Objective-C头文件：

#import <Foundation/Foundation.h>

@interface OCBase16 : NSObject

+ (NSString*) encodeWithNSData:(NSData*)data;

@end

Objective-C实现文件：

#import "OCBase16.h"

@implementation OCBase16

+ (NSString*) encodeWithNSData:(NSData*)data {
    NSString *table = @"0123456789ABCDEF";

    Byte *inputBytes = (Byte*)[data bytes];
    NSInteger inputLength = data.length;

    NSInteger outputLength = 2 * inputLength;
    Byte outputBytes[outputLength];

    for (int i = 0; i < inputLength; i++) {
        //向右移动4bit，获得高4bit
        NSInteger highByte = inputBytes[i] >> 4;
        //与0x0F做位与运算，获得低4bit
        NSInteger lowByte = inputBytes[i] & 0x0F;
        outputBytes[2 * i] = [table characterAtIndex: highByte];
        outputBytes[2 * i + 1] = [table characterAtIndex: lowByte];
    }

    NSData *outputData = [[NSData alloc] initWithBytes:outputBytes length:outputLength];
    return [[NSString alloc] initWithData:outputData encoding:NSUTF8StringEncoding];
}

@end

Objective-C头文件：

#import <Foundation/Foundation.h>

@interface OCBase16 : NSObject

+ (NSData*) decodeWithNSString:(NSString*)input;

@end

Objective-C实现文件：

#import "OCBase16.h"

@implementation OCBase16

//把16进制字符转换成10进制表示的数字
+ (NSInteger) hex2dec : (unichar) c {
    if ('0' <= c && c <= '9') {
        return c - '0';
    } else if ('a' <= c && c <= 'f') {
        return c - 'a' + 10;
    } else if ('A' <= c && c <= 'F') {
        return c - 'A' + 10;
    } else {
        return 0;
    }
}

//input指向的字符串长度必须是2的整数倍
+ (NSData *) decodeWithNSString : (NSString *) input {
    if (input) {
        NSInteger inputLength = input.length;
        NSInteger halfInputLength = inputLength / 2;
        Byte outputBytes[halfInputLength];
        for (int i = 0; i < halfInputLength; i++) {
            unichar c1 = [input characterAtIndex:2 * i];
            unichar c0 = [input characterAtIndex:2 * i + 1];
            //16进制数字转换为10进制数字的过程
            outputBytes[i] = [OCBase16 hex2dec:c1] * 16 + [OCBase16 hex2dec:c0];
        }
        return [[NSData alloc] initWithBytes:outputBytes length:halfInputLength];
    }
    return nil;
}

@end