NextMyDay/internal/core/httpx/presession.go

package httpx

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"net/http"
	"strings"
)

// PreSessionCookieName is the cookie carrying the anonymous double-submit CSRF
// token (A15). It is issued on the enroll/login screens and verified on the
// anonymous actions — before any session exists (SPEC-006 §5).
const PreSessionCookieName = "myday_csrf"

// ErrNoSigningKey is returned when the signing secret is empty — fail closed.
var ErrNoSigningKey = errors.New("httpx: signing key is required")

// Signer signs and verifies opaque cookie values with HMAC-SHA256. It is the
// primitive behind the pre-session CSRF token and the WebAuthn ceremony state:
// both are tamper-evident without a server-side table. The key comes from
// `secrets.cookie_secret`.
type Signer struct{ key []byte }

// NewSigner builds a Signer from the configured key. An empty key fails closed.
func NewSigner(key []byte) (*Signer, error) {
	if len(key) == 0 {
		return nil, ErrNoSigningKey
	}
	k := make([]byte, len(key))
	copy(k, key)
	return &Signer{key: k}, nil
}

// Sign returns `base64url(value).base64url(HMAC)`. Encoding the value keeps the
// "." delimiter unambiguous even when value contains one.
func (s *Signer) Sign(value string) string {
	payload := base64.RawURLEncoding.EncodeToString([]byte(value))
	return payload + "." + s.mac(payload)
}

// Unsign verifies a token and returns the original value. A tampered or
// malformed token reports ok=false.
func (s *Signer) Unsign(token string) (value string, ok bool) {
	payload, sig, found := strings.Cut(token, ".")
	if !found || payload == "" || sig == "" {
		return "", false
	}
	if !hmac.Equal([]byte(sig), []byte(s.mac(payload))) {
		return "", false
	}
	raw, err := base64.RawURLEncoding.DecodeString(payload)
	if err != nil {
		return "", false
	}
	return string(raw), true
}

// mac is the HMAC over the domain-separated payload.
func (s *Signer) mac(payload string) string {
	m := hmac.New(sha256.New, s.key)
	m.Write([]byte("myday.sign.v1:"))
	m.Write([]byte(payload))
	return base64.RawURLEncoding.EncodeToString(m.Sum(nil))
}

// PreSessionCSRF is the signed double-submit token for the anonymous
// enroll/login forms (A15). SPEC-003 §6's session-bound CSRF does not apply
// before a session exists, so this fills the gap: the server issues a signed
// random token in an HttpOnly cookie AND embeds the same token in the form; on
// submit both must match and the signature must verify.
type PreSessionCSRF struct {
	signer *Signer
	secure bool
}

// NewPreSessionCSRF builds the helper from `secrets.cookie_secret`.
func NewPreSessionCSRF(secret []byte, secure bool) (*PreSessionCSRF, error) {
	s, err := NewSigner(secret)
	if err != nil {
		return nil, err
	}
	return &PreSessionCSRF{signer: s, secure: secure}, nil
}

// Signer exposes the underlying signer so the flows can sign their own
// short-lived ceremony state with the same key.
func (p *PreSessionCSRF) Signer() *Signer { return p.signer }

// Token mints a fresh signed token.
func (p *PreSessionCSRF) Token() (string, error) {
	var b [32]byte
	if _, err := rand.Read(b[:]); err != nil {
		return "", fmt.Errorf("httpx: csrf entropy: %w", err)
	}
	return p.signer.Sign(base64.RawURLEncoding.EncodeToString(b[:])), nil
}

// Ensure returns a valid token for the request, reusing the cookie when it is
// present and valid, else issuing a fresh one.
func (p *PreSessionCSRF) Ensure(w http.ResponseWriter, r *http.Request) (string, error) {
	if c, err := r.Cookie(PreSessionCookieName); err == nil && c.Value != "" {
		if v, ok := p.signer.Unsign(c.Value); ok && v != "" {
			return c.Value, nil
		}
	}
	tok, err := p.Token()
	if err != nil {
		return "", err
	}
	http.SetCookie(w, &http.Cookie{
		Name:     PreSessionCookieName,
		Value:    tok,
		Path:     "/",
		HttpOnly: true,
		Secure:   p.secure,
		SameSite: http.SameSiteLaxMode,
	})
	return tok, nil
}

// Valid verifies the double-submit: the submitted token must equal the cookie's
// value AND carry a valid signature. The comparison is constant-time.
func (p *PreSessionCSRF) Valid(r *http.Request, submitted string) bool {
	if submitted == "" {
		return false
	}
	c, err := r.Cookie(PreSessionCookieName)
	if err != nil || c.Value == "" {
		return false
	}
	if !hmac.Equal([]byte(c.Value), []byte(submitted)) {
		return false
	}
	v, ok := p.signer.Unsign(submitted)
	return ok && v != ""
}