1 files changed, 98 insertions, 0 deletions
diff --git a/signerverifier/ed25519.go b/signerverifier/ed25519.go
new file mode 100644
index 0000000..0a2210c
--- /dev/null
+++ b/signerverifier/ed25519.go
@@ -0,0 +1,98 @@
+package signerverifier
+
+import (
+	"context"
+	"crypto"
+	"crypto/ed25519"
+	"encoding/hex"
+	"fmt"
+	"os"
+)
+
+const ED25519KeyType = "ed25519"
+
+// ED25519SignerVerifier is a dsse.SignerVerifier compliant interface to sign
+// and verify signatures using ED25519 keys.
+type ED25519SignerVerifier struct {
+	keyID   string
+	private ed25519.PrivateKey
+	public  ed25519.PublicKey
+}
+
+// NewED25519SignerVerifierFromSSLibKey creates an Ed25519SignerVerifier from an
+// SSLibKey.
+func NewED25519SignerVerifierFromSSLibKey(key *SSLibKey) (*ED25519SignerVerifier, error) {
+	if len(key.KeyVal.Public) == 0 {
+		return nil, ErrInvalidKey
+	}
+
+	public, err := hex.DecodeString(key.KeyVal.Public)
+	if err != nil {
+		return nil, fmt.Errorf("unable to create ED25519 signerverifier: %w", err)
+	}
+
+	var private []byte
+	if len(key.KeyVal.Private) > 0 {
+		private, err = hex.DecodeString(key.KeyVal.Private)
+		if err != nil {
+			return nil, fmt.Errorf("unable to create ED25519 signerverifier: %w", err)
+		}
+
+		// python-securesystemslib provides an interface to generate ed25519
+		// keys but it differs slightly in how it serializes the key to disk.
+		// Specifically, the keyval.private field includes _only_ the private
+		// portion of the key while libraries such as crypto/ed25519 also expect
+		// the public portion. So, if the private portion is half of what we
+		// expect, we append the public portion as well.
+		if len(private) == ed25519.PrivateKeySize/2 {
+			private = append(private, public...)
+		}
+	}
+
+	return &ED25519SignerVerifier{
+		keyID:   key.KeyID,
+		public:  ed25519.PublicKey(public),
+		private: ed25519.PrivateKey(private),
+	}, nil
+}
+
+// Sign creates a signature for `data`.
+func (sv *ED25519SignerVerifier) Sign(ctx context.Context, data []byte) ([]byte, error) {
+	if len(sv.private) == 0 {
+		return nil, ErrNotPrivateKey
+	}
+
+	signature := ed25519.Sign(sv.private, data)
+	return signature, nil
+}
+
+// Verify verifies the `sig` value passed in against `data`.
+func (sv *ED25519SignerVerifier) Verify(ctx context.Context, data []byte, sig []byte) error {
+	if ok := ed25519.Verify(sv.public, data, sig); ok {
+		return nil
+	}
+	return ErrSignatureVerificationFailed
+}
+
+// KeyID returns the identifier of the key used to create the
+// ED25519SignerVerifier instance.
+func (sv *ED25519SignerVerifier) KeyID() (string, error) {
+	return sv.keyID, nil
+}
+
+// Public returns the public portion of the key used to create the
+// ED25519SignerVerifier instance.
+func (sv *ED25519SignerVerifier) Public() crypto.PublicKey {
+	return sv.public
+}
+
+// LoadED25519KeyFromFile returns an SSLibKey instance for an ED25519 key stored
+// in a file in the custom securesystemslib format.
+func LoadED25519KeyFromFile(path string) (*SSLibKey, error) {
+	contents, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("unable to load ED25519 key from file: %w", err)
+	}
+
+	return LoadKeyFromSSLibBytes(contents)
+}