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;
keygenerator = KeyPairGenerator.getInstance("RSA");
keygenerator.initialize(4096);

KeyPair keypair = keygenerator.generateKeyPair();
PrivateKey privateKey = keypair.getPrivate().getEncoded();
PublicKey publicKey = keypair.getPublic().getEncoded();

I have successfully read and used the public key, using this method, which strips some preamble from the key.

I now want to use the private key. The same method doesn’t work, I assumed the preamble is different somehow. The blog suggested that it was importing PKCS#1 PEM keys, but then says they’re binary, so I think they just mean Base64-encoded DER keys. I also found that maybe the keys I have are PKCS#8 encoded instead.

Certainly I can use

openssl pkcs8 -nocrypt -inform der < pk8.der > pvt.pem

on a sample private key and openssl doesn’t complain.

Would it make sense that the public key was PKCS#1 and the private PKCS#8?

But I really would like to use CommonCrypto and the Security Framework rather than linking against OpenSSL if I possibly can. On Mac OS there are functions in libsecurity to read PKCS#8, but this hasn’t made it to iOS yet. I did, honestly, try reading the source but I can’t work out where they actually strip the key.

[TL;DR] How can I strip the version and algorithm PKCS#8 fields from the DER private key, and just get the plain key, using either CommonCrypto or some C/C++/ObjC?

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);
}