CRC64 check

Last updated: 2020-07-15 10:53:55

    Overview

    Errors may occur when data is transferred between the client and the server. COS can verify data integrity using MD5 and custom attributes, or the CRC64 check code.

    COS will calculate the CRC64 of the newly uploaded object and store the result as object attributes. It will carry x-cos-hash-crc64ecma in the returned response header, which indicates the CRC64 value of the uploaded object calculated according to ECMA-182 standard. If an object already has a CRC64 value stored before this feature is activated, COS will not calculate its CRC64 value, nor will it be returned when the object is obtained.

    Directions

    APIs that currently support CRC64 are:

    • Simple upload APIs
      • PUT Object and POST Object: you can get the CRC64 check value for your file in the returned response headers.
    • Multipart upload APIs
      • Upload Part: you can compare and verify the CRC64 value returned by COS against the value calculated locally.
      • Complete Multipart Upload: returns a CRC64 value for the entire object only if each part has a CRC64 attribute. Otherwise, no value is returned.
    • The Upload Part - Copy operation returns a corresponding CRC64 value.
    • When you call the PUT Object - Copy, the CRC64 value is returned only if the source object has one.
    • The HEAD Object and GET Object operations return the CRC64 value provided the object has one. You can compare and verify the CRC64 value returned by COS against that calculated locally.

    API Samples

    Multipart upload response

    The following is an example of the response obtained after a user sends an Upload Part request. The x-cos-hash-crc64ecma header indicates the CRC64 value of the part, and the user can compare the value with the locally calculated CRC64 value to verify the integrity of the part.

    HTTP/1.1 200 OK
    content-length: 0
    connection: close
    date: Thu, 05 Dec 2019 01:58:03 GMT
    etag: "358e8c8b1bfa35ee3bd44cb3d2cc416b"
    server: tencent-cos
    x-cos-hash-crc64ecma: 15060521397700495958
    x-cos-request-id: NWRlODY0MmJfMjBiNDU4NjRfNjkyZl80ZjZi****

    Completing the multipart upload response

    The following is an example of the response obtained after a user sends a Complete Multipart Upload request. The x-cos-hash-crc64ecma header indicates the CRC64 value of the part, and the user can compare the value with the locally calculated CRC64 value to verify the integrity of the part.

    HTTP/1.1 200 OK
    content-type: application/xml
    transfer-encoding: chunked
    connection: close
    date: Thu, 05 Dec 2019 02:01:17 GMT
    server: tencent-cos
    x-cos-hash-crc64ecma: 15060521397700495958
    x-cos-request-id: NWRlODY0ZWRfMjNiMjU4NjRfOGQ4Ml81MDEw****
    
    [Object Content]

    SDK Samples

    The following uses the Python SDK as an example to demonstrate how to verify an object. The complete sample code is as follows.

    The code is based on Python 2.7. For more information on how to use the Python SDK, see Object Operations for Python SDK..

    1. Initial configuration

    Configure user attributes, including SecretId, SecretKey, and region, and create a client object.

    # -*- coding=utf-8
    from qcloud_cos import CosConfig
    from qcloud_cos import CosS3Client
    from qcloud_cos import CosServiceError
    from qcloud_cos import CosClientError
    import sys
    import logging
    import hashlib
    
    logging.basicConfig(level=logging.INFO, stream=sys.stdout)
    
    # Configure user attributes, including SecretId, SecretKey, and region
    # APPID has been removed from the configuration. Please add the APPID to the `Bucket` parameter in the format of `BucketName-APPID`.
    secret_id = COS_SECRETID           # Replace with your own SecretId
    secret_key = COS_SECRETKEY         # Replace with your own SecretKey
    region = 'ap-beijing'      # Replace with your own region (which is Beijing in this sample)
    token = None                # If a temporary key is used, token needs to be passed in. This is optional and is null by default
    config = CosConfig(Region=region, SecretId=secret_id, SecretKey=secret_key, Token=token)  # Get the configured object
    client = CosS3Client(config)

    2. Calculate the object checksum

    Simulate object parts and calculate the checksum of the entire object.

    OBJECT_PART_SIZE = 1024 * 1024      # Size of each simulated part
    OBJECT_TOTAL_SIZE = OBJECT_PART_SIZE * 1 + 123      # Total size of the object
    object_body = '1' * OBJECT_TOTAL_SIZE       # Object content
    
    #Calculate the checksum of the entire object.
    c64 = crcmod.mkCrcFun(0x142F0E1EBA9EA3693L, initCrc=0L, xorOut=0xffffffffffffffffL, rev=True)
    local_crc64 =str(c64(object_body))

    1. Initialize multipart upload

    # Initialize the multipart upload
    response = client.create_multipart_upload(
        Bucket='examplebucket-1250000000',  #Replaced with your own bucket name. 'examplebucket' is a sample bucket, and '1250000000' is a sample APPID.
        Key='exampleobject',              # Replaced with the key value of your uploaded object 
        StorageClass='STANDARD',            # Storage class of the object
    )
    # Get the UploadId of the multipart upload
    upload_id = response['UploadId']

    4. Upload the object via multipart upload

    During a multipart upload, an object is divided into multiple (up to 10,000) parts for upload. The size of each part can range from 1 MB to 5 GB, and the last part can be less than 1 MB. When uploading parts, configure the PartNumber of each part and its corresponding CRC64 value. After the parts are successfully uploaded, check the returned CRC64 value with the locally calculated value to verify the integrity of each part.

    # Upload an object in parts where the size of each part is OBJECT_PART_SIZE except the last part which may be smaller
    part_list = list()
    position = 0 
    left_size = OBJECT_TOTAL_SIZE
    part_number = 0 
    while left_size > 0:
        part_number += 1
        if left_size >= OBJECT_PART_SIZE:
            body = object_body[position:position+OBJECT_PART_SIZE]
        else:
            body = object_body[position:]
        position += OBJECT_PART_SIZE
        left_size -= OBJECT_PART_SIZE
        local_part_crc_64 = c64(body)#Calculate CRC64 locally
    
        response = client.upload_part(
            Bucket='examplebucket-1250000000',
            Key=‘exampleobject’,
            Body=body,
            PartNumber=part_number,
            UploadId=upload_id,
        )   
        part_crc_64 = response['x-cos-hash-crc64ecma']# CRC64 returned by the server
        if local_part_crc64 != part_crc_64:# Data Check
            print 'crc64 check FAIL'
            exit(-1)
        etag = response['ETag']
        part_list.append({'ETag' : etag, 'PartNumber' : part_number})

    5. Complete multipart upload

    After all parts are successfully uploaded, you need to complete the multipart upload. The CRC64 returned by COS and the CRC64 of the local object can be used for comparison and verification.

    # Complete the multipart upload
    response = client.complete_multipart_upload(
        Bucket='examplebucket-1250000000',      # Replace with your own bucket name, examplebucket is a sample bucket, and 1250000000 is a sample APPID
        Key=‘exampleobject’,             #Key value of the object 
        UploadId=upload_id,
        MultipartUpload={       #Requires one-to-one correspondence between ETag and PartNumber for each part
            'Part' : part_list    
        },
    )
    crc64ecma = response['x-cos-hash-crc64ecma']
    if crc64ecma != local_crc64:# Data Check
        print 'check crc64 Failed'
        exit(-1)

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