diff --git a/did/crypto.go b/did/crypto.go
new file mode 100644
index 0000000..0947322
--- /dev/null
+++ b/did/crypto.go
@@ -0,0 +1,39 @@
+package did
+
+import (
+	"errors"
+
+	crypto "github.com/libp2p/go-libp2p/core/crypto"
+	"github.com/libp2p/go-libp2p/core/crypto/pb"
+	"github.com/multiformats/go-multicodec"
+	"github.com/multiformats/go-varint"
+)
+
+func FromPrivKey(privKey crypto.PrivKey) (DID, error) {
+	return FromPubKey(privKey.GetPublic())
+}
+
+func FromPubKey(pubKey crypto.PubKey) (DID, error) {
+	code, ok := map[pb.KeyType]multicodec.Code{
+		pb.KeyType_Ed25519:   multicodec.Ed25519Pub,
+		pb.KeyType_RSA:       multicodec.RsaPub,
+		pb.KeyType_Secp256k1: multicodec.Secp256k1Pub,
+		pb.KeyType_ECDSA:     multicodec.Es256,
+	}[pubKey.Type()]
+	if !ok {
+		return Undef, errors.New("Blah")
+	}
+
+	buf := varint.ToUvarint(uint64(code))
+
+	pubBytes, err := pubKey.Raw()
+	if err != nil {
+		return Undef, err
+	}
+
+	return DID{
+		str:  string(append(buf, pubBytes...)),
+		code: uint64(code),
+		key:  true,
+	}, nil
+}
diff --git a/did/crypto_test.go b/did/crypto_test.go
new file mode 100644
index 0000000..98e38a9
--- /dev/null
+++ b/did/crypto_test.go
@@ -0,0 +1,33 @@
+package did_test
+
+import (
+	"bytes"
+	"encoding/binary"
+	"testing"
+
+	"github.com/libp2p/go-libp2p/core/crypto"
+	"github.com/multiformats/go-multicodec"
+	"github.com/stretchr/testify/require"
+	"github.com/ucan-wg/go-ucan/v1/did"
+)
+
+func TestFromPubKey(t *testing.T) {
+	t.Parallel()
+
+	const example = "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK"
+
+	id, err := did.Parse(example)
+	require.NoError(t, err)
+
+	buf := bytes.NewBuffer(id.Bytes())
+
+	code, err := binary.ReadUvarint(buf)
+	require.NoError(t, err)
+	require.Equal(t, uint64(multicodec.Ed25519Pub), code)
+
+	pubKey, err := crypto.UnmarshalEd25519PublicKey(buf.Bytes())
+	require.NoError(t, err)
+
+	id2, err := did.FromPubKey(pubKey)
+	require.Equal(t, id, id2)
+}
diff --git a/did/did.go b/did/did.go
index 483485c..1b32a60 100644
--- a/did/did.go
+++ b/did/did.go
@@ -5,6 +5,7 @@ import (
 	"strings"
 
 	mbase "github.com/multiformats/go-multibase"
+	"github.com/multiformats/go-multicodec"
 	varint "github.com/multiformats/go-varint"
 )
 
@@ -13,12 +14,16 @@ const KeyPrefix = "did:key:"
 
 const DIDCore = 0x0d1d
 const Ed25519 = 0xed
+const RSA = uint64(multicodec.RsaPub)
 
 var MethodOffset = varint.UvarintSize(uint64(DIDCore))
 
+//
+// [did:key format]: https://w3c-ccg.github.io/did-method-key/
 type DID struct {
-	key bool
-	str string
+	key  bool
+	code uint64
+	str  string
 }
 
 // Undef can be used to represent a nil or undefined DID, using DID{}
@@ -36,6 +41,10 @@ func (d DID) Bytes() []byte {
 	return []byte(d.str)
 }
 
+func (d DID) Code() uint64 {
+	return d.code
+}
+
 func (d DID) DID() DID {
 	return d
 }
@@ -54,8 +63,8 @@ func Decode(bytes []byte) (DID, error) {
 	if err != nil {
 		return Undef, err
 	}
-	if code == Ed25519 {
-		return DID{str: string(bytes), key: true}, nil
+	if code == Ed25519 || code == RSA {
+		return DID{str: string(bytes), code: code, key: true}, nil
 	} else if code == DIDCore {
 		return DID{str: string(bytes)}, nil
 	}