You can also get the same error (SecKeyCreate init(RSAPrivateKey) failed: -50) from a RSA key if you:

1. Have not stripped the headers -----[BEGIN/END] PRIVATE KEY----- from the .pem file contents

2. Loaded the file into string as a format other than ASCII (it's probably a common mistake to try to load it as UTF8)

3. Have not created a Data object with the string using the base64Encoded parameter (which will fail if you have loaded the file in a non-ASCII format)

So if you put all of this together, the full answer is this, which successfully creates the SecKey from a .key or .pem file (with .key, assuming you chose the correct options for public key):

let privateKeyData = try! Data(contentsOf: Bundle.main.url(forResource: "rsaPrivateKey", withExtension: "pem")!)
let privateKeyString = String(data: privateKeyData, encoding: .ascii)!
    .replacingOccurrences(of: "-----BEGIN PRIVATE KEY-----", with: "")
    .replacingOccurrences(of: "-----END PRIVATE KEY-----", with: "")
    .replacingOccurrences(of: "\n", with: "")

let privateKeyStrippedData = Data(base64Encoded: publicKeyString, options: .ignoreUnknownCharacters)!
        
let options: [NSObject: NSObject] = [
    kSecAttrKeyType: kSecAttrKeyTypeRSA,
    kSecAttrKeyClass: kSecAttrKeyClassPrivate,
    kSecAttrKeySizeInBits: NSNumber(value: 2048)
]
var error: Unmanaged<CFError>?
let privateKey = SecKeyCreateWithData(privateKeyStrippedData as CFData, options as CFDictionary, &error)