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

I couldn't solve the problem without OpenSSL. So here is a solution that does use OpenSSL.

Assuming you have a NSData called privateKey with the key, and another called signableData which you want to sign.

#import <openssl/x509.h>
#import <openssl/pem.h>

NSURL *cacheDir = [[[NSFileManager defaultManager] URLsForDirectory:NSCachesDirectory inDomains:NSUserDomainMask] lastObject];
NSString *infile = [[cacheDir URLByAppendingPathComponent:@"privkey.der"] path];

NSError *error;
[privateKey writeToFile:infile options:NSDataWritingFileProtectionComplete error:&error];
if (error) {
    NSLog(@"%@", error);
} else {
    BIO *in = BIO_new_file([infile cStringUsingEncoding:NSUTF8StringEncoding], "rb");
    PKCS8_PRIV_KEY_INFO *p8inf = d2i_PKCS8_PRIV_KEY_INFO_bio(in, NULL);
    NSLog(@"%i", p8inf->broken);
    EVP_PKEY *pkey = EVP_PKCS82PKEY(p8inf);
    PKCS8_PRIV_KEY_INFO_free(p8inf);
    BIO_free(in);

    uint8_t * cipherBuffer = NULL;

    // Calculate the buffer sizes.
    unsigned int cipherBufferSize = RSA_size(pkey->pkey.rsa);
    unsigned int signatureLength;

    // Allocate some buffer space. I don't trust calloc.
    cipherBuffer = malloc(cipherBufferSize);
    memset((void *)cipherBuffer, 0x0, cipherBufferSize);

    unsigned char *openSSLHash = SHA1(signableData.bytes, signableData.length, NULL);
    int success = RSA_sign(NID_sha1, openSSLHash, 20, cipherBuffer, &signatureLength, pkey->pkey.rsa);
    if (success) NSLog(@"WIN");


    NSData *signed = [NSData dataWithBytes:(const void*)cipherBuffer length:signatureLength];    

    EVP_PKEY_free(pkey);
}

Answer 2

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))
    }