted to make it. |
| 134 | // |
| 135 | // Owner-scoped through Get: whether a document is published is part of the |
| 136 | // share page, which is the owner's alone. |
| 137 | func (s *Store) IsPublished(ctx context.Context, owner int64, sqid string) (bool, error) { |
| 138 | doc, err := s.Get(ctx, owner, sqid) |
| 139 | if err != nil { |
| 140 | return false, err |
| 141 | } |
| 142 | var count int64 |
| 143 | if err := s.db.WithContext(ctx).Model(&Link{}). |
| 144 | Where("doc_id = ?", doc.ID).Count(&count).Error; err != nil { |
| 145 | return false, err |
| 146 | } |
| 147 | return count > 0, nil |
| 148 | } |
| 149 | |
| 150 | // ByToken resolves a public link to its document, for the one route in |
| 151 | // this app that no session guards. |
| 152 | // |
| 153 | // An unknown token and a token whose document has been deleted answer |
| 154 | // identically — gorm.ErrRecordNotFound, which the handler turns into the |
| 155 | // same 404 a nonexistent URL gets. That is §4.2's rule reaching the public |
| 156 | // route: without it, the difference between the two answers is an oracle |
| 157 | // for which documents are published. |
| 158 | // |
| 159 | // The empty token is rejected before the query, so a request to a route |
| 160 | // that somehow lost its path value cannot match a row with an empty |
| 161 | // column. |
| 162 | func (s *Store) ByToken(ctx context.Context, token string) (*Document, error) { |
| 163 | if token == "" { |
| 164 | return nil, gorm.ErrRecordNotFound |
| 165 | } |
| 166 | var doc Document |
| 167 | err := s.db.WithContext(ctx). |
| 168 | Joins("JOIN links ON links.doc_id = documents.id"). |
| 169 | Where("links.token_hash = ? AND documents.deleted_at IS NULL AND documents.owner_id > 0", |
| 170 | hashToken(token)). |
| 171 | First(&doc).Error |
| 172 | if err != nil { |
| 173 | return nil, err |
| 174 | } |
| 175 | return &doc, nil |
| 176 | } |
| 177 | |
amadan