commit 23c2b1f459835795743b9469f2b59874bf505ff7
Author: Alan Shaw <alan.shaw@protocol.ai>
Date:   Wed Sep 13 22:32:07 2023 +0100

    first commit

diff --git a/did.go b/did.go
new file mode 100644
index 0000000..b093ce4
--- /dev/null
+++ b/did.go
@@ -0,0 +1,72 @@
+package did
+
+import (
+	"fmt"
+	"strings"
+
+	mbase "github.com/multiformats/go-multibase"
+	varint "github.com/multiformats/go-varint"
+)
+
+const Prefix = "did:"
+const KeyPrefix = "did:key:"
+
+const Ed25519 = 0xed
+
+type DID struct {
+	str string
+}
+
+// Undef can be used to represent a nil or undefined DID, using DID{}
+// directly is also acceptable.
+var Undef = DID{}
+
+func (d DID) Defined() bool {
+	return d.str != ""
+}
+
+func (d DID) Bytes() []byte {
+	if !d.Defined() {
+		return nil
+	}
+	return []byte(d.str)
+}
+
+func (d DID) DID() DID {
+	return d
+}
+
+func (d DID) String() string {
+	key, _ := mbase.Encode(mbase.Base58BTC, []byte(d.str))
+	return "did:key:" + key
+}
+
+func Decode(bytes []byte) (DID, error) {
+	code, _, err := varint.FromUvarint(bytes)
+	if err != nil {
+		return Undef, err
+	}
+	if code == Ed25519 {
+		return DID{str: string(bytes)}, nil
+	}
+	return Undef, fmt.Errorf("decoding DID: unsupported DID encoding: 0x%x", code)
+}
+
+func Parse(str string) (DID, error) {
+	if !strings.HasPrefix(str, Prefix) {
+		return Undef, fmt.Errorf("parsing DID: must start with 'did:'")
+	}
+
+	if strings.HasPrefix(str, KeyPrefix) {
+		code, bytes, err := mbase.Decode(str[len(KeyPrefix):])
+		if err != nil {
+			return Undef, err
+		}
+		if code != mbase.Base58BTC {
+			return Undef, fmt.Errorf("parsing DID key: not Base58BTC encoded")
+		}
+		return Decode(bytes)
+	}
+
+	return Undef, fmt.Errorf("parsing DID: unsupported method")
+}
diff --git a/did_test.go b/did_test.go
new file mode 100644
index 0000000..720e733
--- /dev/null
+++ b/did_test.go
@@ -0,0 +1,53 @@
+package did
+
+import (
+	"testing"
+)
+
+func TestParseDIDKey(t *testing.T) {
+	str := "did:key:z6Mkod5Jr3yd5SC7UDueqK4dAAw5xYJYjksy722tA9Boxc4z"
+	d, err := Parse(str)
+	if err != nil {
+		t.Fatalf("%v", err)
+	}
+	if d.String() != str {
+		t.Fatalf("expected %v to equal %v", d.String(), str)
+	}
+}
+
+func TestDecodeDIDKey(t *testing.T) {
+	str := "did:key:z6Mkod5Jr3yd5SC7UDueqK4dAAw5xYJYjksy722tA9Boxc4z"
+	d0, err := Parse(str)
+	if err != nil {
+		t.Fatalf("%v", err)
+	}
+	d1, err := Decode(d0.Bytes())
+	if err != nil {
+		t.Fatalf("%v", err)
+	}
+	if d1.String() != str {
+		t.Fatalf("expected %v to equal %v", d1.String(), str)
+	}
+}
+
+func TestEquivalence(t *testing.T) {
+	u0 := DID{}
+	u1 := Undef
+	if u0 != u1 {
+		t.Fatalf("undef DID not equivalent")
+	}
+
+	d0, err := Parse("did:key:z6Mkod5Jr3yd5SC7UDueqK4dAAw5xYJYjksy722tA9Boxc4z")
+	if err != nil {
+		t.Fatalf("%v", err)
+	}
+
+	d1, err := Parse("did:key:z6Mkod5Jr3yd5SC7UDueqK4dAAw5xYJYjksy722tA9Boxc4z")
+	if err != nil {
+		t.Fatalf("%v", err)
+	}
+
+	if d0 != d1 {
+		t.Fatalf("two equivalent DID not equivalent")
+	}
+}