0. Download source code

http://www.openssl.org/source
Get the latest version, for example, openssl-1.0.2e.

1. Modify ui_openssl.c

Replace static volatile sig_atomic_t intr_signal; with static volatile int intr_signal; in crypto/ui/ui_openssl.c for preventing building error.

2. Execute configure

$ cd Desktop; mkdir ssllibs; cd ssllibs
$ mkdir openssl_i386 openssl_x86_64 openssl_armv7 openssl_armv7s openssl_arm64
$ cd openssl-1.0.2e
$ ./configure BSD-generic32 --openssldir=/Users/GeekRRK/Desktop/ssllibs/openssl_i386

3. Modify Makefile and Compile

Replace CC= gcc with CC= /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -miphoneos-version-min=7.0 -arch i386
Append -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk to CFLAG=
$ make
$ make install

4. Clean configure, and repeat step 2 and 3 for other platforms

$ make clean
Remember replace i386 with other platforms, and replace iPhoneSimulator with iPhoneOS for iPhones.

5. Combine different architectures into one

Go into directory /Users/GeekRRK/Desktop/ssllibs/openssl_xxx/lib to find libcrypto.a and libssl.a and combine each of them as the steps in blog Static library

6. Use OpenSSL in project

Drag the two .a files and include directory into project.
Set Always Search User Paths to YES and User Header Search Paths to $(SRCROOT)/include
Add the following code to our code and build run.

#include <openssl/md5.h>

void Md5( NSString * string){
    unsigned char *inStrg = ( unsigned char *)[[string dataUsingEncoding : NSASCIIStringEncoding ] bytes];
    unsigned long lngth = [string length ];
    unsigned char result[ MD5_DIGEST_LENGTH ];
    NSMutableString *outStrg = [NSMutableString string];
    MD5 (inStrg, lngth, result);
    unsigned int i;
    for (i = 0 ; i < MD5_DIGEST_LENGTH ; i++)
    {
        [outStrg appendFormat : @"%02x" , result[i]];
    }
    NSLog ( @"input string:%@" ,string);
    NSLog ( @"md5:%@" ,outStrg);
}

- (void)viewDidLoad {
    [super viewDidLoad];

    Md5(@"12345");
}

Refer to:
http://blog.csdn.net/kmyhy/article/details/6534067
http://m.blog.csdn.net/blog/lsq19871207/46535597
This is a script to compile the OpenSSL for all architectures:
https://github.com/x2on/OpenSSL-for-iPhone/blob/master/build-libssl.sh

Functions of OpenSSL

Common interface of X509: http://blog.csdn.net/wanjie518/article/details/6570141