SecKeyCreateWithData Returns ‘Nil’ with Error

Question

We are trying to create a ’SecKey’ from a ‘.der’ file. But ‘SecKeyCreateWithData’ always throw ‘Nil ’ with Error. 

Steps Followed::

First we created a ECDSA Private & Public key pair with the below Commands , then converted the .pem file holding the private key to ‘.der’file.  Finally used the ‘.der’ file to generate a ‘SecKey’ via code.

Commands Used to Generate Private & Public Keys::

openssl ecparam -genkey -name prime256v1 -noout -out ec-key-pair.pem

openssl ec -in ec-key-pair.pem -pubout -out ec-key-pair.pub

Command Used to Generate .Der file:: 

openssl pkey -outform der -in ec-key-pair.pem -out ec-key-pair.der

Content Inside .pem file::

Error Received::

Unmanaged - _value : Error Domain=NSOSStatusErrorDomain Code=-50 "EC private key creation from data failed" UserInfo={NSDescription=EC private key creation from data failed}

Minimum Deployment Target Used::

iOS 14.0

Code Used::

    if let certificateData = NSData(contentsOf:Bundle.main.url(forResource: "ec-key-pair", withExtension: "der")! ) {
            var error: Unmanaged<CFError>? = nil
            let privateSecKey = SecKeyCreateWithData(certificateData , [
                   kSecAttrKeyType: kSecAttrKeyTypeEC,
                   kSecAttrKeyClass: kSecAttrKeyClassPrivate] as NSDictionary, &error)
}

idz · Accepted Answer · 2021-07-08 18:45:53Z

0

It looks like you're not passing the key in the correct format. You're passing ASN.1 DER and the format SecKeyCreateWithData expects is explained here: https://developer.apple.com/documentation/security/1643698-seckeycopyexternalrepresentation

For an elliptic curve private key, the output is formatted as the public key concatenated with the big endian encoding of the secret scalar, or 04 || X || Y || K.

I do not know if there is a way to directly generate the format required from OpenSSL. You may have to do some parsing!

answered Jul 8, 2021 at 18:45

idz

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