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SSH can actually do that.

FTP does FXP without either server knowing credentials of the other.

How does what you say "SSH" accomplish server to server transfer without exposing credentials of one server to another?

scp bob@host1:/tmp/x john@host2:/tmp/

Also SSH can be configured to use different keys for each server.

now do that without dragging all the data through your workstation.

Recent versions of OpenSsh don't drag the data through your workstation unless you pass "-3" (for triangle routing).

Don't you still need credentials for server A to set up an SSH connection to server B? The question was to avoid that.

The scp protocol is deprecated, the scp command is alive and well.

One poor answer is SSH agent forwarding [1]. At best some SSH agents will prompt at each use, but that's questionable security.

Using something like magic wormhole[2] could work. SSH to both sides and run the required commands.