There may be circumstances that necessitate the transfer of data between the object storage service and a provider substrate extension (PSE) 224. For example, the object storage service may store machine images used to launch VMs, as well as snapshots representing point-in-time backups of volumes. The object gateway can be provided on a PSE server or a specialized storage device, and provide customers with configurable, per-bucket caching of object storage bucket contents in their PSE 224 to minimize the impact of PSE-region latency on the customer's workloads. The object gateway can also temporarily store snapshot data from snapshots of volumes in the PSE 224 and then sync with the object servers in the region when possible. The object gateway can also store machine images that the customer designates for use within the PSE 224 or on the customer's premises. In some implementations, the data within the PSE 224 may be encrypted with a unique key, and the cloud provider can limit keys from being shared from the region to the PSE 224 for security reasons. Accordingly, data exchanged between the object store servers and the object gateway may utilize encryption, decryption, and/or re-encryption in order to preserve security boundaries with respect to encryption keys or other sensitive data. The transformation intermediary can perform these operations, and a PSE bucket can be created (on the object store servers) to store snapshot data and machine image data using the PSE encryption key.