文件在下載完成之后通常會進行Hash校驗，因此客戶端通過計算文件Hash值驗證機制來確保文件上傳下載后的完整性.計算方式比較常用的有MD5和SHA，兩種計算方式不可逆和無沖突，其中MD5計算文件的Hash值較為普遍.

SHA加密

導入頭文件

#include <CommonCrypto/CommonDigest.h>

+ (NSString *)computeHashForFile:(NSURL *)fileURL {
    NSString *fileContentsHash;
    if ([[NSFileManager defaultManager] fileExistsAtPath:[fileURL path]]) {
        NSData *fileContents = [NSData dataWithContentsOfURL:fileURL];
        fileContentsHash = [self computeHashForData:fileContents];
    }
    return fileContentsHash;
}

+ (NSString *)computeHashForData:(NSData *)inputData {
    uint8_t digest[CC_SHA256_DIGEST_LENGTH];
    CC_SHA256(inputData.bytes, (CC_LONG)inputData.length, digest);
    NSMutableString* inputHash = [NSMutableString stringWithCapacity:CC_SHA256_DIGEST_LENGTH * 2];
    for (int i = 0; i < CC_SHA256_DIGEST_LENGTH; i++) {
        [inputHash appendFormat:@"%02x", digest[i]];
    }
    return inputHash;
}

MD5計算

+ (NSString *)computeHashForFile:(NSURL *)fileURL {
    NSString *fileContentsHash;
    if ([[NSFileManager defaultManager] fileExistsAtPath:[fileURL path]]) {
        NSData *fileContents = [NSData dataWithContentsOfURL:fileURL];
        fileContentsHash = [self computeHashForData:fileContents];
    }
    return fileContentsHash;
}

+ (NSString *)computeHashForData:(NSData *)inputData {
    uint8_t digest[CC_MD5_DIGEST_LENGTH];
    CC_MD5(inputData.bytes, (CC_LONG)inputData.length, digest);
    NSMutableString *inputHash = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
    for (int i = 0; i < CC_MD5_DIGEST_LENGTH; i++) {
        [inputHash appendFormat:@"%02x", digest[i]];
    }
    return inputHash;
}

字符串Hash值計算:

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