1 files changed, 76 insertions, 1 deletions
diff --git a/lib/paseto/message.go b/lib/paseto/message.go
index 47718f9e..e9dca297 100644
--- a/lib/paseto/message.go
+++ b/lib/paseto/message.go
@@ -3,8 +3,83 @@
 
 package paseto
 
+import (
+	"crypto/ed25519"
+	"encoding/base64"
+	"fmt"
+	"strings"
+	"time"
+)
+
 // Message defines the payload be signed and verified by sender/receiver.
 type Message struct {
-	Payload Payload
+	// The following fields are filled after Unpack and MUST not be used
+	// directly.
+	RawPayload []byte
+	RawFooter  []byte
+	Sig        []byte
+	PAE        []byte
+
+	// The Payload and optional Footer to be packed and signed into token.
 	Footer  Footer
+	Payload Payload
+}
+
+// NewMessage returns [Message] where all fields except [Payload.Data] and
+// [Footer.Data] are set using send, receiver, subject, and current time.
+func NewMessage(sender, receiver Peer, subject string) (msg *Message) {
+	now := time.Now().UTC().Unix()
+	msg = &Message{
+		Payload: Payload{
+			Issuer:    sender.ID,
+			Audience:  receiver.ID,
+			Subject:   subject,
+			IssuedAt:  now,
+			NotBefore: now,
+			ExpiredAt: now + DefaultTTL,
+		},
+		Footer: Footer{
+			PeerID: sender.ID,
+		},
+	}
+	return msg
+}
+
+// Unpack returns the decoded token into a [Message].
+func (msg *Message) Unpack(header, token string, implicit []byte) (err error) {
+	logp := `Unpack`
+
+	// Step 3: verify the header and unpack the footer if it exists.
+	token, found := strings.CutPrefix(token, header)
+	if !found {
+		return fmt.Errorf(`%s: invalid header, want %s`, logp, header)
+	}
+	token, footerb64, found := strings.Cut(token, `.`)
+	if found {
+		msg.RawFooter, err = base64.RawURLEncoding.DecodeString(footerb64)
+		if err != nil {
+			return fmt.Errorf(`%s: invalid footer: %w`, logp, err)
+		}
+	}
+
+	// Step 4: Decodes the payload.
+	paysig, err := base64.RawURLEncoding.DecodeString(token)
+	if err != nil {
+		return fmt.Errorf(`%s: %w`, logp, err)
+	}
+	lenpaysig := len(paysig)
+	if lenpaysig <= ed25519.SignatureSize {
+		return fmt.Errorf(`%s: invalid payload size %d`, logp, lenpaysig)
+	}
+	msg.RawPayload = paysig[:lenpaysig-64]
+	msg.Sig = paysig[lenpaysig-64:]
+
+	// Step 5: Generate PAE.
+	msg.PAE, err = PreAuthEncode([]byte(header), msg.RawPayload,
+		msg.RawFooter, implicit)
+	if err != nil {
+		return fmt.Errorf(`%s: %w`, logp, err)
+	}
+
+	return nil
 }