| 1 | // The device handoff: PROTOCOL.md's rung 1, and the only route in CONTRA that |
| 2 | // carries a secret from one device to another. |
| 3 | // |
| 4 | // It carries it sealed. The waiting device mints an ephemeral P-256 keypair |
| 5 | // and posts only the point; an enrolled device seals the Account Key to that |
| 6 | // point with ECIES; the server stores an envelope it has no key for and hands |
| 7 | // it over once. The AAD inside names the waiting device's own public key, so |
| 8 | // even a server that swapped one envelope for another cannot make a second |
| 9 | // device open it. |
| 10 | // |
| 11 | // # Why every route here needs a session |
| 12 | // |
| 13 | // slopbox's equivalent leaves /start and /claim open, because its waiting |
| 14 | // device has no account yet. CONTRA's does: a fresh browser signs in with the |
| 15 | // email and passphrase alone — the login key comes from S_pw, never from the |
| 16 | // Account Key — and only then finds it cannot unlock. So both ends are |
| 17 | // already authenticated, and scoping every query to the session's account |
| 18 | // means a stranger cannot see that a code exists, let alone spend one. |
| 19 | // |
| 20 | // That is what buys the short code. Its job is to tell the person which |
| 21 | // device they are approving, not to prove who is asking; the session already |
| 22 | // did that. |
| 23 | |
| 24 | package vault |
| 25 | |
| 26 | import ( |
| 27 | "encoding/hex" |
| 28 | "errors" |
| 29 | "net/http" |
| 30 | "time" |
| 31 | |
| 32 | "crypto/sha256" |
| 33 | |
| 34 | "github.com/carlosframework/contra/internal/contra/crypto" |
| 35 | "github.com/carlosframework/contra/internal/contra/store" |
| 36 | ) |
| 37 | |
| 38 | // codeHashOf decodes the hash the client sends. |
| 39 | // |
| 40 | // The client hashes; the server never receives the code at all. slopbox posts |
| 41 | // the code and hashes it server-side, which stores nothing but does mean the |
| 42 | // value passes through the server's memory and any request log in front of |
| 43 | // it. Hashing on the client costs one line and makes "the code exists only on |
| 44 | // the two screens the person is looking at" literally true. |
| 45 | // |
| 46 | // Nothing is lost by it: whoever can compute the hash already knows the code. |
| 47 | func codeHashOf(hexHash string) ([]byte, bool) { |
| 48 | h, err := hex.DecodeString(hexHash) |
| 49 | if err != nil || len(h) != sha256.Size { |
| 50 | return nil, false |
| 51 | } |
| 52 | return h, true |
| 53 | } |
| 54 | |
| 55 | // StartLink is the waiting device saying "here is my public key, here is the |
| 56 | // hash of the code I am showing the person". |
| 57 | // |
| 58 | // The server does not choose the code. A server-chosen code would be a value |
| 59 | // it knows in full for every live handoff; this way the only copies are on |
| 60 | // the two screens a person is looking at. |
| 61 | func (v *Vault) StartLink(w http.ResponseWriter, r *http.Request) { |
| 62 | acct, ok := accountID(r) |
| 63 | if !ok { |
| 64 | notFound(w) |
| 65 | return |
| 66 | } |
| 67 | var req struct { |
| 68 | CodeHashHex string `json:"code_hash_hex"` |
| 69 | DevicePublicHex string `json:"device_public_hex"` |
| 70 | } |
| 71 | if !readJSON(w, r, &req) { |
| 72 | return |
| 73 | } |
| 74 | codeHash, ok := codeHashOf(req.CodeHashHex) |
| 75 | if !ok { |
| 76 | writeJSON(w, http.StatusBadRequest, map[string]string{"error": "malformed code"}) |
| 77 | return |
| 78 | } |
| 79 | devicePublic, err := hex.DecodeString(req.DevicePublicHex) |
| 80 | if err != nil || len(devicePublic) != crypto.PublicKeyLen { |
| 81 | writeJSON(w, http.StatusBadRequest, map[string]string{"error": "malformed device key"}) |
| 82 | return |
| 83 | } |
| 84 | id, err := store.NewLinkID() |
| 85 | if err != nil { |
| 86 | writeJSON(w, http.StatusInternalServerError, map[string]string{"error": "could not start"}) |
| 87 | return |
| 88 | } |
| 89 | |
| 90 | link, err := v.store.StartLink(r.Context(), id, codeHash, acct, devicePublic, time.Now().UTC()) |
| 91 | if errors.Is(err, store.ErrTooManyLinks) { |
| 92 | writeJSON(w, http.StatusTooManyRequests, |
| 93 | map[string]string{"error": "too many handoffs open — finish or cancel one first"}) |
| 94 | return |
| 95 | } |
| 96 | if err != nil { |
| 97 | // A duplicate code hash lands here too. The client draws again. |
| 98 | writeJSON(w, http.StatusBadRequest, map[string]string{"error": "could not start"}) |
| 99 | return |
| 100 | } |
| 101 | writeJSON(w, http.StatusOK, map[string]any{ |
| 102 | "expires_at": link.ExpiresAt.Format(time.RFC3339), |
| 103 | }) |
| 104 | } |
| 105 | |
| 106 | // PendingLink is the enrolled device asking what a typed code refers to. It |
| 107 | // answers with the waiting device's public key and nothing else — there is |
| 108 | // nothing else in the row. |
| 109 | func (v *Vault) PendingLink(w http.ResponseWriter, r *http.Request) { |
| 110 | acct, ok := accountID(r) |
| 111 | if !ok { |
| 112 | notFound(w) |
| 113 | return |
| 114 | } |
| 115 | codeHash, ok := codeHashOf(r.URL.Query().Get("h")) |
| 116 | if !ok { |
| 117 | notFound(w) |
| 118 | return |
| 119 | } |
| 120 | link, err := v.store.PendingLink(r.Context(), acct, codeHash, time.Now().UTC()) |
| 121 | if err != nil { |
| 122 | notFound(w) |
| 123 | return |
| 124 | } |
| 125 | writeJSON(w, http.StatusOK, map[string]any{ |
| 126 | "device_public_hex": hex.EncodeToString(link.DevicePublic), |
| 127 | "approved": len(link.Sealed) > 0, |
| 128 | }) |
| 129 | } |
| 130 | |
| 131 | // ApproveLink takes the sealed envelope from an enrolled device. |
| 132 | // |
| 133 | // The envelope is opaque here and stays opaque. What this handler checks is |
| 134 | // that it is long enough to be one at all — a caller that posted an empty |
| 135 | // body would otherwise spend a live code on nothing, and the waiting device |
| 136 | // would sit there until the code expired with no way to tell why. |
| 137 | func (v *Vault) ApproveLink(w http.ResponseWriter, r *http.Request) { |
| 138 | acct, ok := accountID(r) |
| 139 | if !ok { |
| 140 | notFound(w) |
| 141 | return |
| 142 | } |
| 143 | var req struct { |
| 144 | CodeHashHex string `json:"code_hash_hex"` |
| 145 | SealedHex string `json:"sealed_hex"` |
| 146 | } |
| 147 | if !readJSON(w, r, &req) { |
| 148 | return |
| 149 | } |
| 150 | codeHash, ok := codeHashOf(req.CodeHashHex) |
| 151 | if !ok { |
| 152 | notFound(w) |
| 153 | return |
| 154 | } |
| 155 | sealed, err := hex.DecodeString(req.SealedHex) |
| 156 | if err != nil { |
| 157 | writeJSON(w, http.StatusBadRequest, map[string]string{"error": "malformed envelope"}) |
| 158 | return |
| 159 | } |
| 160 | if err := v.store.ApproveLink(r.Context(), acct, codeHash, sealed, time.Now().UTC()); err != nil { |
| 161 | notFound(w) |
| 162 | return |
| 163 | } |
| 164 | w.WriteHeader(http.StatusNoContent) |
| 165 | } |
| 166 | |
| 167 | // ClaimLink is the waiting device polling. Once there is an envelope it is |
| 168 | // handed over and the row is deleted in the same transaction, so it leaves |
| 169 | // exactly once. |
| 170 | func (v *Vault) ClaimLink(w http.ResponseWriter, r *http.Request) { |
| 171 | acct, ok := accountID(r) |
| 172 | if !ok { |
| 173 | notFound(w) |
| 174 | return |
| 175 | } |
| 176 | codeHash, ok := codeHashOf(r.URL.Query().Get("h")) |
| 177 | if !ok { |
| 178 | notFound(w) |
| 179 | return |
| 180 | } |
| 181 | sealed, err := v.store.ClaimLink(r.Context(), acct, codeHash, time.Now().UTC()) |
| 182 | if err != nil { |
| 183 | notFound(w) |
| 184 | return |
| 185 | } |
| 186 | if len(sealed) == 0 { |
| 187 | writeJSON(w, http.StatusOK, map[string]any{"approved": false}) |
| 188 | return |
| 189 | } |
| 190 | writeJSON(w, http.StatusOK, map[string]any{ |
| 191 | "approved": true, |
| 192 | "sealed_hex": hex.EncodeToString(sealed), |
| 193 | }) |
| 194 | } |
| 195 | |
| 196 | // CancelLink drops a code the waiting device gave up on, so closing the tab |
| 197 | // does not leave a live handoff behind. |
| 198 | func (v *Vault) CancelLink(w http.ResponseWriter, r *http.Request) { |
| 199 | acct, ok := accountID(r) |
| 200 | if !ok { |
| 201 | notFound(w) |
| 202 | return |
| 203 | } |
| 204 | var req struct { |
| 205 | CodeHashHex string `json:"code_hash_hex"` |
| 206 | } |
| 207 | if !readJSON(w, r, &req) { |
| 208 | return |
| 209 | } |
| 210 | codeHash, ok := codeHashOf(req.CodeHashHex) |
| 211 | if !ok { |
| 212 | notFound(w) |
| 213 | return |
| 214 | } |
| 215 | if err := v.store.CancelLink(r.Context(), acct, codeHash); err != nil { |
| 216 | notFound(w) |
| 217 | return |
| 218 | } |
| 219 | w.WriteHeader(http.StatusNoContent) |
| 220 | } |
| 221 | |
amadan