=============蘋果自帶方法=====================

typedef void (^keyPair)(SecKeyRef publicKey ,SecKeyRef privateKey);

#pragma mark -  =============蘋果自帶方法=====================

+ (void)getRSAKeyPairWithKeySize:(int)keySize keyPair:(keyPair)pair;
{
    
    OSStatus status = noErr;
    if (keySize == 512 || keySize == 1024 || keySize == 2048) {
        
        //定義dictionary，用于傳遞SecKeyGeneratePair函數中的第1個參數。
        NSMutableDictionary *privateKeyAttr = [[NSMutableDictionary alloc] init];
        NSMutableDictionary *publicKeyAttr = [[NSMutableDictionary alloc] init];
        NSMutableDictionary *keyPairAttr = [[NSMutableDictionary alloc] init];
        
        //把第1步中定義的字符串轉換為NSData對象。
        NSData * publicTag = [NSData dataWithBytes:publicKeyIdentifier
                                            length:strlen((const char *)publicKeyIdentifier)];
        NSData * privateTag = [NSData dataWithBytes:privateKeyIdentifier
                                             length:strlen((const char *)privateKeyIdentifier)];
        //為公／私鑰對準備SecKeyRef對象。
        SecKeyRef publicKey = NULL;
        SecKeyRef privateKey = NULL;
        //
        //設置密鑰對的密鑰類型為RSA。
        [keyPairAttr setObject:(id)kSecAttrKeyTypeRSA forKey:(id)kSecAttrKeyType];
        //設置密鑰對的密鑰長度為1024。
        [keyPairAttr setObject:[NSNumber numberWithInt:keySize] forKey:(id)kSecAttrKeySizeInBits];
        
        //設置私鑰的持久化屬性（即是否存入鑰匙串）為YES。
        [privateKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:(id)kSecAttrIsPermanent];
        [privateKeyAttr setObject:privateTag forKey:(id)kSecAttrApplicationTag];
        
        //設置公鑰的持久化屬性（即是否存入鑰匙串）為YES。
        [publicKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:(id)kSecAttrIsPermanent];
        [publicKeyAttr setObject:publicTag forKey:(id)kSecAttrApplicationTag];
        
        // 把私鑰的屬性集（dictionary）加到密鑰對的屬性集（dictionary）中。
        [keyPairAttr setObject:privateKeyAttr forKey:(id)kSecPrivateKeyAttrs];
        [keyPairAttr setObject:publicKeyAttr forKey:(id)kSecPublicKeyAttrs];
        
        //生成密鑰對
        status = SecKeyGeneratePair((CFDictionaryRef)keyPairAttr,&publicKey, &privateKey); // 13
        if (status == noErr && publicKey != NULL && privateKey != NULL) {
            pair(publicKey,privateKey);
        }
        else
            pair(publicKey,privateKey);
    }
    
}

==============OpenSSL 方式=================

#pragma mark - ==============OpenSSL 方式=================
#pragma mark ---生成密鑰對
+ (BOOL)generateRSAKeyPairWithKeySize:(int)keySize publicKey:(RSA **)publicKey privateKey:(RSA **)privateKey {
   
   if (keySize == 512 || keySize == 1024 || keySize == 2048) {
       
       /* 產生RSA密鑰 */
       RSA *rsa = RSA_new();
       BIGNUM* e = BN_new();
       
       /* 設置隨機數長度 */
       BN_set_word(e, 65537);
       
       /* 生成RSA密鑰對 */
       RSA_generate_key_ex(rsa, keySize, e, NULL);
       
       if (rsa) {
           *publicKey = RSAPublicKey_dup(rsa);
           *privateKey = RSAPrivateKey_dup(rsa);
           return YES;
       }
   }
   return NO;
}
#pragma mark ---RSA 轉化為字符串
+ (NSString *)PEMFormatRSAKey:(RSA *)rsaKey isPublic:(BOOL)isPublickey
{
   if (!rsaKey) {
       return nil;
   }
   
   BIO *bio = BIO_new(BIO_s_mem());
   if (isPublickey)
       PEM_write_bio_RSA_PUBKEY(bio, rsaKey);
   
   else
   {
       //此方法生成的是pkcs1格式的,IOS中需要pkcs8格式的,因此通過PEM_write_bio_PrivateKey 方法生成
       // PEM_write_bio_RSAPrivateKey(bio, rsaKey, NULL, NULL, 0, NULL, NULL);
       
       EVP_PKEY* key = NULL;
       key = EVP_PKEY_new();
       EVP_PKEY_assign_RSA(key, rsaKey);
       PEM_write_bio_PrivateKey(bio, key, NULL, NULL, 0, NULL, NULL);
   }
   
   BUF_MEM *bptr;
   BIO_get_mem_ptr(bio, &bptr);
   BIO_set_close(bio, BIO_NOCLOSE); /* So BIO_free() leaves BUF_MEM alone */
   BIO_free(bio);
   return [NSString stringWithUTF8String:bptr->data];
   
}

注意:
openssl 生成的私鑰為 pkcs1格式的,我們加密需要的是 pkcs8格式的,暫時還沒有找到轉換方法.
更新:

 //此方法生成的是pkcs1格式的,IOS中需要pkcs8格式的
   PEM_write_bio_RSAPrivateKey(bio, rsaKey, NULL, NULL, 0, NULL, NULL);

因此通過PEM_write_bio_PrivateKey 方法生成

        EVP_PKEY* key = NULL;
        key = EVP_PKEY_new();
        EVP_PKEY_assign_RSA(key, rsaKey);
        PEM_write_bio_PrivateKey(bio, key, NULL, NULL, 0, NULL, NULL);

Demo地址:https://github.com/yuying2012/WJDStudyLibrary
這是一個大工程,請從工程中尋找相關模塊代碼.