UUIDをなるべく短い文字列に変換する ref: http://qiita.com/sgr@github/items/1e720c916f0c80dff4f4

NSUUID+Base64.h

//
//  NSUUID+Base64.h
//
//

#import <Foundation/Foundation.h>

@interface NSUUID (Base64)

- (instancetype)initWithBase64String:(NSString*)string withPadding:(BOOL)padding;
- (NSString*)Base64StringWithPadding:(BOOL)padding;

@end

NSUUID+Base64.m

//
//  NSUUID+Base64.m
//
//

#import "NSUUID+Base64.h"

@implementation NSUUID (Base64)

- (instancetype)initWithBase64String:(NSString*)string withPadding:(BOOL)padding
{
    if (padding == NO) {
        string = [string stringByPaddingToLength:(string.length + (string.length % 4)) withString:@"=" startingAtIndex:0];
    }

    NSData* d = [[NSData alloc] initWithBase64EncodedString:string options:NSDataBase64DecodingIgnoreUnknownCharacters];
    uuid_t bytes;
    [d getBytes:bytes];
    return [self initWithUUIDBytes:bytes];
}

- (NSString*)Base64StringWithPadding:(BOOL)padding
{
    uuid_t bytes;
    [self getUUIDBytes:bytes];
    NSString* string = [[NSData dataWithBytes:bytes length:sizeof(uuid_t)]
                        base64EncodedStringWithOptions:NSDataBase64Encoding76CharacterLineLength];
    if (padding == NO) {
        return [string stringByTrimmingCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:@"="]];
    } else {
        return string;
    }
}

@end