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For the ETag, yes and no that it's an MD5 of the object depending on encryption and multipart upload: https://docs.aws.amazon.com/AmazonS3/latest/API/RESTCommonRe...

For the object upload, AWSv4 request signature uses SHA256 on the payload/object, but I don't know if S3 also computes & compares the digest or just uses the x-amz-content-sha256 header value.

AWS support via Twitter said that the SHA256 digest of the payload is calculated and compared for S3 when using AWSv4 signature requests.

Yes, S3 uses the content-md5 header to verify data integrity for puts.

https://docs.aws.amazon.com/AmazonS3/latest/API/API_PutObjec...

The request signature (assuming version 4) is SHA256, the object eTag is MD5.