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internal/session/session.go
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| 1 | // Package session mints and verifies the signed, stateless cookies gsg |
| 2 | // uses to recognize a logged-in browser. There is no server-side session |
| 3 | // store: everything needed to verify a cookie travels inside it, HMAC-signed |
| 4 | // against a secret that lives only in this process's memory. A restart |
| 5 | // invalidates every outstanding session -- an accepted trade-off for a |
| 6 | // login this lightweight. |
| 7 | // |
| 8 | // The master secret is never used to sign anything directly. Each purpose |
| 9 | // (today: the user session cookie; later: share-link tokens) gets its own |
| 10 | // key derived from the secret via deriveKey, so a value signed for one |
| 11 | // purpose can never be replayed as another -- that's enforced by |
| 12 | // construction, not by a discriminator field a bug could skip. |
| 13 | package session |
| 14 | |
| 15 | import ( |
| 16 | "crypto/hmac" |
| 17 | "crypto/rand" |
| 18 | "crypto/sha256" |
| 19 | "crypto/subtle" |
| 20 | "encoding/base64" |
| 21 | "encoding/json" |
| 22 | "fmt" |
| 23 | "net/http" |
| 24 | "strconv" |
| 25 | "strings" |
| 26 | "time" |
| 27 | ) |
| 28 | |
| 29 | const ( |
| 30 | // CookieName is the browser cookie carrying the signed session. |
| 31 | CookieName = "gsg_user" |
| 32 | |
| 33 | // TTL is how long a session lasts after login before it must be |
| 34 | // re-established. Embedded inside the signed payload, not left to the |
| 35 | // cookie's own Max-Age/Expires alone, so a copied cookie can't outlive |
| 36 | // what the server considers valid. |
| 37 | TTL = 30 * 24 * time.Hour |
| 38 | |
| 39 | // ShareCookieName is the browser cookie a redeemed share link mints. |
| 40 | ShareCookieName = "gsg_share" |
| 41 | |
| 42 | purposeSession = "session" |
| 43 | purposeShareURL = "share-url" |
| 44 | purposeShareCookie = "share-cookie" |
| 45 | ) |
| 46 | |
| 47 | // Manager issues and verifies gsg's signed cookies. Safe for concurrent |
| 48 | // use; a nil *Manager verifies every request as anonymous, so wiring it up |
| 49 | // is optional. |
| 50 | type Manager struct { |
| 51 | secret []byte |
| 52 | } |
| 53 | |
| 54 | // NewManager generates a fresh in-memory secret. |
| 55 | func NewManager() *Manager { |
| 56 | secret := make([]byte, 32) |
| 57 | if _, err := rand.Read(secret); err != nil { |
| 58 | panic("session: failed to generate secret: " + err.Error()) |
| 59 | } |
| 60 | return &Manager{secret: secret} |
| 61 | } |
| 62 | |
| 63 | // deriveKey derives a purpose-scoped signing key from the master secret. |
| 64 | // This is a single-step HMAC-based derivation (not full RFC 5869 HKDF), |
| 65 | // which is sufficient here: the input is already a uniformly random 32-byte |
| 66 | // secret, so there's no entropy-extraction step to do, only labeling. |
| 67 | func (m *Manager) deriveKey(purpose string) []byte { |
| 68 | mac := hmac.New(sha256.New, m.secret) |
| 69 | mac.Write([]byte(purpose)) |
| 70 | return mac.Sum(nil) |
| 71 | } |
| 72 | |
| 73 | // Issue mints a signed session cookie for username. |
| 74 | func (m *Manager) Issue(username string) *http.Cookie { |
| 75 | exp := time.Now().Add(TTL).Unix() |
| 76 | payload := username + "|" + strconv.FormatInt(exp, 10) |
| 77 | return &http.Cookie{ |
| 78 | Name: CookieName, |
| 79 | Value: sign(m.deriveKey(purposeSession), payload, ""), |
| 80 | Path: "/", |
| 81 | HttpOnly: true, |
| 82 | Secure: true, |
| 83 | SameSite: http.SameSiteLaxMode, |
| 84 | Expires: time.Unix(exp, 0), |
| 85 | } |
| 86 | } |
| 87 | |
| 88 | // Verify reads and checks the session cookie on r, returning the |
| 89 | // authenticated username, or "" if there is none, it's malformed, it's |
| 90 | // signed for a different purpose, or it has expired. |
| 91 | func (m *Manager) Verify(r *http.Request) string { |
| 92 | if m == nil { |
| 93 | return "" |
| 94 | } |
| 95 | c, err := r.Cookie(CookieName) |
| 96 | if err != nil { |
| 97 | return "" |
| 98 | } |
| 99 | payload, ok := verify(m.deriveKey(purposeSession), c.Value, "") |
| 100 | if !ok { |
| 101 | return "" |
| 102 | } |
| 103 | user, expStr, ok := strings.Cut(payload, "|") |
| 104 | if !ok { |
| 105 | return "" |
| 106 | } |
| 107 | exp, err := strconv.ParseInt(expStr, 10, 64) |
| 108 | if err != nil || time.Now().Unix() > exp { |
| 109 | return "" |
| 110 | } |
| 111 | return user |
| 112 | } |
| 113 | |
| 114 | // Clear returns a cookie that immediately expires the session. |
| 115 | func (m *Manager) Clear() *http.Cookie { |
| 116 | return &http.Cookie{ |
| 117 | Name: CookieName, |
| 118 | Value: "", |
| 119 | Path: "/", |
| 120 | HttpOnly: true, |
| 121 | Secure: true, |
| 122 | SameSite: http.SameSiteLaxMode, |
| 123 | MaxAge: -1, |
| 124 | } |
| 125 | } |
| 126 | |
| 127 | // ShareClaims is the payload embedded in both a share-URL token and the |
| 128 | // gsg_share cookie minted on redemption. Slugs is the set of repo slugs the |
| 129 | // grant unlocks; IAT/Exp are unix seconds. Pwd records only whether the |
| 130 | // URL token requires a password -- it carries no password material itself. |
| 131 | type ShareClaims struct { |
| 132 | V int `json:"v"` |
| 133 | Slugs []string `json:"slugs"` |
| 134 | IAT int64 `json:"iat"` |
| 135 | Exp int64 `json:"exp"` |
| 136 | Pwd bool `json:"pwd"` |
| 137 | } |
| 138 | |
| 139 | func (c ShareClaims) expired() bool { return c.Exp != 0 && time.Now().Unix() > c.Exp } |
| 140 | |
| 141 | // IssuedAt is c.IAT as a time.Time, for comparison against a repo's |
| 142 | // gsg.share-revoked-after. |
| 143 | func (c ShareClaims) IssuedAt() time.Time { return time.Unix(c.IAT, 0) } |
| 144 | |
| 145 | // NewShareToken signs a share-URL token. password is "" for an unprotected |
| 146 | // link; when set, it is folded into the signature, never stored in the |
| 147 | // token -- see the package doc. |
| 148 | func (m *Manager) NewShareToken(slugs []string, ttl time.Duration, password string) (string, ShareClaims) { |
| 149 | now := time.Now() |
| 150 | claims := ShareClaims{V: 1, Slugs: slugs, IAT: now.Unix(), Exp: now.Add(ttl).Unix(), Pwd: password != ""} |
| 151 | b, _ := json.Marshal(claims) // ShareClaims always marshals; no error path |
| 152 | return sign(m.deriveKey(purposeShareURL), string(b), password), claims |
| 153 | } |
| 154 | |
| 155 | // PeekShareToken decodes a share-URL token's claims WITHOUT verifying its |
| 156 | // signature. It exists only so the redemption page can decide whether to |
| 157 | // show a password prompt before it knows the password; the result must |
| 158 | // never be used to grant access -- only VerifyShareToken's result may do |
| 159 | // that. |
| 160 | func PeekShareToken(tok string) (ShareClaims, bool) { |
| 161 | encPayload, _, found := strings.Cut(tok, ".") |
| 162 | if !found { |
| 163 | return ShareClaims{}, false |
| 164 | } |
| 165 | b, err := dec(encPayload) |
| 166 | if err != nil { |
| 167 | return ShareClaims{}, false |
| 168 | } |
| 169 | var c ShareClaims |
| 170 | if err := json.Unmarshal(b, &c); err != nil || c.V != 1 { |
| 171 | return ShareClaims{}, false |
| 172 | } |
| 173 | return c, true |
| 174 | } |
| 175 | |
| 176 | // VerifyShareToken checks a share-URL token against the password the |
| 177 | // visitor supplied (pass "" for a token that doesn't require one) and |
| 178 | // returns its claims. Wrong password, tampering, or expiry all fail the |
| 179 | // same way -- there is nothing here for an attacker to distinguish. |
| 180 | func (m *Manager) VerifyShareToken(tok, password string) (ShareClaims, bool) { |
| 181 | payload, ok := verify(m.deriveKey(purposeShareURL), tok, password) |
| 182 | if !ok { |
| 183 | return ShareClaims{}, false |
| 184 | } |
| 185 | var c ShareClaims |
| 186 | if err := json.Unmarshal([]byte(payload), &c); err != nil || c.V != 1 || c.expired() { |
| 187 | return ShareClaims{}, false |
| 188 | } |
| 189 | return c, true |
| 190 | } |
| 191 | |
| 192 | // IssueShareCookie mints the gsg_share cookie from already-verified claims. |
| 193 | // IAT and Exp are copied verbatim from the URL token, not restamped to |
| 194 | // "now": the revocation check compares a repo's gsg.share-revoked-after |
| 195 | // against IAT, and resetting it here would let an already-redeemed browser |
| 196 | // outlive a revocation aimed at it. |
| 197 | func (m *Manager) IssueShareCookie(claims ShareClaims) *http.Cookie { |
| 198 | b, _ := json.Marshal(claims) |
| 199 | return &http.Cookie{ |
| 200 | Name: ShareCookieName, |
| 201 | Value: sign(m.deriveKey(purposeShareCookie), string(b), ""), |
| 202 | Path: "/", |
| 203 | HttpOnly: true, |
| 204 | Secure: true, |
| 205 | SameSite: http.SameSiteLaxMode, |
| 206 | Expires: time.Unix(claims.Exp, 0), |
| 207 | } |
| 208 | } |
| 209 | |
| 210 | // VerifyShareCookie reads and checks the gsg_share cookie on r. |
| 211 | func (m *Manager) VerifyShareCookie(r *http.Request) (ShareClaims, bool) { |
| 212 | if m == nil { |
| 213 | return ShareClaims{}, false |
| 214 | } |
| 215 | c, err := r.Cookie(ShareCookieName) |
| 216 | if err != nil { |
| 217 | return ShareClaims{}, false |
| 218 | } |
| 219 | payload, ok := verify(m.deriveKey(purposeShareCookie), c.Value, "") |
| 220 | if !ok { |
| 221 | return ShareClaims{}, false |
| 222 | } |
| 223 | var claims ShareClaims |
| 224 | if err := json.Unmarshal([]byte(payload), &claims); err != nil || claims.V != 1 || claims.expired() { |
| 225 | return ShareClaims{}, false |
| 226 | } |
| 227 | return claims, true |
| 228 | } |
| 229 | |
| 230 | // sign produces "base64url(payload).base64url(hmac-sha256(payload||0||extra))". |
| 231 | // Compact and self-contained like a JWT, without pulling in a JWT library or |
| 232 | // its algorithm-confusion surface (alg:none, HS/RS key confusion) -- the key |
| 233 | // and the hash are fixed by this package, not negotiable per token. extra is |
| 234 | // "" for every purpose except a password-protected share-URL token, where |
| 235 | // it's the password: folding it into the signature input (rather than |
| 236 | // storing a hash of it in the payload) means nobody holding just the token |
| 237 | // can crack it offline, and a guess can only be checked by asking the |
| 238 | // server, which can rate-limit attempts. |
| 239 | func sign(key []byte, payload, extra string) string { |
| 240 | mac := hmac.New(sha256.New, key) |
| 241 | mac.Write([]byte(payload)) |
| 242 | mac.Write([]byte{0}) |
| 243 | mac.Write([]byte(extra)) |
| 244 | return enc(payload) + "." + enc(string(mac.Sum(nil))) |
| 245 | } |
| 246 | |
| 247 | // verify checks tok against key and extra and, on success, returns the |
| 248 | // payload. |
| 249 | func verify(key []byte, tok, extra string) (payload string, ok bool) { |
| 250 | encPayload, encSig, found := strings.Cut(tok, ".") |
| 251 | if !found { |
| 252 | return "", false |
| 253 | } |
| 254 | payloadBytes, err := dec(encPayload) |
| 255 | if err != nil { |
| 256 | return "", false |
| 257 | } |
| 258 | sig, err := dec(encSig) |
| 259 | if err != nil { |
| 260 | return "", false |
| 261 | } |
| 262 | mac := hmac.New(sha256.New, key) |
| 263 | mac.Write(payloadBytes) |
| 264 | mac.Write([]byte{0}) |
| 265 | mac.Write([]byte(extra)) |
| 266 | if subtle.ConstantTimeCompare(sig, mac.Sum(nil)) != 1 { |
| 267 | return "", false |
| 268 | } |
| 269 | return string(payloadBytes), true |
| 270 | } |
| 271 | |
| 272 | func enc(s string) string { return base64.RawURLEncoding.EncodeToString([]byte(s)) } |
| 273 | |
| 274 | func dec(s string) ([]byte, error) { |
| 275 | b, err := base64.RawURLEncoding.DecodeString(s) |
| 276 | if err != nil { |
| 277 | return nil, fmt.Errorf("session: bad encoding: %w", err) |
| 278 | } |
| 279 | return b, nil |
| 280 | } |