diff --git a/db.go b/db.go
index 117445d..dc3786e 100644
--- a/db.go
+++ b/db.go
@@ -1,10 +1,13 @@
 package rendezvous
 
 import (
+	"bytes"
 	"context"
 	"crypto/rand"
+	"crypto/sha256"
 	"database/sql"
-	"errors"
+	"encoding/binary"
+	"fmt"
 	"os"
 	"time"
 
@@ -242,7 +245,7 @@ func (db *DB) Discover(ns string, cookie []byte, limit int) ([]RegistrationRecor
 	)
 
 	if cookie != nil {
-		counter, err = cookieToCounter(cookie)
+		counter, err = unpackCookie(cookie)
 		if err != nil {
 			log.Errorf("error unpacking cookie: %s", err.Error())
 			return nil, nil, err
@@ -293,15 +296,15 @@ func (db *DB) Discover(ns string, cookie []byte, limit int) ([]RegistrationRecor
 			log.Errorf("error decoding peer id: %s", err.Error())
 			continue
 		}
-		reg.Id = p
 
-		addrs, err = unpackAddrs(raddrs)
+		addrs, err := unpackAddrs(raddrs)
 		if err != nil {
 			log.Errorf("error unpacking address: %s", err.Error())
 			continue
 		}
-		reg.Addrs = addrs
 
+		reg.Id = p
+		reg.Addrs = addrs
 		reg.Ttl = int(expire - now)
 
 		if ns == "" {
@@ -317,15 +320,14 @@ func (db *DB) Discover(ns string, cookie []byte, limit int) ([]RegistrationRecor
 	}
 
 	if counter > 0 {
-		cookie = counterToCookie(counter, ns, db.nonce)
+		cookie = packCookie(counter, ns, db.nonce)
 	}
 
 	return regs, cookie, nil
 }
 
 func (db *DB) ValidCookie(ns string, cookie []byte) bool {
-	// XXX
-	return false
+	return validCookie(cookie, ns, db.nonce)
 }
 
 func (db *DB) background(ctx context.Context) {
@@ -345,21 +347,97 @@ func (db *DB) background(ctx context.Context) {
 }
 
 func packAddrs(addrs [][]byte) []byte {
-	// XXX
-	return nil
+	packlen := 0
+	for _, addr := range addrs {
+		packlen = packlen + 2 + len(addr)
+	}
+
+	packed := make([]byte, packlen)
+	buf := packed
+	for _, addr := range addrs {
+		binary.BigEndian.PutUint16(buf, uint16(len(addr)))
+		buf = buf[2:]
+		copy(buf, addr)
+		buf = buf[len(addr):]
+	}
+
+	return packed
 }
 
-func unpackAddrs(maddrs []byte) ([][]byte, error) {
-	// XXX
-	return nil, errors.New("IMPLEMENTME: unpackAddrs")
+func unpackAddrs(packed []byte) ([][]byte, error) {
+	var addrs [][]byte
+
+	buf := packed
+	for len(buf) > 1 {
+		l := binary.BigEndian.Uint16(buf)
+		buf = buf[2:]
+		if len(buf) < int(l) {
+			return nil, fmt.Errorf("bad packed address: not enough bytes %v %v", packed, buf)
+		}
+		addr := make([]byte, l)
+		copy(addr, buf[:l])
+		buf = buf[l:]
+		addrs = append(addrs, addr)
+	}
+
+	if len(buf) > 0 {
+		return nil, fmt.Errorf("bad packed address: unprocessed bytes: %v %v", packed, buf)
+	}
+
+	return addrs, nil
 }
 
-func cookieToCounter(cookie []byte) (int64, error) {
-	// XXX
-	return 0, errors.New("IMPLEMENTME: cookieToCounter")
+// cookie: counter:SHA256(nonce + ns + counter)
+func packCookie(counter int64, ns string, nonce []byte) []byte {
+	cbits := make([]byte, 8)
+	binary.BigEndian.PutUint64(cbits, uint64(counter))
+
+	hash := sha256.New()
+	_, err := hash.Write(nonce)
+	if err != nil {
+		panic(err)
+	}
+	_, err = hash.Write([]byte(ns))
+	if err != nil {
+		panic(err)
+	}
+	_, err = hash.Write(cbits)
+	if err != nil {
+		panic(err)
+	}
+
+	return hash.Sum(cbits)
 }
 
-func counterToCookie(counter int64, ns string, nonce []byte) []byte {
-	// XXX
-	return nil
+func unpackCookie(cookie []byte) (int64, error) {
+	if len(cookie) < 8 {
+		return 0, fmt.Errorf("bad packed cookie: not enough bytes: %v", cookie)
+	}
+
+	counter := binary.BigEndian.Uint64(cookie[:8])
+	return int64(counter), nil
+}
+
+func validCookie(cookie []byte, ns string, nonce []byte) bool {
+	if len(cookie) != 40 {
+		return false
+	}
+
+	cbits := cookie[:8]
+	hash := sha256.New()
+	_, err := hash.Write(nonce)
+	if err != nil {
+		panic(err)
+	}
+	_, err = hash.Write([]byte(ns))
+	if err != nil {
+		panic(err)
+	}
+	_, err = hash.Write(cbits)
+	if err != nil {
+		panic(err)
+	}
+	hbits := hash.Sum(nil)
+
+	return bytes.Equal(cookie[8:], hbits)
 }