DateKeys/capsule/inspect.go

package capsule

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"time"

	"filippo.io/age"

	datekeys "g.activething.com/go/DateKeys"
	"g.activething.com/go/DateKeys/agewrap"
	"g.activething.com/go/DateKeys/datekey"
	"g.activething.com/go/DateKeys/extension"
	"g.activething.com/go/DateKeys/profile"
)

// InspectOptions configures Inspect.
type InspectOptions struct {
	// Registry holds the locally pinned profiles. Required.
	Registry profile.Registry
	// Extensions lists the extensions the application implements. Nil knows
	// none, the state of the base protocol V1. When it is also an
	// extension.DataValidator, the data of the known extensions is checked
	// (spec §54).
	Extensions extension.Registry
}

// CheckResult records one step of the flow of spec §63.
type CheckResult struct {
	Step   int    `json:"step"`
	Name   string `json:"name"`
	OK     bool   `json:"ok"`
	Detail string `json:"detail,omitempty"`
	// Error is the normative code of a failed step, for example
	// "ERR_ROUND_MISMATCH".
	Error string `json:"error,omitempty"`
}

// StanzaInfo is the visible part of an age recipient stanza.
type StanzaInfo struct {
	Type string   `json:"type"`
	Args []string `json:"args"`
}

// Inspection is the result of the pre-unlock validation, steps 1 to 8 of
// spec §63. It is produced without network access and without secrets.
type Inspection struct {
	Prelude        Prelude
	PublicHeader   []byte // exact PUBLIC_HEADER bytes
	Header         *Header
	Profile        *profile.Profile
	UnlockAt       time.Time // effective round time of the DateKey
	PayloadOffset  int64
	OuterStanzas   []StanzaInfo // OUTER_TIME_AGE
	PayloadStanzas []StanzaInfo // PAYLOAD_AGE
	// UnusableExtensions are the known noncritical PUBLIC_HEADER extensions
	// whose data InspectOptions.Extensions rejects. The capsule stays valid;
	// the application must not use them (spec §54).
	UnusableExtensions []extension.Unusable
	Checks             []CheckResult
}

func (in *Inspection) pass(step int, name, detail string) {
	in.Checks = append(in.Checks, CheckResult{Step: step, Name: name, OK: true, Detail: detail})
}

func (in *Inspection) fail(step int, name string, err error) error {
	in.Checks = append(in.Checks, CheckResult{Step: step, Name: name, Detail: err.Error(), Error: datekeys.Code(err)})
	return err
}

// parsed carries what Open needs after the inspection.
type parsed struct {
	prelude [PreludeSize]byte
	sealed  []byte    // OUTER_TIME_AGE
	payload io.Reader // positioned at the start of PAYLOAD_AGE
}

// Inspect runs steps 1 to 8 of spec §63 on the .dkc read from r: framing,
// canonical PUBLIC_HEADER, canonical DateKey, pinned profile, known critical
// extensions, the stanza structure of OUTER_TIME_AGE and PAYLOAD_AGE, and the
// round and chain hash of the tlock stanza. It never contacts a release
// source and never uses a secret, so an invalid capsule is rejected before it
// can cause an observable query (spec §27, §63).
//
// Inspect reads the prelude, the header, SEALED_CONTROL and the age header of
// the payload; it does not read the rest of the payload. On failure it
// returns the partial Inspection together with the error.
func Inspect(r io.Reader, opts InspectOptions) (*Inspection, error) {
	in, _, err := inspect(r, opts)
	return in, err
}

func inspect(r io.Reader, opts InspectOptions) (*Inspection, *parsed, error) {
	in := &Inspection{}
	if opts.Registry == nil {
		return in, nil, errors.New("capsule: InspectOptions.Registry is required")
	}

	// Steps 1 and 2: parse DKC1 and validate the prelude.
	var pre [PreludeSize]byte
	n, err := io.ReadFull(r, pre[:])
	if err != nil && n >= 4 && string(pre[:4]) == Magic {
		return in, nil, in.fail(1, "parse DKC1", fmt.Errorf("capsule: truncated prelude: %w", datekeys.ErrIntegrity))
	}
	prelude, err := ParsePrelude(pre[:n])
	if errors.Is(err, datekeys.ErrInvalidMagic) {
		return in, nil, in.fail(1, "parse DKC1", err)
	}
	in.pass(1, "parse DKC1", "magic DKC1")
	if err != nil {
		return in, nil, in.fail(2, "prelude", err)
	}
	in.Prelude = prelude
	in.PayloadOffset = prelude.PayloadOffset()
	in.pass(2, "prelude", fmt.Sprintf("DKC1 v%d, PUBLIC_HEADER_LEN=%d, SEALED_CONTROL_LEN=%d",
		FramingVersion, prelude.PublicHeaderLen, prelude.SealedControlLen))

	// Step 3: read the exact PUBLIC_HEADER bytes.
	hb, err := readExactly(r, int64(prelude.PublicHeaderLen))
	if err != nil {
		return in, nil, in.fail(3, "public header", fmt.Errorf("capsule: truncated PUBLIC_HEADER: %w", datekeys.ErrIntegrity))
	}
	in.PublicHeader = hb
	in.pass(3, "public header", fmt.Sprintf("%d bytes", len(hb)))

	// Step 4: canonical CBOR, canonical DateKey, pinned profile, known
	// critical extensions with valid data.
	h, err := DecodeHeader(hb)
	if err != nil {
		return in, nil, in.fail(4, "header validation", err)
	}
	in.Header = h
	p, ok := opts.Registry.Lookup(h.DateKey.ProfileID)
	if !ok {
		return in, nil, in.fail(4, "header validation", fmt.Errorf("capsule: profile %q is not pinned: %w", h.DateKey.ProfileID, datekeys.ErrUnknownProfile))
	}
	in.Profile = p
	if err := extension.CheckCritical(h.Critical, opts.Extensions); err != nil {
		return in, nil, in.fail(4, "header validation", fmt.Errorf("capsule: PUBLIC_HEADER: %w", err))
	}
	in.UnusableExtensions = extension.CheckNoncritical(h.Noncritical, opts.Extensions)
	in.pass(4, "header validation", fmt.Sprintf("capsule_id=%s datekey=%s policy=%s profile=%s%s",
		h.CapsuleIDHex(), h.DateKey.Compact(), h.Policy, p.ID, unusable(in.UnusableExtensions)))

	// Step 5: OUTER_TIME_AGE holds exactly one stanza, of type tlock.
	sealed, err := readExactly(r, int64(prelude.SealedControlLen))
	if err != nil {
		return in, nil, in.fail(5, "sealed control structure", fmt.Errorf("capsule: truncated SEALED_CONTROL: %w", datekeys.ErrIntegrity))
	}
	outer, err := agewrap.Stanzas(bytes.NewReader(sealed))
	if err != nil {
		return in, nil, in.fail(5, "sealed control structure", fmt.Errorf("capsule: SEALED_CONTROL: %w", err))
	}
	in.OuterStanzas = infos(outer)
	if len(outer) != 1 || outer[0].Type != agewrap.StanzaTLock {
		err := fmt.Errorf("capsule: OUTER_TIME_AGE must hold exactly one tlock stanza, found %d: %w", len(outer), datekeys.ErrPolicyStructureMismatch)
		return in, nil, in.fail(5, "sealed control structure", err)
	}
	in.pass(5, "sealed control structure", "one tlock stanza")

	// Step 6: PAYLOAD_AGE holds exactly one stanza, of type X25519. Only its
	// age header is read; the bytes consumed are replayed for decryption.
	var captured bytes.Buffer
	payloadStanzas, err := agewrap.Stanzas(io.TeeReader(r, &captured))
	if err != nil {
		return in, nil, in.fail(6, "payload structure", fmt.Errorf("capsule: PAYLOAD_AGE: %w", err))
	}
	in.PayloadStanzas = infos(payloadStanzas)
	if err := agewrap.CheckPayloadStanzas(payloadStanzas); err != nil {
		return in, nil, in.fail(6, "payload structure", err)
	}
	in.pass(6, "payload structure", "one X25519 stanza")

	// Step 7: resolve and verify the time condition locally.
	if err := h.DateKey.Validate(p); err != nil {
		return in, nil, in.fail(7, "condition", err)
	}
	unlock, err := datekey.RoundTime(p, h.DateKey.Round)
	if err != nil {
		return in, nil, in.fail(7, "condition", err)
	}
	in.UnlockAt = unlock
	in.pass(7, "condition", fmt.Sprintf("round %d, unlock at %s", h.DateKey.Round, unlock.Format(time.RFC3339)))

	// Step 8: the tlock stanza names the DateKey round and the pinned chain.
	if err := agewrap.CheckTimeStanzas(outer, p, h.DateKey.Round); err != nil {
		return in, nil, in.fail(8, "tlock stanza", err)
	}
	in.pass(8, "tlock stanza", fmt.Sprintf("round %d, chain %s", h.DateKey.Round, p.ChainHashHex()))

	return in, &parsed{
		prelude: pre,
		sealed:  sealed,
		payload: io.MultiReader(bytes.NewReader(captured.Bytes()), r),
	}, nil
}

// readExactly reads n bytes. The buffer grows with the data actually read, so
// a short file that declares a large length (within the §57 limits) does not
// force an allocation of that size.
func readExactly(r io.Reader, n int64) ([]byte, error) {
	var b bytes.Buffer
	if _, err := io.CopyN(&b, r, n); err != nil {
		return nil, err
	}
	return b.Bytes(), nil
}

// unusable describes the unusable extensions for a check detail.
func unusable(u []extension.Unusable) string {
	if len(u) == 0 {
		return ""
	}
	return fmt.Sprintf(", %d unusable noncritical extensions", len(u))
}

func infos(stanzas []*age.Stanza) []StanzaInfo {
	out := make([]StanzaInfo, len(stanzas))
	for i, s := range stanzas {
		out[i] = StanzaInfo{Type: s.Type, Args: s.Args}
	}
	return out
}