| 1 | package calls |
| 2 | |
| 3 | import ( |
| 4 | "context" |
| 5 | "errors" |
| 6 | "fmt" |
| 7 | "log/slog" |
| 8 | "strings" |
| 9 | |
| 10 | "amadan.net/rastrillo/rastrillo/mail" |
| 11 | ) |
| 12 | |
| 13 | // RedactMailErrors wraps a mail sender so a delivery failure never |
| 14 | // carries the recipient's address out of this package (review C1-2, |
| 15 | // C2-1). |
| 16 | // |
| 17 | // rastrillo/mail wraps every SMTP failure as "send to <address>: <err>" |
| 18 | // (mail/mail.go:163) and rastrillo/auth logs that error verbatim |
| 19 | // (auth/handlers.go:51). An outsider who asks for a magic link during an |
| 20 | // SMTP outage would therefore leave their plaintext address in the log, |
| 21 | // before they have signed in and before Calls has decided to admit them — |
| 22 | // which spec §0.1's falsifiable claim covers as squarely as the database |
| 23 | // does. So the wrapper logs the failure itself and hands rastrillo an |
| 24 | // error that says only that delivery failed. |
| 25 | // |
| 26 | // What the log line keeps is the root cause's *type* and a boolean |
| 27 | // saying whether the address appeared in the message. It does not keep |
| 28 | // the message, and never calls err.Error() for logging purposes — only |
| 29 | // to test it for the address, and that result is thrown away, kept only |
| 30 | // as a bool. That is a real loss — an SMTP reply's text is the most |
| 31 | // useful thing about it — and it is the deliberate trade: a remote |
| 32 | // server's error text is arbitrary bytes that may carry the address in a |
| 33 | // shape no scrubber anticipated (the local part alone, the domain alone, |
| 34 | // re-encoded, split across a fold). A type plus a yes/no is small, and |
| 35 | // it cannot leak. |
| 36 | // |
| 37 | // The first version of this file tried to do better: replace every |
| 38 | // occurrence of the address in the message with "[address]" and log the |
| 39 | // result. That slices the message at byte offsets found in a |
| 40 | // strings.ToLower copy, and ToLower can change a string's byte length |
| 41 | // (the Kelvin sign U+212A is three bytes and lowercases to one-byte "k"; |
| 42 | // capital dotted I U+0130 is two bytes and lowercases to two-byte "i̇"), |
| 43 | // so an offset found in the lowered copy does not address the same bytes |
| 44 | // in the original — an address containing either could leave a fragment |
| 45 | // of itself in the "redacted" text (codex round 2, C2-1). Logging |
| 46 | // nothing derived from the message at all, only a boolean computed with |
| 47 | // no slicing, has no such trap: strings.Contains never needs its |
| 48 | // operands to line up byte-for-byte with anything else. |
| 49 | // |
| 50 | // The wrapper forwards Delivers and SendList when the inner sender has |
| 51 | // them, so mail.IsConfigured still sees through it (mail/mail.go:59) and |
| 52 | // a ListSender does not silently lose its list headers. |
| 53 | func RedactMailErrors(inner mail.Sender, logger *slog.Logger) mail.Sender { |
| 54 | if inner == nil { |
| 55 | return nil |
| 56 | } |
| 57 | if logger == nil { |
| 58 | logger = slog.Default() |
| 59 | } |
| 60 | s := &redactingSender{inner: inner, logger: logger} |
| 61 | list, isList := inner.(mail.ListSender) |
| 62 | deliverer, isDeliverer := inner.(mail.Deliverer) |
| 63 | switch { |
| 64 | case isList && isDeliverer: |
| 65 | return &redactingListDeliverer{redactingListSender{*s, list}, deliverer} |
| 66 | case isList: |
| 67 | return &redactingListSender{*s, list} |
| 68 | case isDeliverer: |
| 69 | return &redactingDeliverer{*s, deliverer} |
| 70 | } |
| 71 | return s |
| 72 | } |
| 73 | |
| 74 | type redactingSender struct { |
| 75 | inner mail.Sender |
| 76 | logger *slog.Logger |
| 77 | } |
| 78 | |
| 79 | func (s *redactingSender) Send(ctx context.Context, to, subject, body string) error { |
| 80 | return s.redact(to, s.inner.Send(ctx, to, subject, body)) |
| 81 | } |
| 82 | |
| 83 | // redact logs a delivery failure and returns the only error this package |
| 84 | // lets out. It never returns anything derived from err. |
| 85 | func (s *redactingSender) redact(to string, err error) error { |
| 86 | if err == nil { |
| 87 | return nil |
| 88 | } |
| 89 | s.logger.Error("calls: mail delivery failed", |
| 90 | "type", fmt.Sprintf("%T", rootCause(err)), |
| 91 | "address_seen", addressSeen(err.Error(), to)) |
| 92 | return errors.New("calls: mail delivery failed") |
| 93 | } |
| 94 | |
| 95 | // redactingListSender forwards SendList; redactingDeliverer forwards |
| 96 | // Delivers; redactingListDeliverer forwards both. Three types rather |
| 97 | // than one with nil checks, so the interface a caller type-asserts for |
| 98 | // is the interface the inner sender actually had. |
| 99 | type redactingListSender struct { |
| 100 | redactingSender |
| 101 | list mail.ListSender |
| 102 | } |
| 103 | |
| 104 | func (s *redactingListSender) SendList(ctx context.Context, m mail.ListMessage) error { |
| 105 | return s.redact(m.To, s.list.SendList(ctx, m)) |
| 106 | } |
| 107 | |
| 108 | type redactingDeliverer struct { |
| 109 | redactingSender |
| 110 | deliverer mail.Deliverer |
| 111 | } |
| 112 | |
| 113 | func (s *redactingDeliverer) Delivers() bool { return s.deliverer.Delivers() } |
| 114 | |
| 115 | type redactingListDeliverer struct { |
| 116 | redactingListSender |
| 117 | deliverer mail.Deliverer |
| 118 | } |
| 119 | |
| 120 | func (s *redactingListDeliverer) Delivers() bool { return s.deliverer.Delivers() } |
| 121 | |
| 122 | // rootCause is the innermost error in the chain. rastrillo/mail wraps |
| 123 | // with fmt.Errorf, so %T on what it returns says "*fmt.wrapError" and |
| 124 | // nothing about what actually went wrong; the cause names the dial |
| 125 | // failure, the TLS refusal or the SMTP reply type. |
| 126 | func rootCause(err error) error { |
| 127 | for { |
| 128 | next := errors.Unwrap(err) |
| 129 | if next == nil { |
| 130 | return err |
| 131 | } |
| 132 | err = next |
| 133 | } |
| 134 | } |
| 135 | |
| 136 | // addressSeen reports whether the recipient's address appears in an |
| 137 | // error message, case-insensitively — the one bit of the message worth |
| 138 | // keeping, and the only thing this package computes from it. No |
| 139 | // slicing: strings.Contains over two independently-lowered strings never |
| 140 | // needs an offset from one to mean anything in the other, so the byte- |
| 141 | // length change strings.ToLower can produce (the Kelvin sign U+212A, |
| 142 | // capital dotted I U+0130) cannot desynchronise anything the way it did |
| 143 | // when the first version of this file used lowered offsets to slice the |
| 144 | // original message. |
| 145 | func addressSeen(msg, address string) bool { |
| 146 | address = strings.TrimSpace(address) |
| 147 | if address == "" { |
| 148 | return false |
| 149 | } |
| 150 | return strings.Contains(strings.ToLower(msg), strings.ToLower(address)) |
| 151 | } |
| 152 | |
amadan