Using SecKeyCreateWithData to Load data from openssl ecparam generated keys

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Created Jan ’20

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I've been reading previous threads such as this about how to import openssl generated keys using SecKeyCreateWithData but always get the error Code=-50 "EC private key creation from data failed". I understand that SecKeyCreateWithData doesn't allow the standard headers that openssl generates, but even after converting the bit string section to a binary file, it refuses to load.

This is the command I use to generate the key:

openssl ecparam -name prime256v1 -genkey -noout -out openssl.key -outform der

Using dumpasn1:

$ ~/Desktop/dumpasn1 -w76 openssl.key
  0 119: SEQUENCE {
  2   1:   INTEGER 1
  5  32:   OCTET STRING
       :     67 BF 75 38 C7 14 38 88 4C DD BC 91 A5 5C 88 10
       :     DE 0C 6F EE A7 2B 85 75 19 EC 71 1D 91 BD FD E3
39  10:   [0] {
41   8:     OBJECT IDENTIFIER prime256v1 (1 2 840 10045 3 1 7)
       :     }
51  68:   [1] {
53  66:     BIT STRING
       :       04 5A C4 BB DE 43 15 25 9E E2 A3 CD D8 80 0E 12
       :       57 24 0D 68 BD 22 61 57 D9 87 F8 E2 16 DD 8C 02
       :       AD 73 4F D0 69 6F F0 61 0D FD FB 4D EB F1 45 C0
       :       AB D4 46 82 B6 DB 69 62 8F C9 7C C9 07 09 0C 91
       :       91
       :     }
       :   }

If I trim off the headers:

$ dd if=openssl.key of=stripped.openssl.key skip=56 bs=1
65+0 records in
65+0 records out
65 bytes transferred in 0.000320 secs (203001 bytes/sec)

$ hexdump -Cv stripped.openssl.key
00000000  04 5a c4 bb de 43 15 25  9e e2 a3 cd d8 80 0e 12  |.Z...C.%........|
00000010  57 24 0d 68 bd 22 61 57  d9 87 f8 e2 16 dd 8c 02  |W$.h."aW........|
00000020  ad 73 4f d0 69 6f f0 61  0d fd fb 4d eb f1 45 c0  |.sO.io.a...M..E.|
00000030  ab d4 46 82 b6 db 69 62  8f c9 7c c9 07 09 0c 91  |..F...ib..|.....|
00000040  91                                                |.|
00000041

It's 65 bytes beginning with 04, which I believe it just the keydata. This refuses to load on iOS with the error:

var error: Unmanaged?
        guard let key = SecKeyCreateWithData(privateKey as CFData,
                                             [kSecAttrKeyType: kSecAttrKeyTypeECSECPrimeRandom,
                                              kSecAttrKeyClass: kSecAttrKeyClassPrivate,
                                              kSecAttrKeySizeInBits: 256] as CFDictionary,
                                             &error)

key length: 65
failed - Failed to create key: Unmanaged(_value: Error Domain=NSOSStatusErrorDomain Code=-50 "EC private key creation from data failed" UserInfo={NSDescription=EC private key creation from data failed})
However, it will load as a public key which I don't understand.

I tried creating a key in iOS then saving it as data and it generates a larger size (97 vs 65) but it will succesfully load.

$ hexdump -Cv ios.private.key
00000000  04 6b 17 c3 46 32 db 10  02 9a 2b 02 de 53 89 c3  |.k..F2....+..S..|
00000010  87 71 d3 bf b6 1a 64 c0  0e e1 35 6e 1c 9f af 5f  |.q....d...5n..._|
00000020  70 d2 05 ba fc 4e fb 5a  e2 93 6a 68 12 b1 18 a8  |p....N.Z..jh....|
00000030  c3 f1 2a db aa 77 a1 e0  57 bc d9 23 6c a4 82 c7  |..*..w..W..#l...|
00000040  75 a9 d0 e0 01 76 6c de  8d f0 22 64 a0 4e 06 bf  |u....vl..."d.N..|
00000050  c3 0e b1 4d 45 3e fc 9f  f1 a4 4b e3 85 e0 0f 07  |...ME>....K.....|
00000060  bd                                                |.|
00000061

I just want to understand what these extra bytes are and how to get a openssl-generated key to load in iOS.

Thanks

Answered by DTS Engineer in 403629022

In

<Security.framework/SecKey.h>

you’ll see this comment:

The requested data format depend on the type of key (kSecAttrKeyType) being created:
 * kSecAttrKeyTypeRSA               PKCS#1 format, public key can be also in x509 public key format
 * kSecAttrKeyTypeECSECPrimeRandom  ANSI X9.63 format (04 || X || Y [ || K])

Look at the last line, which describes the EC key format. The

item is in square brackets, indicating it’s optional. This is the difference between a public key (without

) and a private key (with

).

I don’t know what spec the OpenSSL key conforms to — I don’t have time tonight to look up the details — but it seems that it stores this

value in first

OCTET STRING

. That is, if you take the 67 BF 75 38 … 91 BD FD E3 bytes and append it to 04 5A C4 BB … 09 0C 91 91 bytes, you end up with 97 bytes that import correctly.

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

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

Boost

Answer 1

DTS Engineer OP

Apple

Jan ’20

Accepted Answer

In

<Security.framework/SecKey.h>

you’ll see this comment:

The requested data format depend on the type of key (kSecAttrKeyType) being created:
 * kSecAttrKeyTypeRSA               PKCS#1 format, public key can be also in x509 public key format
 * kSecAttrKeyTypeECSECPrimeRandom  ANSI X9.63 format (04 || X || Y [ || K])

Look at the last line, which describes the EC key format. The

item is in square brackets, indicating it’s optional. This is the difference between a public key (without

) and a private key (with

).

I don’t know what spec the OpenSSL key conforms to — I don’t have time tonight to look up the details — but it seems that it stores this

value in first

OCTET STRING

. That is, if you take the 67 BF 75 38 … 91 BD FD E3 bytes and append it to 04 5A C4 BB … 09 0C 91 91 bytes, you end up with 97 bytes that import correctly.

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

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

0

Answer 2

jasongz OP

Jan ’20

Wow that works! thanks

I wrote a simple ruby script to convert it if anyone needs it: https://gist.github.com/niveus/f9ed2010dadeaf1df72a2d81d1317722

1

Answer 3

maciekish OP

3w

Hi, thank you for making that script, however i still can't get it to work.

I created a key like mentioned in the script:

openssl ecparam -name prime256v1 -genkey -noout -out openssl.key -outform der

And converted it using the script:

./convert-ec-private-key-openssl-to-apple.rb openssl.key out.der
Private key start: 6, length: 32
Private key: 5...3
Public key start: 56, length: 66
Public key: 047481669ec2e9835109c55c574c78237b7a98e8743e0eec41e44ff3c496a648f7665bbdc0e1ff530a100796a3763a4b4768e6fec2538edb3ffe5ce5eeaa208cec

The filesize is 97bytes suggesting a correct conversion, and it starts with 0x04:

hexdump -Cv out.der
00000000  04 74 81 66 9e c2 e9 83  51 09 c5 5c 57 4c 78 23  |.t.f....Q..\WLx#|
00000010  7b 7a 98 e8 74 3e 0e ec  41 e4 4f f3 c4 96 a6 48  |{z..t>..A.O....H|
00000020  f7 66 5b bd c0 e1 ff 53  0a 10 07 96 a3 76 3a 4b  |.f[....S.....v:K|
00000030  47 68 e6 fe c2 53 8e db  3f fe 5c e5 ee aa 20 8c  |Gh...S..?.\... .|
00000040  ec 57 a4 64 ed dc a2 ee  13 ef 12 5f e2 2b 5a 22  |.W.d......._.+Z"|
00000050  c6 64 2b 9c 54 61 6f 1b  75 00 53 13 e7 90 71 6a  |.d+.Tao.u.S...qj|
00000060  73                                                |s|
00000061

But when i try to load it, i still get the dreaded code -50 with "EC public key creation from data failed". What am i doing wrong?

Here is my loading code:

    private static func loadPublicKey() -> SecKey? {
        guard let publicKeyData = try? Data(contentsOf: Bundle.main.url(forResource: "out", withExtension: "der")!) else {
            print("Failed to load public key data from file")
            return nil
        }
        
        print("Public key data length: \(publicKeyData.count)")
        
        let byteArray = publicKeyData.map { String(format: "%02x", $0) }
        print("Public key data bytes (loaded in iOS): \(byteArray.joined(separator: " "))")

        let attributes: [String: Any] = [
            kSecAttrKeyType as String: kSecAttrKeyTypeECSECPrimeRandom,
            kSecAttrKeyClass as String: kSecAttrKeyClassPublic,
            kSecAttrKeySizeInBits as String: 256
        ]

        var error: Unmanaged<CFError>?
        guard let secKey = SecKeyCreateWithData(publicKeyData as CFData, attributes as CFDictionary, &error) else {
            if let error = error {
                print("SecKeyCreate init failed: \(error.takeRetainedValue() as Error)")
            } else {
                print("SecKeyCreate init failed with unknown error")
            }
            return nil
        }

        return secKey
    }

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