最近在Android和IOS上都需要对用户的某些输入进行简单的加密，于是采用MD5加密方式。

• 首先将目的字符串加密一次，获得32位字符串
• 然后将32位字符串拆为2段，分别加密1次
• 最后将加密后的2段拼接，加密100次

下面是Android的Java部分和IOS的Objective-C部分

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class MD5Util
{
    public static String getMD5(String content) {
        String s = makeMD5(content);
        String s1 = null;
        if (s != null) {
            s1 = s.substring(0, 16);
        }
        String s2 = null;
        if (s != null) {
            s2 = s.substring(16, 32);
        }
        s1 = makeMD5(s1);
        s2 = makeMD5(s2);
        s = s1 + s2;
        for (int i = 0; i < 100; i++) {
            s = makeMD5(s);
        }
        return s;
    }

    private static String makeMD5(String content) {
        try {
            MessageDigest messageDigest = MessageDigest.getInstance("MD5");
            messageDigest.update(content.getBytes());
            return getHashString(messageDigest);
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }
        return null;
    }

    private static String getHashString(MessageDigest messageDigest) {
        StringBuilder builder = new StringBuilder();
        for (byte b : messageDigest.digest()) {
            builder.append(Integer.toHexString(  ( b >> 4 ) & 0xf )  );
            builder.append(Integer.toHexString(  b & 0xf )           );
        }
        return builder.toString();
    }

}

#import "MD5Utils.h"
#import <CommonCrypto/CommonDigest.h>

@implementation MD5Utils

+(NSString*)MD5:(NSString *)input{
    NSString* s = [self makeMd5:input];
    NSString* s1 = [s substringToIndex:16];
    NSString* s2 =[s substringFromIndex:16];
    s1 = [self makeMd5:s1];
    s2 = [self makeMd5:s2];
    NSMutableString* md5String = [NSMutableString stringWithString:s1];
    [md5String appendString:s2];
    for(int i =0;i<100;i++){
        md5String = [self makeMd5WithMutaleString:md5String];
    }
    return md5String;
}

+ (NSString*)makeMd5:(NSString*) str{
    const char * pointer = [str UTF8String];
    unsigned char md5Buffer[CC_MD5_DIGEST_LENGTH];
    CC_MD5(pointer, (CC_LONG)strlen(pointer), md5Buffer);
    NSMutableString *string = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
    for (int i = 0; i < CC_MD5_DIGEST_LENGTH; i++)
        [string appendFormat:@"%02x",md5Buffer[i]];
    return string;
}

+ (NSMutableString*)makeMd5WithMutaleString:(NSMutableString*) str{
    const char * pointer = [str UTF8String];
    unsigned char md5Buffer[CC_MD5_DIGEST_LENGTH];
    CC_MD5(pointer, (CC_LONG)strlen(pointer), md5Buffer);
    NSMutableString *string = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
    for (int i = 0; i < CC_MD5_DIGEST_LENGTH; i++)
        [string appendFormat:@"%02x",md5Buffer[i]];
    return string;
}

@end