Flexible Checksums¶

Default behavior
Choosing an algorithm
Reading the checksum back
Restoring the old Content-MD5 behavior

As of paws.common >= 0.9.0, Paws supports AWS's flexible checksum feature for S3 and other services whose operations advertise an httpChecksum trait, aligning with the default behavior of boto3 and the other AWS SDKs.

Default behavior¶

Operations that support a request checksum (e.g. PutObject, UploadPart) now have one computed and attached automatically, even if you don't pass ChecksumAlgorithm yourself. The default algorithm is CRC32.

library(paws)

svc <- s3(region = "us-west-2")

# A CRC32 checksum is computed and sent automatically.
svc$put_object(Bucket = "my-bucket", Key = "file.txt", Body = "hello")

Choosing an algorithm¶

Pass ChecksumAlgorithm on supporting operations to choose a different algorithm. Supported values are CRC32, CRC32C, SHA1, SHA256, SHA512, MD5, XXHASH64, XXHASH3, and XXHASH128.

svc$put_object(
    Bucket = "my-bucket",
    Key = "file.txt",
    Body = "hello",
    ChecksumAlgorithm = "SHA256"
)

CRC64NVME is not supported, since it requires AWS's CRT, which Paws has no equivalent of.

Reading the checksum back¶

To have a checksum returned and validated on download, set ChecksumMode = "ENABLED" on operations that support it (e.g. GetObject). The checksum is returned in the matching typed field of the response, e.g. ChecksumSHA256 or ChecksumCRC32, not as a raw header.

resp <- svc$get_object(Bucket = "my-bucket", Key = "file.txt", ChecksumMode = "ENABLED")
resp$ChecksumSHA256

Restoring the old Content-MD5 behavior¶

Before paws.common 0.9.0, Paws only ever sent a Content-MD5 header and never a flexible checksum. To restore that behavior, set request_checksum_calculation to "when_required" in the service config (the default is "when_supported"). This is a plain config list entry, not one of the config()/credentials() helper function's named arguments.

svc <- s3(config = list(request_checksum_calculation = "when_required"))

# Sends Content-MD5 only; no x-amz-checksum-* header is added.
svc$put_object(Bucket = "my-bucket", Key = "file.txt", Body = "hello")

A flexible checksum and Content-MD5 are mutually exclusive on a given request — whenever a flexible checksum is resolved, Content-MD5 is skipped.