| 1 | // SPDX-License-Identifier: AGPL-3.0-or-later |
| 2 | |
| 3 | // Package pat owns personal-access-token minting, hashing, scope |
| 4 | // constants, and the auth-middleware verification path. |
| 5 | // |
| 6 | // Format: shithub_pat_<32-char-base62>. The fixed `shithub_pat_` prefix is |
| 7 | // recognized by secret-scanning tooling and by our own log redactor — so |
| 8 | // even if someone accidentally pastes a raw token into a log line or a |
| 9 | // public PR, downstream tooling has a chance to spot it. |
| 10 | package pat |
| 11 | |
| 12 | import ( |
| 13 | "crypto/rand" |
| 14 | "crypto/sha256" |
| 15 | "crypto/subtle" |
| 16 | "errors" |
| 17 | "fmt" |
| 18 | "strings" |
| 19 | ) |
| 20 | |
| 21 | // Prefix is the fixed marker prepended to every minted token. |
| 22 | const Prefix = "shithub_pat_" |
| 23 | |
| 24 | // PayloadLen is the length of the random payload (chars), not bytes. |
| 25 | // 32 base62 characters carry log2(62)*32 ≈ 190 bits of entropy — well |
| 26 | // beyond any plausible brute-force budget. |
| 27 | const PayloadLen = 32 |
| 28 | |
| 29 | // DisplayPrefixLen is how much of the raw token we keep for display |
| 30 | // (e.g. on the listing page). It includes the literal `shithub_pat_` |
| 31 | // plus four characters of payload, enough to disambiguate at a glance. |
| 32 | const DisplayPrefixLen = len("shithub_pat_") + 4 |
| 33 | |
| 34 | // MaxTokensPerUser bounds the listing page and the auth-lookup table size. |
| 35 | const MaxTokensPerUser = 50 |
| 36 | |
| 37 | // alphabet is the base62 character set used for the random payload. |
| 38 | const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789" |
| 39 | |
| 40 | // ErrMalformed is returned when the supplied string isn't a well-formed |
| 41 | // shithub PAT. Distinct from "valid format but unknown" so the auth |
| 42 | // middleware can short-circuit without touching the DB. |
| 43 | var ErrMalformed = errors.New("pat: malformed token") |
| 44 | |
| 45 | // Mint generates a fresh raw token, its sha256 hash (for storage), and |
| 46 | // its display prefix. The raw token MUST be shown to the user exactly |
| 47 | // once and never stored. |
| 48 | func Mint() (raw string, hash []byte, prefix string, err error) { |
| 49 | payload, err := randomBase62(PayloadLen) |
| 50 | if err != nil { |
| 51 | return "", nil, "", fmt.Errorf("pat: mint payload: %w", err) |
| 52 | } |
| 53 | raw = Prefix + payload |
| 54 | sum := sha256.Sum256([]byte(raw)) |
| 55 | prefix = raw[:DisplayPrefixLen] |
| 56 | return raw, sum[:], prefix, nil |
| 57 | } |
| 58 | |
| 59 | // HashOf computes the canonical lookup hash for raw, after verifying |
| 60 | // the prefix and length. Use this on every middleware lookup so a |
| 61 | // malformed input never reaches the DB index. |
| 62 | func HashOf(raw string) ([]byte, error) { |
| 63 | if !strings.HasPrefix(raw, Prefix) { |
| 64 | return nil, ErrMalformed |
| 65 | } |
| 66 | if len(raw) != len(Prefix)+PayloadLen { |
| 67 | return nil, ErrMalformed |
| 68 | } |
| 69 | for _, r := range raw[len(Prefix):] { |
| 70 | if !isBase62(r) { |
| 71 | return nil, ErrMalformed |
| 72 | } |
| 73 | } |
| 74 | sum := sha256.Sum256([]byte(raw)) |
| 75 | return sum[:], nil |
| 76 | } |
| 77 | |
| 78 | // EqualHash compares two stored hashes in constant time. The auth path |
| 79 | // already keys by hash via UNIQUE-index lookup, but we also expose this |
| 80 | // for any future code that compares hashes outside a DB lookup. |
| 81 | func EqualHash(a, b []byte) bool { |
| 82 | return subtle.ConstantTimeCompare(a, b) == 1 |
| 83 | } |
| 84 | |
| 85 | // LooksLike reports whether s resembles a PAT (cheap structural check). |
| 86 | // Used by URL-credential redaction in the log layer. |
| 87 | func LooksLike(s string) bool { |
| 88 | if !strings.HasPrefix(s, Prefix) { |
| 89 | return false |
| 90 | } |
| 91 | if len(s) != len(Prefix)+PayloadLen { |
| 92 | return false |
| 93 | } |
| 94 | for _, r := range s[len(Prefix):] { |
| 95 | if !isBase62(r) { |
| 96 | return false |
| 97 | } |
| 98 | } |
| 99 | return true |
| 100 | } |
| 101 | |
| 102 | func isBase62(r rune) bool { |
| 103 | return (r >= 'A' && r <= 'Z') || (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9') |
| 104 | } |
| 105 | |
| 106 | // randomBase62 returns n base62 characters from crypto/rand. Reads more |
| 107 | // bytes than strictly needed and rejection-samples to keep the |
| 108 | // distribution uniform. |
| 109 | func randomBase62(n int) (string, error) { |
| 110 | out := make([]byte, 0, n) |
| 111 | buf := make([]byte, n*2) |
| 112 | for len(out) < n { |
| 113 | if _, err := rand.Read(buf); err != nil { |
| 114 | return "", err |
| 115 | } |
| 116 | for _, b := range buf { |
| 117 | // 256 mod 62 == 8; reject the top range to avoid bias. |
| 118 | if b >= 248 { |
| 119 | continue |
| 120 | } |
| 121 | out = append(out, alphabet[b%62]) |
| 122 | if len(out) == n { |
| 123 | break |
| 124 | } |
| 125 | } |
| 126 | } |
| 127 | return string(out), nil |
| 128 | } |
| 129 |