Unique RSA KeyChain item

Question

Created Feb ’19

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During the creation of several key items, I noticed that there are several 'label' or 'tag' options. I did some investigation and I found three different, interesting, values.

kSecAttrApplicationTag - A key whose value indicates the item's private tag.

kSecAttrApplicationLabel - A key whose value indicates the item's application label.

kSecAttrLabel - A key whose value is a string indicating the item's label.

I read that the kSecAttrLabel is "human readable data". But what exactly is meant with the description of the kSecAttrApplicationTag? What exactly is the private tag.

Another question I have is, how can I uniquely identify a key. Say I want to have a single key to encrypt a specific file, how would I go about doing so? Theoretically, I could set the kSecAttrApplicationLabel, as this has to be a unique value, meaing if I were to set the value to "com.app.appname.someidentifier".data(using: .utf8)! an error would occur if the key would (accidentally) be created again (which is what I want to prevent). However the discussion says "in particular, for keys of class

kSecAttrKeyClassPublic

and

kSecAttrKeyClassPrivate

, the value of this attribute is the hash of the public key", and RSA keys do have the public/private class, so the value would no longer be the hash of the public key. Am I actually allowed to overwrite the kSecAttrApplicationLabel? If not, do I have to check if a key for kSecAttrApplicationTag/kSecAttrLabel already exists and delete it first, before adding a new 'unique' key?

Thanks in advance!

Answered by DTS Engineer in 350430022

As I read that the errSecDuplicateItem will never be triggered, as the
kSecAttrApplicationLabel
will differ each time for a random key

Correct. As mentioned in the post I referenced earlier, the expected value for

kSecAttrApplicationLabel

for an asymmetric key is the public key hash. If you’re generating a new key pair, the publish key hash is essentially a large random number, and thus this attribute will be globally unique.

the uniqueness of the key relies on the entire dictionary/property list and not on a single property, correct?

Not the entire attribute list, but the class-specific list of attributes given in the

errSecDuplicateItem

documentation.

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Answer 1

DTS Engineer OP

Apple

Feb ’19

I struggle to remember this info myself, so a while back I posted a summary of their meaning for the benefit of Future Quinn™. Please read that through and then post a follow-up here if there’s still anything unclear.

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Answer 2

Craz1k0ek OP

Feb ’19

Thanks for your reply. I do read that the kSecApplicationTag "Is a component of key uniqueness". However, how does this component work towards key uniqueness? See the example below

let keyAttributes: [String: Any] = [
    String(kSecAttrKeyType)         : kSecAttrKeyTypeRSA,
    String(kSecAttrKeySizeInBits)   : 4096,
    String(kSecAttrApplicationTag)  : kKeyTag,
    String(kSecPrivateKeyAttrs)     :
    [
        String(kSecAttrIsPermanent)     : true
    ]
]
var error: Unmanaged? = nil
let privateKey = SecKeyCreateRandomKey(keyAttributes as CFDictionary, &error)

The kKeyTag is a constant, it does not change. However, everytime I trigger this block of code, another RSA key gets added to the KeyChain with the kKeyTag, in other words, the error object remains nill and no errSecDuplicateItem is created. Therefore, I don't believe the kKeyTag works towards key uniqueness, as I can not unique identify the key via the tag. Can you elaborate further?

0

Answer 3

DTS Engineer OP

Apple

Feb ’19

You only get

errSecDuplicateItem

if every attribute that contributes to key uniqueness is the same. You can find a list of such attributes, which is different for each keychain items class, in the

errSecDuplicateItem

documentation.

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Apple Developer Relations, Developer Technical Support, Core OS/Hardware

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Answer 4

Craz1k0ek OP

Feb ’19

Ah, right. However, question remains, unless my assumption of earlier is correct:

"do I have to check if a key for kSecAttrApplicationTag/kSecAttrLabel already exists and delete it first, before adding a new 'unique' key?"

As I read that the errSecDuplicateItem will never be triggered, as the kSecAttrApplicationLabel will differ each time for a random key, thus meaning it's technically not a duplicate. In other words, the uniqueness of the key relies on the entire dictionary/property list and not on a single property, correct?

0

Answer 5

DTS Engineer OP

Apple

Feb ’19

Accepted Answer

As I read that the errSecDuplicateItem will never be triggered, as the
kSecAttrApplicationLabel
will differ each time for a random key

Correct. As mentioned in the post I referenced earlier, the expected value for

kSecAttrApplicationLabel

for an asymmetric key is the public key hash. If you’re generating a new key pair, the publish key hash is essentially a large random number, and thus this attribute will be globally unique.

the uniqueness of the key relies on the entire dictionary/property list and not on a single property, correct?

Not the entire attribute list, but the class-specific list of attributes given in the

errSecDuplicateItem

documentation.

Share and Enjoy
—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

0