1 files changed, 58 insertions, 0 deletions
diff --git a/pkg/oaddr/oaddr.go b/pkg/oaddr/oaddr.go
new file mode 100644
index 0000000..f065ec9
--- /dev/null
+++ b/pkg/oaddr/oaddr.go
@@ -0,0 +1,58 @@
+// Package oaddr provides onion address formatting
+package oaddr
+
+import (
+	"crypto"
+	"crypto/ed25519"
+	"encoding/base32"
+	"fmt"
+	"strings"
+
+	"golang.org/x/crypto/sha3"
+)
+
+// OnionAddress is an Ed25519 public key that represents a v3 onion address
+type OnionAddress [ed25519.PublicKeySize]byte
+
+// New outputs an onion address from a public key as defined in RFC 8032
+func New(pub []byte) (addr OnionAddress, err error) {
+	if got, want := len(pub), ed25519.PublicKeySize; got != want {
+		return addr, fmt.Errorf("invalid public key size: %d", got)
+	}
+
+	copy(addr[:], pub)
+	return addr, nil
+}
+
+// NewFromSigner outputs an onion address for a given signer
+func NewFromSigner(s crypto.Signer) (addr OnionAddress, err error) {
+	switch t := s.Public().(type) {
+	case ed25519.PublicKey:
+		addr, err = New(s.Public().(ed25519.PublicKey))
+	default:
+		err = fmt.Errorf("unknown key type: %v", t)
+	}
+	return
+}
+
+// String formats addr as defined in rend-spec-v3, see:
+// https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt#n2160
+func (addr OnionAddress) String() string {
+	b := addr[:]
+	b = append(b, addr.checksum()...)
+	b = append(b, addr.version()...)
+	return strings.ToLower(base32.StdEncoding.EncodeToString(b)) + ".onion"
+}
+
+func (addr OnionAddress) checksum() []byte {
+	h := sha3.New256()
+	h.Write([]byte(".onion checksum"))
+	h.Write(addr[:])
+	h.Write(addr.version())
+	sum := h.Sum(nil)
+	return sum[:2]
+}
+
+func (addr OnionAddress) version() []byte {
+	return []byte{0x03}
+}