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S3 supports multipart uploads which don’t necessarily send all the parts to the same server.

Why does it matter where the bytes are stored at rest? Isn't everything you need for SHA-256 just the results of the SHA-256 algorithm on every 4096-byte block? I think you could just calculate that as the data is streamed in.

The data is not necessarily "streamed" in! That's a significant design feature to allow parallel uploads of a single object using many parts ("blocks"). See: https://docs.aws.amazon.com/AmazonS3/latest/API/API_CreateMu...

> Isn't everything you need for SHA-256 just the results of the SHA-256 algorithm on every 4096-byte block?

No, you need the hash of the previous block before you can start processing the next block.

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