Retrieve SecKey from NSData - ios

Question

I encountered an issue with creating SecKey from NSData. Basically my client-server communication is based on signature created with private key and verified on the server with public key.
I am implementing session transfer between two devices and in order to continue communication I need those keys to be transferred as well. I am converting SecKey to NSData and sending it via bluetooth, but on other side I cannot convert NSData to SecKey back to use encryption.
Could you help please?

Answer 1

More complete example (swift 4, iOS 10+) - assuming you have a Base64 encoded string. Note that the other side of the connection needs to also be creating key payloads using the same format (i.e. RSA - PKCS #1, also verify key size ). This function handles public or private keys (security caveats omitted for brevity).
// Extract secKey from encoded string - defaults to extracting public keys
func decodeSecKeyFromBase64(encodedKey: String, isPrivate: Bool = false) -> SecKey? {
var keyClass = kSecAttrKeyClassPublic
if isPrivate {
keyClass = kSecAttrKeyClassPrivate
}
let attributes: [String:Any] =
[
kSecAttrKeyClass as String: keyClass,
kSecAttrKeyType as String: kSecAttrKeyTypeRSA,
kSecAttrKeySizeInBits as String: 2048,
]
guard let secKeyData = Data.init(base64Encoded: encodedKey) else {
print("Error: invalid encodedKey, cannot extract data")
return nil
}
guard let secKey = SecKeyCreateWithData(secKeyData as CFData, attributes as CFDictionary, nil) else {
print("Error: Problem in SecKeyCreateWithData()")
return nil
}
return secKey
}

Answer 2

Starting from iOS 10 you can use the following code:
let attributes: [String:Any] = [
kSecAttrKeyClass as String: kSecAttrKeyClassPublic,
kSecAttrKeyType as String: kSecAttrKeyTypeRSA,
kSecAttrKeySizeInBits as String: blockSize,
]
secKey = SecKeyCreateWithData(secKeyData as CFData, attributes as CFDictionary, nil)

Answer 3

what I use with success ...
func obtainKeyData(tag: String) -> NSData? {
var keyRef: AnyObject?
let query: Dictionary<String, AnyObject> = [
String(kSecAttrKeyType): kSecAttrKeyTypeRSA,
String(kSecReturnData): kCFBooleanTrue as CFBoolean,
String(kSecClass): kSecClassKey as CFStringRef,
String(kSecAttrApplicationTag): tag as CFStringRef,
]
let result: NSData?
switch SecItemCopyMatching(query, &keyRef) {
case noErr:
result = keyRef as? NSData
default:
result = nil
}
return result
}
func insertPublicKey(publicTag: String, data: NSData) -> SecKeyRef? {
let query: Dictionary<String, AnyObject> = [
String(kSecAttrKeyType): kSecAttrKeyTypeRSA,
String(kSecClass): kSecClassKey as CFStringRef,
String(kSecAttrApplicationTag): publicTag as CFStringRef,
String(kSecValueData): data as CFDataRef,
String(kSecReturnPersistentRef): true as CFBooleanRef]
var persistentRef: AnyObject?
let status = SecItemAdd(query, &persistentRef)
if status != noErr && status != errSecDuplicateItem {
return nil
}
return obtainKey(publicTag)
}
func obtainKey(tag: String) -> SecKey? {
var keyRef: AnyObject?
let query: Dictionary<String, AnyObject> = [
String(kSecAttrKeyType): kSecAttrKeyTypeRSA,
String(kSecReturnRef): kCFBooleanTrue as CFBoolean,
String(kSecClass): kSecClassKey as CFStringRef,
String(kSecAttrApplicationTag): tag as CFStringRef,
]
let status = SecItemCopyMatching(query, &keyRef)
switch status {
case noErr:
if let ref = keyRef {
return (ref as! SecKeyRef)
}
default:
break
}
return nil
}
There is no easy way to transfer private part of the key pair( it is possible, but try to avoid it )