Add last night's progress.

Basic functionality for HOTP, TOTP, and YubiKey OTP. Still need YubiKey
HMAC, serialisation, check, and scan.

yubikey.go

@@ -0,0 +1,56 @@
+package twofactor
+
+// Implement YubiKey OTP and YubiKey HOTP.
+
+import (
+	"github.com/conformal/yubikey"
+	"github.com/gokyle/twofactor/modhex"
+	"hash"
+)
+
+// YubiKey is an implementation of the YubiKey hard token. Note
+// that the internal counter only actually uses 32 bits.
+type YubiKey struct {
+	token   yubikey.Token
+	counter uint64
+	key     yubikey.Key
+	public  []byte
+}
+
+func (yk *YubiKey) Public() []byte {
+	return yk.public[:]
+}
+
+func (yk *YubiKey) Counter() uint64 {
+	return yk.counter
+}
+
+func (yk *YubiKey) SetCounter(counter uint64) {
+	yk.counter = counter & 0xffffffff
+}
+
+func (yk *YubiKey) Key() []byte {
+	return yk.key[:]
+}
+
+func (yk *YubiKey) Size() int {
+	return yubikey.OTPSize + len(yk.public)
+}
+
+func (yk *YubiKey) OTP() string {
+	otp := yk.token.Generate(yk.key)
+	if otp == nil {
+		return ""
+	}
+	return modhex.StdEncoding.EncodeToString(otp.Bytes())
+}
+
+// Hash always returns nil, as the YubiKey tokens do not use a hash
+// function.
+func (yk *YubiKey) Hash() func() hash.Hash {
+	return nil
+}
+
+func (yk *YubiKey) Type() Type {
+	return YUBIKEY
+}