Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/google/go-tpm/tpm2/create_credential.go

@@ -0,0 +1,40 @@
+package tpm2
+
+import (
+	"io"
+)
+
+var (
+	labelIdentity = "IDENTITY"
+)
+
+// CreateCredential creates an encrypted secret that can be recovered using ActivateCredential as part of a key-attestation flow.
+func CreateCredential(rand io.Reader, pub LabeledEncapsulationKey, name []byte, credentialValue []byte) (idObject []byte, encSecret []byte, err error) {
+	secret, ciphertext, err := pub.Encapsulate(rand, labelIdentity)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Marshal the credentialValue as a TPM2B_DIGEST before encrypting it.
+	// See Part 1, "Credential Protection", and Part 2, "TPMS_ID_OBJECT".
+	credential2B := Marshal(TPM2BDigest{Buffer: credentialValue})
+
+	// Encrypt the credentialValue as encIdentity.
+	encIdentity, err := deriveAndEncrypt(pub, secret, name, credential2B)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Compute the HMAC of (encIdentity || name)
+	identityHMAC, err := deriveAndHMAC(pub, secret, nil, encIdentity, name)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Marshal the virtual TPMS_ID_OBJECT ourselves. We have to do this since encIdentity's size is encrypted.
+	idObject = make([]byte, 0, 2+len(identityHMAC)+len(encIdentity))
+	idObject = append(idObject, Marshal(TPM2BDigest{Buffer: identityHMAC})...)
+	idObject = append(idObject, encIdentity...)
+
+	return idObject, ciphertext, nil
+}