Objective-C code to generate a CSR via OpenSSL

CertificateSigningRequest.h

//
//  CertificateSigningRequest.h
//  Cloak
//
//  Created by Dave Peck on 10/31/16.
//  Copyright © 2016 Bourgeois Bits LLC. All rights reserved.
//  Licensed under the MIT license
//

#import <Foundation/Foundation.h>


@interface CertificateSigningRequest: NSObject

+ (nullable NSData *)csrWithPublicKey:(nonnull NSData *)publicKey privateKey:(nonnull NSData *)privateKey;

@end

CertificateSigningRequest.m

//
//  CertificateSigningRequest.m
//  Cloak
//
//  Created by Dave Peck on 10/31/16.
//  Copyright © 2016 Bourgeois Bits LLC. All rights reserved.
//  Licensed under the MIT license
//

#import "CertificateSigningRequest.h"
#import <openssl/x509.h>


@interface CertificateSigningRequest ()

@property (nonatomic, readwrite) NSData *publicKey;
@property (nonatomic, readwrite) NSData *privateKey;

// Intermediate structures.
@property (nonatomic) EVP_PKEY *evpKeyPair;
@property (nonatomic) X509_REQ *request;
@property (nonatomic) BIO *basicOutput;

@end


@implementation CertificateSigningRequest

+ (nullable NSData *)csrWithPublicKey:(nonnull NSData *)publicKey privateKey:(nonnull NSData *)privateKey;
{
    return [[[CertificateSigningRequest alloc] initWithPublicKey:publicKey privateKey:privateKey] generate];
}


- (id)initWithPublicKey:(nonnull NSData *)publicKey privateKey:(nonnull NSData *)privateKey
{
    self = [super init];

    if (self != nil)
    {
        _publicKey = publicKey;
        _privateKey = privateKey;
    }

    return self;
}


- (void)dealloc
{
    if (self.evpKeyPair != NULL) {
        EVP_PKEY_free(self.evpKeyPair);
    }

    if (self.request != NULL) {
        X509_REQ_free(self.request);
    }

    if (self.basicOutput != NULL) {
        BIO_free(self.basicOutput);
    }
}


- (nullable NSData *)generate
{
    NSData *data = nil;

    @try
    {
        [self importKeys];
        [self createRequest];
        data = [self exportRequest];
    }
    @catch (NSException *exception)
    {
        NSLog(@"Error generating CSR: %@", exception.reason);
    }

    return data;
}


- (void)importKeys
{
    EVP_PKEY *evpKeyPair = NULL;
    
    // OpenSSL has terrifying interfaces. Read the implementations of d2i_*Key and cry.    
    const unsigned char *letOpenSSLOverwriteMe = NULL;

    // Wrap the key in an EVP_PKEY.
    letOpenSSLOverwriteMe = (const unsigned char *)self.publicKey.bytes;
    evpKeyPair = d2i_PublicKey(EVP_PKEY_RSA, NULL, &letOpenSSLOverwriteMe, self.publicKey.length);
    if (evpKeyPair == NULL) {
        [self failWithReason:@"Failed to add public key data to EVP_PKEY object."];
    }

    letOpenSSLOverwriteMe = (const unsigned char *)self.privateKey.bytes;
    evpKeyPair = d2i_PrivateKey(EVP_PKEY_RSA, &evpKeyPair, &letOpenSSLOverwriteMe, self.privateKey.length);
    if (evpKeyPair == NULL) {
        [self failWithReason:@"Failed to add private key data to EVP_PKEY object."];
    }

    self.evpKeyPair = evpKeyPair;
}


- (void)createRequest
{
    int success = 0;

    self.request = X509_REQ_new();
    if (self.request == NULL) {
        [self failWithReason:@"Failed to allocate X509_REQ object."];
    }

    success = X509_REQ_set_pubkey(self.request, self.evpKeyPair);
    if (!success) {
        [self failWithReason:@"Failed to add keys to the X509_REQ."];
    }

    success = X509_REQ_sign(self.request, self.evpKeyPair, EVP_sha256());
    if (!success) {
        [self failWithReason:@"Failed to sign X509_REQ."];
    }
}


- (nonnull NSData *)exportRequest
{
    BUF_MEM *bufferMemory = NULL;
    NSData *data = nil;
    int success = 0;


    self.basicOutput = BIO_new(BIO_s_mem());
    if (self.basicOutput == NULL) {
        [self failWithReason:@"Failed to allocate mem BIO."];
    }

    success = i2d_X509_REQ_bio(self.basicOutput, self.request);
    if (!success) {
        [self failWithReason:@"Failed to export X509_REQ to BIO."];
    }

    BIO_get_mem_ptr(self.basicOutput, &bufferMemory);
    if (bufferMemory == NULL) {
        [self failWithReason:@"Can't get BIO buffer."];
    }

    data = [NSData dataWithBytes:bufferMemory->data length:bufferMemory->length];

    return data;
}


#pragma mark - Utils

- (void)failWithReason:(nonnull NSString *)reason
{
    @throw [NSException exceptionWithName:NSGenericException reason:reason userInfo:nil];
}

@end