How to reuse existing private key to generate a digital signature using smartcard and PKCS#11

Question

I want to create a digital signature using pkcs11 standard. Lets suppose that I already has a public and private key pair that is stored on my smart card. This keys was generated by using next code:

byte[] ckaId = session.GenerateRandom(20);

// Prepare attribute template of new public key
var publicKeyAttributes = new List<ObjectAttribute>();
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_TOKEN, true));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_PRIVATE, false));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_LABEL, PKCS11Settings.ApplicationName));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_ID, ckaId));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_ENCRYPT, true));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_VERIFY, true));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_VERIFY_RECOVER, true));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_WRAP, true));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_MODULUS_BITS, 1024));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_PUBLIC_EXPONENT, new byte[] { 0x01, 0x00, 0x01 }));

// Prepare attribute template of new private key
var privateKeyAttributes = new List<ObjectAttribute>();
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_TOKEN, true));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_PRIVATE, true));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_LABEL, PKCS11Settings.ApplicationName));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_ID, ckaId));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_SENSITIVE, true));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_DECRYPT, true));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_SIGN, true));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_SIGN_RECOVER, true));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_UNWRAP, true));

// Specify key generation mechanism
Mechanism mechanism = new Mechanism(CKM.CKM_RSA_PKCS_KEY_PAIR_GEN);

// Generate key pair
session.GenerateKeyPair(mechanism, publicKeyAttributes, privateKeyAttributes, out publicKeyHandle, out privateKeyHandle);

Now I can use these keys to sign some data. For example:

var mechanism = new Mechanism(CKM.CKM_RSA_PKCS);
byte[] byteContent = (ConvertUtils.Utf8StringToBytes("Hello World!!!"));
byte[] signature = session.Sign(mechanism, derivedKey, byteContent);

This code works perfect when you want to create keys and then use it in C_sign method

But how to get an access to the already existing keys to do the similar operation? As I understand I should derive a private key from existing one by using the C_Derrive() method and than use it in C_Sign() method. For this purpose I wrote next code:

// Prepare attribute template of new key
List<ObjectAttribute> objectAttributes = new List<ObjectAttribute>();
objectAttributes.Add(new ObjectAttribute(CKA.CKA_CLASS, CKO.CKO_SECRET_KEY));
objectAttributes.Add(new ObjectAttribute(CKA.CKA_KEY_TYPE, CKK.CKK_DES3));
objectAttributes.Add(new ObjectAttribute(CKA.CKA_ENCRYPT, true));
objectAttributes.Add(new ObjectAttribute(CKA.CKA_DECRYPT, true));
objectAttributes.Add(new ObjectAttribute(CKA.CKA_DERIVE, true));
objectAttributes.Add(new ObjectAttribute(CKA.CKA_EXTRACTABLE, true));

// Specify key generation mechanism
Mechanism mechanism = new Mechanism(CKM.CKM_RSA_PKCS);

// Generate key
ObjectHandle baseKey = session.GenerateKey(mechanism, objectAttributes);

byte[] dt = session.GenerateRandom(24);

// Specify mechanism parameters
var mechanismParams = new CkKeyDerivationStringData(dt);

// Specify derivation mechanism with parameters
Mechanism mech = new Mechanism(CKM.CKM_RSA_PKCS, mechanismParams);

// Derive key
ObjectHandle derivedKey = session.DeriveKey(mech, baseKey, null);


byte[] byteContent = (ConvertUtils.Utf8StringToBytes("Hello World!"));
byte[] signature = session.Sign(mech, derivedKey, byteContent);

But when I run this code it will throw the next error:

Method C_GenerateKey returned CKR_MECHANISM_INVALID

Could anybody tell me what I'm doing wrong and how to solve this problem ?

Answer 1

If you want to acquire ObjectHandle for an existing key you need to find the key by its attributes such as label etc. Key derivation is completely different cryptographic operation.

Following code sample searches for the keys you have generated by the code present in your question:

// Prepare attribute template that defines search criteria for public key
List<ObjectAttribute> publicKeyAttributes = new List<ObjectAttribute>();
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_CLASS, CKO.CKO_PUBLIC_KEY));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_KEY_TYPE, CKK.CKK_RSA));
publicKeyAttributes.Add(new ObjectAttribute(CKA.CKA_LABEL, PKCS11Settings.ApplicationName));

// Find all objects that match provided attributes
List<ObjectHandle> foundPublicKeys = session.FindAllObjects(publicKeyAttributes);
if (foundPublicKeys == null || foundPublicKeys.Count != 1)
    throw new Exception("Unable to find public key");

// Prepare attribute template that defines search criteria for private key
List<ObjectAttribute> privateKeyAttributes = new List<ObjectAttribute>();
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_CLASS, CKO.CKO_PRIVATE_KEY));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_KEY_TYPE, CKK.CKK_RSA));
privateKeyAttributes.Add(new ObjectAttribute(CKA.CKA_LABEL, PKCS11Settings.ApplicationName));

// Find all objects that match provided attributes
List<ObjectHandle> foundPrivateKeys = session.FindAllObjects(publicKeyAttributes);
if (foundPrivateKeys == null || foundPrivateKeys.Count != 1)
    throw new Exception("Unable to find private key");

// Use found object handles
ObjectHandle publicKeyHandle = foundPublicKeys[0];
ObjectHandle privateKeyHandle = foundPrivateKeys[0];