Generate base64 url-encoded X.509 format 2048-bit RSA public key with Swift?

Question

Working in Apple Swift for iOS. I have to generate this for the backend as it's a secure app.

I'm new to security and certificates and have been searching for a day now with no results.

How can I generate base64 url-encoded X.509 format 2048-bit RSA public key with swift?

Any help is highly appreciated.

Answer 1

There's a library for handling public-private key pairs in Swift that I recently created called Heimdall , which allows you to easily export the X.509 formatted Base64 string of the public key.

To comply with SO rules, I will also include the implementation in this answer (so that it is self-explanatory)

public func X509PublicKey() -> NSString? {
    // Fetch the key, so that key = NSData of the public key
    let result = NSMutableData()

    let encodingLength: Int = {
        if key.length + 1 < 128 {
            return 1
        } else {
            return ((key.length + 1) / 256) + 2
        }
    }()

    let OID: [CUnsignedChar] = [0x30, 0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86,
        0xf7, 0x0d, 0x01, 0x01, 0x01, 0x05, 0x00]

    var builder: [CUnsignedChar] = []

    // ASN.1 SEQUENCE
    builder.append(0x30)

    // Overall size, made of OID + bitstring encoding + actual key
    let size = OID.count + 2 + encodingLength + key.length
    let encodedSize = encodeLength(size)
    builder.extend(encodedSize)
    result.appendBytes(builder, length: builder.count)
    result.appendBytes(OID, length: OID.count)
    builder.removeAll(keepCapacity: false)

    builder.append(0x03)
    builder.extend(encodeLength(key.length + 1))
    builder.append(0x00)
    result.appendBytes(builder, length: builder.count)

    // Actual key bytes
    result.appendData(key)

    // Convert to Base64 and make safe for URLs
    var string = result.base64EncodedStringWithOptions(.allZeros)
    string = string.stringByReplacingOccurrencesOfString("/", withString: "_")
    string = string.stringByReplacingOccurrencesOfString("+", withString: "-")

    return string
}

public func encodeLength(length: Int) -> [CUnsignedChar] {
    if length < 128 {
        return [CUnsignedChar(length)];
    }

    var i = (length / 256) + 1
    var len = length
    var result: [CUnsignedChar] = [CUnsignedChar(i + 0x80)]

    for (var j = 0; j < i; j++) {
        result.insert(CUnsignedChar(len & 0xFF), atIndex: 1)
        len = len >> 8
    }

    return result
}

I have removed the data fetching code, refer to either source of Heimdall or Jeff Hay's answer

Answer 2

If the public key is already in your keychain, you can look up the public key and return the data as base64 with something similar to the following:

// Create dictionary to specify attributes for the key we're
// searching for.  Swift will automatically bridge native values 
// to to right types for the SecItemCopyMatching() call.
var queryAttrs = [NSString:AnyObject]() 
queryAttrs[kSecClass] = kSecClassKey 
queryAttrs[kSecAttrApplicationTag] = publicKeyTag
queryAttrs[kSecAttrKeyType] = kSecAttrKeyTypeRSA
queryAttrs[kSecReturnData] = true

var publicKeyBits = Unmanaged<AnyObject>?()
SecItemCopyMatching(queryAttrs, &publicKeyBits)

// Work around a compiler bug with Unmanaged<AnyObject> types
//  the following two lines should simply be 
//  let publicKeyData : NSData = publicKeyRef!.takeRetainedValue() as NSData
let opaqueBits = publicKeyBits?.toOpaque() 
let publicKeyData = Unmanaged<NSData>.fromOpaque(opaqueBits).takeUnretainedValue()

let publicKeyBase64 = publicKeyData.base64EncodedData(NSDataBase64EncodingOptions.Encoding64CharacterLineLength)

If you need to generate the keypair and store it in the keychain, use something along these lines:

// Create dictionaries to specify key attributes.  Swift will
// automatically bridge native values to to right types for the
// SecKeyGeneratePair() call.
var pairAttrs = [NSString:AnyObject]()
var privateAttrs = [NSString:AnyObject]()
var publicAttrs = [NSString:AnyObject]()

privateAttrs[kSecAttrIsPermanent] = true
privateAttrs[kSecAttrApplicationTag] = privateKeyTag

publicAttrs[kSecAttrIsPermanent] = true
publicAttrs[kSecAttrApplicationTag] = publicKeyTag

pairAttrs[kSecAttrKeyType] = kSecAttrKeyTypeRSA
pairAttrs[kSecAttrKeySizeInBits] = 2048
pairAttrs[(kSecPrivateKeyAttrs.takeUnretainedValue() as! String)] = privateAttrs
pairAttrs[(kSecPublicKeyAttrs.takeUnretainedValue() as! String)] = publicAttrs

var publicKeyPtr = Unmanaged<SecKey>?()
var privateKeyPtr = Unmanaged<SecKey>?()

let status = SecKeyGeneratePair(pairAttrs, &publicKeyPtr, &privateKeyPtr)

Note: publicKeyTag and privateKeyTag are strings used to identify the key in the keystore. Apple recommends reverse-dns notation (com.company.key.xxx), but as long as they are unique, all should be good.

Answer 3

Similar question with answer: Generate key pair on iphone and print to log as NSString

Although the answer there is in Objective-C, Apple reference shows that the functions (especially the most important one, SecKeyGeneratePair ) can also be called directly from Swift (as long as you can do the type conversions from all those UnsafeMutablePointers etc).