add otphyp package

provides a loose implementation of TOTP for hyp to interface with

otphyp/README.md

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+# otp-hyp
+
+otp-hyp is a purpose built implementation of One-Time-Passwords for HYP (Hide Your Ports).  It supports generating secret keys, as well as time-based 64 bits of spread spectrum output using the secret key.
+
+### References
+
+* RFC 4226 - HOTP: An HMAC-Based One-Time Password Algorithm
+* RFC 6238 - TOTP: Time-Based One-Time Password Alogorithm

otphyp/go.mod

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+module deadbeef.codes/steven/hyp/otphyp
+
+go 1.22.0

otphyp/otphyp.go

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+// Package otphyp is a loose implementation of hotp standard RFC4226
+// it is not a full hotp implementation and only contains specific features required for hyp
+package otphyp
+
+import (
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/sha1"
+	"encoding/base32"
+	"encoding/binary"
+	"fmt"
+	"math"
+	"time"
+)
+
+// A loose implementation of hotp meant for our specific purposes of generating four random port numbers
+// Accepts a base32 encoded shared secret and a time
+func GeneratePorts(sharedSecret string, t time.Time) (ports [4]uint16, err error) {
+	// 30 second key rotation
+	movingFactor := uint64(math.Floor(float64(t.Unix()) / float64(30)))
+
+	sharedSecretBytes, err := base32.StdEncoding.DecodeString(sharedSecret)
+	if err != nil {
+		return [4]uint16{0, 0, 0, 0}, fmt.Errorf("failed to base32 decode shared secret string to bytes: %v", err)
+	}
+
+	buf := make([]byte, 8)
+	binary.BigEndian.PutUint64(buf, movingFactor)
+
+	mac := hmac.New(sha1.New, sharedSecretBytes)
+	mac.Write(buf)
+	sum := mac.Sum(nil)
+	offset := sum[len(sum)-1] & 0xf
+
+	// deviation from RFC4226's dynamic truncate and modulo reduction algorithm
+	// we don't need base10 human friendliness and instead just care about 64 bits / 4
+	ports = [4]uint16{
+		uint16((int(sum[offset]) & 0xff) << 8),
+		uint16((int(sum[offset+1] & 0xff)) << 8),
+		uint16((int(sum[offset+2] & 0xff)) << 8),
+		uint16((int(sum[offset+3] & 0xff)) << 8),
+	}
+
+	return ports, err
+}
+
+func GenerateSecret() (sharedSecret string, err error) {
+	sharedSecretBytes := make([]byte, 20)
+	r := rand.Reader
+	_, err = r.Read([]byte(sharedSecretBytes))
+	if err != nil {
+		return "", fmt.Errorf("failed to read from RNG to sharedSecret byte slice: %v", err)
+	}
+
+	return base32.StdEncoding.WithPadding(base32.NoPadding).EncodeToString([]byte(sharedSecretBytes)), nil
+}