Im trying to export EC Public keys from android to iOS, what is the best way to do this?

Question

I'm trying to perform an EC key exchange between iOS and Android clients of my app. I have successfully transported and generated them from iOS to Android. But I'm unable to use the keys generated in the android app in iOS.

I'm using the SecKeyCreateWithData method in swift to generate keys from 'Data' type, but I'm getting this error:

Error Domain=NSOSStatusErrorDomain Code=-50 "EC public key creation from data failed"

I used the follwing encoding in the android client, it produces a base64 string which I process and pass as Data SecKeyCreateWithData in swift

byte [] encodedPublicKey = PubKey.getEncoded();
String b64PublicKey = Base64.encodeToString(encodedPublicKey,Base64.DEFAULT);

I'd like to generate a SecKeyRef public key, please help

score 1 · Accepted Answer · answered Apr 22 '20 at 18:05

Apple APIs don't follow the standards exactly to play nicely with other platforms. Exporting an EC public key on iOS doesn't include a fixed header identifier. Therefore, the import functions also expect that the header is missing. This means you must remove the header that Android creates when importing the Android EC key.

Here's a portable example:

CFMutableDataRef mutableData = CFDataCreateMutable(kCFAllocatorDefault, 0);
if (mutableData)
{
    //Fixed schema header
    const UInt8 headerBytes[] = { 0x30,0x59,0x30,0x13,0x06,0x07,0x2a,0x86,
    0x48,0xce,0x3d,0x02,0x01,0x06,0x08,0x2a,
    0x86,0x48,0xce,0x3d,0x03,0x01,0x07,0x03,
        0x42,0x00 };
    const CFIndex headerSize = sizeof(headerBytes);

    //Uncomment for exporting (such as via SecKeyCopyExternalRepresentation)
    //CFDataAppendBytes(mutableData, headerBytes, headerSize);
    //CFDataAppendBytes(mutableData, CFDataGetBytePtr(iosKeyData), CFDataGetLength(iosKeyData));

    //For importing Android key data
    CFDataAppendBytes(mutableData, CFDataGetBytePtr(androidKeyData), CFDataGetLength(androidKeyData));
    CFDataDeleteBytes(mutableData, CFRangeMake(0, headerSize));

    //Use the mutableData here (SecKeyCreateWithData)

    CFRelease(mutableData);
}

Swift version:

    let mutableData = CFDataCreateMutable(kCFAllocatorDefault, CFIndex(0))
    if mutableData != nil 
    {
        //Fixed schema header
        //var headerBytes = [0x30, 0x59, 0x30, 0x13, 0x06, 0x07, 0x2a, 0x86, 0x48, 0xce, 0x3d, 0x02, 0x01, 0x06, 0x08, 0x2a, 0x86, 0x48, 0xce, 0x3d, 0x03, 0x01, 0x07, 0x03, 0x42, 0x00] as [UInt8]
        let headerSize = 26

        //Uncomment for exporting (such as via SecKeyCopyExternalRepresentation)
        //CFDataAppendBytes(mutableData, &headerBytes, headerSize)
        //CFDataAppendBytes(mutableData, CFDataGetBytePtr(iosKeyData), CFDataGetLength(iosKeyData))

        //For importing Android key data
        CFDataAppendBytes(mutableData, CFDataGetBytePtr(androidKeyData), CFDataGetLength(androidKeyData))
        CFDataDeleteBytes(mutableData, CFRangeMake(CFIndex(0), headerSize))

        //Use the mutableData here (SecKeyCreateWithData)
    }

Can you explain what the headerBytes mean though or point me to some resources on the topic? — AdmiralAnk, Apr 29 '20 at 13:44
@AdmiralAnk SecKeyCreateWithData expects raw public key bits. Other systems instead expect those raw bits wrapped in a structure called a SubjectPublicKeyInfo, defined by RFC 3280. It's this structure that contains a header with an object identifier (OID) so a parser can understand what kind of data lies ahead. This is in a standard language called ASN.1 but the iOS SDK doesn't include an ASN.1 parser. So this answer is hacking the bits directly. — NSDestr0yer, Apr 29 '20 at 19:51
Hacking the bits directly like this means if you use a different key size that "fixed header" is different. Examples for 384 or 521 are: `var header384r1: [UInt8] = [0x30, 0x76, 0x30, 0x10, 0x06, 0x07, 0x2A, 0x86, 0x48, 0xCE, 0x3D, 0x02, 0x01, 0x06, 0x05, 0x2B, 0x81, 0x04, 0x00, 0x22, 0x03, 0x62, 0x00]` `var header521r1: [UInt8] = [0x30, 0x81, 0x9B, 0x30, 0x10, 0x06, 0x07, 0x2A, 0x86, 0x48, 0xCE, 0x3D, 0x02, 0x01, 0x06, 0x05, 0x2B, 0x81, 0x04, 0x00, 0x23, 0x03, 0x81, 0x86, 0x00]` If you're interested in ASN.1 I wrote about it [here](https://kolinsturt.github.io/lessons/2014/06/01/asn1]) — NSDestr0yer, Apr 29 '20 at 19:56

Im trying to export EC Public keys from android to iOS, what is the best way to do this?

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