composite-checksum · shipping · also Multipart composite checksum, COMPOSITE checksum

Composite checksum

A composite checksum is an object-level integrity value formed by combining the ordered checksums of the object's upload parts under a specified construction.

Amazon S3 distinguishes COMPOSITE from FULL_OBJECT. [1] For multipart SHA-256, its documented construction hashes the concatenated binary part digests and returns the Base64 digest with a part-count suffix; hashing the assembled file once is a different construction. [2]

Relations

related

checksum, full-object-checksum, multipart-upload, entity-tag, object-storage

Advantages

• Part checksums can be validated independently before their ordered digests are combined into the object's composite checksum. [2]

Limitations

• Verification requires the original part boundaries, part order and specified algorithm; a different multipart layout can change the value without changing the assembled bytes. [2]
• For S3 GetObject, request ChecksumMode ENABLED and inspect ChecksumType alongside the algorithm field; the ETag is a separate validator, not a substitute checksum. [3]

Citations

1. Amazon S3: Checking object integrity for data uploads, full-object and composite checksum types (accessed 2026-10-16)
2. Amazon S3 tutorial: Upload an object through multipart upload and verify its data integrity, step 9 (accessed 2026-10-16)
3. Amazon S3 API Reference: GetObject, checksum mode, checksum type and ETag (accessed 2026-10-16)

Further reading

4. Hesela: Object ETags, multipart checksum scope and download verification (accessed 2026-10-16)

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