How can I use a PKCS8 RSA DER Private Key in iOS?

Question

At run time, my iOS application receives a file with a public-private RSA key-pair, generated by someone else\'s Java:

KeyPairGenerator keygenerator;
keygene         


        

Answer 1

You can see how a der key looks in ASN1 structure in this webpage: https://lapo.it/asn1js/

Here is a code from the SwCrypt library, which strips the PKCS8 header from a private key. This is Swift, but you can rewrite it any other language easily.

    static private func stripHeaderIfAny(keyData: NSData) throws -> NSData {
        var bytes = keyData.arrayOfBytes()

        var offset = 0
        guard bytes[offset] == 0x30 else {
            throw SwError.ASN1Parse
        }
        offset += 1

        if bytes[offset] > 0x80 {
            offset += Int(bytes[offset]) - 0x80
        }
        offset += 1

        guard bytes[offset] == 0x02 else {
            throw SwError.ASN1Parse
        }
        offset += 3

        //without PKCS8 header
        if bytes[offset] == 0x02 {
            return keyData
        }

        let OID: [UInt8] = [0x30, 0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86,
                            0xf7, 0x0d, 0x01, 0x01, 0x01, 0x05, 0x00]
        let slice: [UInt8] = Array(bytes[offset..<(offset + OID.count)])

        guard slice == OID else {
            throw SwError.ASN1Parse
        }

        offset += OID.count
        guard bytes[offset] == 0x04 else {
            throw SwError.ASN1Parse
        }

        offset += 1
        if bytes[offset] > 0x80 {
            offset += Int(bytes[offset]) - 0x80
        }
        offset += 1

        guard bytes[offset] == 0x30 else {
            throw SwError.ASN1Parse
        }

        return keyData.subdataWithRange(NSRange(location: offset, length: keyData.length - offset))
    }