Generating an OpenSSL Certificate Signing Request in iOS with Keychain stored keys

Question

I'm trying to generate a CSR in iOS. Since apparently the Apple security framework for iOS doesn't include methods for CSR generation I had to compile the OpenSSL source code for my project.

Now I want to know how to use these methods with the keys I've generated in the Keychain previously. That is, I need to convert SecKeyRef type into OpenSSL types like EVP_PKEY. That will allow me to call the OpenSSL method X509_REQ_set_pubkey.

Does anyone know a way to achieve this?

Answer 1

Found the solution myself.

First of all you need to extract the key from the Keychain as NSData.

- (NSData *) getKeyDataWithIdentifier:(NSString *) identifier
{
    NSData * keyBits = nil;
    NSMutableDictionary * keyQuery = [[NSMutableDictionary alloc] init];
    NSData * encodedId = [identifier dataUsingEncoding:NSUTF8StringEncoding];
    [keyQuery setObject:encodedId forKey:kSecAttrApplicationTag];
    [keyQuery setObject:kSecClassKey forKey:kSecClass];
    [keyQuery setObject:[NSNumber numberWithBool:YES] forKey:kSecReturnData];
    [keyQuery setObject:kSecAttrKeyTypeRSA forKey:kSecAttrKeyType];

    OSStatus sanityCheck = SecItemCopyMatching((CFDictionaryRef)keyQuery, (CFTypeRef *)&keyBits);

    if (sanityCheck != noErr) {
        NSLog(@"Error: %ld", sanityCheck);
    }

    return keyBits;
}

Now we need to cast this data as unsigned char and give it to the method d2i_RSAPublicKey

- (void) generateCSR:(NSData *) keyData
{
    X509_REQ *req = NULL;
    X509_NAME *name= NULL;
    EVP_PKEY *key;
    const unsigned char * bits = (unsigned char *) [keyData bytes];
    int length = [keyData length];

    if ((req=X509_REQ_new()) == NULL) {
        NSLog(@"big error");
        return;
    }

    RSA * rsa = NULL;
    key=EVP_PKEY_new();
    d2i_RSAPublicKey(&rsa, &bits, length);
    EVP_PKEY_assign_RSA(key,rsa);
    name = X509_REQ_get_subject_name(req);
    X509_REQ_set_pubkey(req, key);

    /* This function creates and adds the entry, working out the
     * correct string type and performing checks on its length.
     * Normally we'd check the return value for errors...
             */
    X509_NAME_add_entry_by_txt(name,"CN",
                               MBSTRING_ASC, "My certificate request", -1, -1, 0);
    X509_REQ_print_fp(stdout, req);
}

That generates a simple CSR in OpenSSL (not signed) with a public key and a common name and prints it to the standard out.

Answer 2

Well, the keystore stuff wasn't working out right for us, so we generated them and stored them as files. If anyone ever needs this, I'll leave it here.

+ (void)generateCsrAndKeyAtPath:(NSString *)csrPath KeyPath:(NSString *)keyPath Username:(NSString *)username {
    int i;
    RSA *rsakey;
    X509_REQ *req;
    X509_NAME *subj;
    EVP_PKEY *pkey;
    EVP_MD *digest;
    FILE *fp;

    structentry[ENTRIES - 1].value = [username UTF8String];

    // standard set up for OpenSSL
    OpenSSL_add_all_algorithms();
    ERR_load_crypto_strings();

    // Generate the RSA key; we don't assign a callback to monitor progress
    // since generating keys is fast enough these days
    rsakey = RSA_generate_key(2048, RSA_F4, NULL, NULL);

    // Create evp obj to hold our rsakey
    if (!(pkey = EVP_PKEY_new()))
        NSLog(@"Could not create EVP object");

    if (!(EVP_PKEY_set1_RSA(pkey, rsakey)))
        NSLog(@"Could not assign RSA key to EVP object");

    // create request object
    if (!(req = X509_REQ_new()))
        NSLog(@"Failed to create X509_REQ object");
    X509_REQ_set_pubkey(req, pkey);

    // create and fill in subject object
    if (!(subj = X509_NAME_new()))
        NSLog(@"Failed to create X509_NAME object");

    for (i = 0; i < ENTRIES; i++) {
        int nid;                  // ASN numeric identifier
        X509_NAME_ENTRY *ent;

        if ((nid = OBJ_txt2nid(structentry[i].key)) == NID_undef) {
            fprintf(stderr, "Error finding NID for %s\n", structentry[i].key);
            NSLog(@"Error on lookup");
        }
        if (!(ent = X509_NAME_ENTRY_create_by_NID(NULL, nid, MBSTRING_ASC,
                structentry[i].value, -1)))
            NSLog(@"Error creating Name fewfwefewf from NID");

        if (X509_NAME_add_entry(subj, ent, -1, 0) != 1)
            NSLog(@"Error adding fewfwefewf to Name");
    }
    if (X509_REQ_set_subject_name(req, subj) != 1)
        NSLog(@"Error adding subject to request");

    // request is filled in and contains our generated public key;
    // now sign it
    digest = (EVP_MD *) EVP_sha1();

    if (!(X509_REQ_sign(req, pkey, digest)))
        NSLog(@"Error signing request");

    // write output files
    if (!(fp = fopen([csrPath UTF8String], "wb")))
        NSLog(@"Error writing to request file");
    if (PEM_write_X509_REQ(fp, req) != 1)
        NSLog(@"Error while writing request");
    fclose(fp);

    if (!(fp = fopen([keyPath UTF8String], "w")))
        NSLog(@"Error writing to private key file");
    if (PEM_write_PrivateKey(fp, pkey, NULL, NULL, 0, 0, NULL) != 1)
        NSLog(@"Error while writing private key");
    fclose(fp);

    EVP_PKEY_free(pkey);
    X509_REQ_free(req);
}