{
  "slug": "composite-checksum",
  "term": "Composite checksum",
  "aliases": [
    "Multipart composite checksum",
    "COMPOSITE checksum"
  ],
  "definition": "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.",
  "note": "Amazon S3 distinguishes COMPOSITE from FULL_OBJECT. [aws-s3-upload-checksums] 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. [aws-s3-checksum-tutorial]",
  "status": "shipping",
  "advantages": [
    {
      "text": "Part checksums can be validated independently before their ordered digests are combined into the object's composite checksum.",
      "citationIds": [
        "aws-s3-checksum-tutorial"
      ]
    }
  ],
  "limitations": [
    {
      "text": "Verification requires the original part boundaries, part order and specified algorithm; a different multipart layout can change the value without changing the assembled bytes.",
      "citationIds": [
        "aws-s3-checksum-tutorial"
      ]
    },
    {
      "text": "For S3 GetObject, request ChecksumMode ENABLED and inspect ChecksumType alongside the algorithm field; the ETag is a separate validator, not a substitute checksum.",
      "citationIds": [
        "aws-s3-get-object"
      ]
    }
  ],
  "related": [
    "checksum",
    "full-object-checksum",
    "multipart-upload",
    "entity-tag",
    "object-storage"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "citations": [
    {
      "id": "aws-s3-upload-checksums",
      "title": "Amazon S3: Checking object integrity for data uploads, full-object and composite checksum types",
      "url": "https://docs.aws.amazon.com/AmazonS3/latest/userguide/checking-object-integrity-upload.html",
      "accessed": "2026-10-16"
    },
    {
      "id": "aws-s3-checksum-tutorial",
      "title": "Amazon S3 tutorial: Upload an object through multipart upload and verify its data integrity, step 9",
      "url": "https://docs.aws.amazon.com/AmazonS3/latest/userguide/tutorial-s3-mpu-additional-checksums.html",
      "accessed": "2026-10-16"
    },
    {
      "id": "aws-s3-get-object",
      "title": "Amazon S3 API Reference: GetObject, checksum mode, checksum type and ETag",
      "url": "https://docs.aws.amazon.com/AmazonS3/latest/API/API_GetObject.html",
      "accessed": "2026-10-16"
    },
    {
      "id": "hesela-checksum-scope",
      "title": "Hesela: Object ETags, multipart checksum scope and download verification",
      "url": "https://hesela.com/analyses/object-etags-checksums-conditional-writes/#download-verification",
      "accessed": "2026-10-16"
    }
  ],
  "updated": "2026-10-16"
}