gpg commands fail without offering password prompts

Issue

• After using the su command to switch users, gpg doesn't allow entering a passphrase -- whether encrypting, decrypting, or generating a new key with gpg --gen-key. Example:

  $ gpg -d testfile.gpg  
  gpg: 3DES encrypted data  
  can't connect to `/root/.gnupg/S.gpg-agent': No such file or directory  
  gpg-agent[8750]: command get_passphrase failed: Operation cancelled  
  gpg: cancelled by user  
  gpg: encrypted with 1 passphrase  
  gpg: decryption failed: No secret key