Generate private key that cannot be exported

Question

I want to generate a key pair on an iPhone such that the private key can only ever be used to sign blocks of data on that particular iPhone.

The SecKeyGeneratePair function looks promising. I can generate a key pair to the keychain (using kSecAttrIsPermanent) and I can disable decryption, derivation and unwrapping with the private key (setting kSecAttrCanDecrypt, kSecAttrCanDerive and kSecAttrCanUnwrap to false).

Two things worry me about key pairs generated with SecKeyGeneratePair:

1. Is it possible to export the private key outside of the keychain into application memory?

2. Is it possible to change a key property (e.g. set kSecAttrCanDecrypt to true) after a private key has been created?

Answer 1

This article provides more details (compared to other answers in this thread):

SecGenerateKeyPair(), which is used to generate RSA and ECDSA key pairs, can now be configured to directly store the generated private key in the device’s Keychain (within the Secure Enclave). This means that the private key can be used without ever leaving the device’s Secure Enclave.

And the important addition:

The kSecAttrTokenIDSecureEnclave attribute needs to be used when generating the key pair.

If you don't specify this attribute the private key will be accessible even on iOS9.

Answer 2

To answer the first question, the private key cannot be retrieved according to this source:

One API call, SecKeyGeneratePair(), creates a public and private key. The public key is returned to the app, and the private key is sent directly to the Secure Enclave. This private key cannot be retrieved.

More information is available here:

The supported keys are Elliptic Curve P256, the private key is not extractible in any form, even protected, and the applications are RawSign and RawVerify.