Digest an ask and an act over named fields in a canonical encoding, not a language's encoding of a struct

The review of 2026-10-09: a digest over Go's JSON of a struct changes when a tag is renamed or a field
added, and another language cannot reproduce it. An act is now a flat map of named strings, and both
digests are over length-prefixed values in a fixed order, versioned; a test fails when a field is added
to Ask or Option without being added to the digest.

go/asks/asks.go

@@ -8,10 +8,11 @@ package asks
 import (
 	"crypto/sha256"
 	"encoding/hex"
-	"encoding/json"
 	"errors"
 	"fmt"
 	"regexp"
+	"sort"
+	"strconv"
 	"strings"
 	"time"
 )
@@ -107,25 +108,67 @@ type Option struct {
 	Binds string `json:"binds,omitempty"`
 }
 
-// ActDigest is the digest an asker puts in Option.Binds: SHA-256 over the act's JSON (Go's encoding sorts a
-// map's keys, so the same act gives the same digest), written "sha256:<hex>".
-func ActDigest(act any) (string, error) {
-	raw, err := json.Marshal(act)
-	if err != nil {
-		return "", fmt.Errorf("the act cannot be digested: %w", err)
+// An Act is what an option binds: named fields, each a string — the verb, the machine, the level, and each
+// argument under a name of its own ("arg.delivery"). Flat on purpose: its digest is over these names and
+// values alone, never over how a language happens to encode a struct.
+type Act map[string]string
+
+// ActDigest is the digest an asker puts in Option.Binds: SHA-256 over the act's canonical encoding (canonical),
+// written "sha256:<hex>". The same names and values give the same digest in any language, whatever order
+// they were set in; a field renamed, added or emptied gives another.
+func ActDigest(act Act) (string, error) {
+	if len(act) == 0 {
+		return "", fmt.Errorf("the act cannot be digested: it names nothing")
 	}
-	sum := sha256.Sum256(raw)
+	keys := make([]string, 0, len(act))
+	for k := range act {
+		if k == "" {
+			return "", fmt.Errorf("the act cannot be digested: a field has no name")
+		}
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	var b strings.Builder
+	b.WriteString("novox.act.v1\n")
+	for _, k := range keys {
+		canonical(&b, k)
+		canonical(&b, act[k])
+	}
+	sum := sha256.Sum256([]byte(b.String()))
 	return "sha256:" + hex.EncodeToString(sum[:]), nil
 }
 
+// canonical writes one value as its length in bytes, a colon, the bytes and a newline: no value can be read
+// as another's end or start, so two different sequences of values never encode the same.
+func canonical(b *strings.Builder, v string) {
+	b.WriteString(strconv.Itoa(len(v)))
+	b.WriteByte(':')
+	b.WriteString(v)
+	b.WriteByte('\n')
+}
+
 // Digest is the digest of the ask exactly as its asker published it: its id, words, options with what each
 // binds, who answers, and its expiry. The router puts it in the warrant (Warrant.AskDigest), and an asker
 // acts only on a warrant whose digest is that of the ask it keeps — so a warrant answers one ask, as the
 // person was shown it, and nothing published under the same id before or after.
+//
+// It is over the ask's named fields in a fixed order, each written canonically, and the expiry as UTC
+// RFC 3339 to the nanosecond — never over a language's encoding of the struct, so a field added to Ask
+// later changes no digest until it is added here, on purpose.
 func (a Ask) Digest() string {
-	a.Expires = a.Expires.UTC()
-	raw, _ := json.Marshal(a)
-	sum := sha256.Sum256(raw)
+	var b strings.Builder
+	b.WriteString("novox.ask.v1\n")
+	for _, v := range []string{a.ID, a.Headline, a.Explanation, a.Who,
+		a.Expires.UTC().Format(time.RFC3339Nano), a.OnExpiry, a.About, strconv.FormatBool(a.Urgent),
+		strconv.Itoa(len(a.Options))} {
+		canonical(&b, v)
+	}
+	for _, o := range a.Options {
+		for _, v := range []string{o.ID, o.Label, o.Does, string(o.Level), o.Binds} {
+			canonical(&b, v)
+		}
+	}
+	sum := sha256.Sum256([]byte(b.String()))
 	return "sha256:" + hex.EncodeToString(sum[:])
 }
 
@@ -341,7 +384,7 @@ func (w Warrant) For(asker string, a Ask) (Option, error) {
 
 // Performs checks that the act an asker is about to perform is the one the chosen option bound when it
 // asked: the act's digest equals the option's Binds. An acknowledge option that bound nothing passes.
-func (o Option) Performs(act any) error {
+func (o Option) Performs(act Act) error {
 	if o.Binds == "" && o.Level == Acknowledge {
 		return nil
 	}