[base version 2.3.4, Yosemite/Xcode6.4] Error: Failed to verify signature for MacPorts source!

[toddwayne (Todd Wayne)]

iMac, OSX: 10.10.5 (Yosemite) with downrev'd Xcode (version 6.4, to avoid other build issues)

Self update started failing yesterday. Seen several bug-reports on ElCapitan marked as duplicates. This is just Yosemite; self-update with this system worked successfully on 11-October (clang-3.8 @3.8-r249977, llvm-3.8 @3.8-r249977, others), and now fails consistently with the following error (I'm logged in as root):

20# port -vd selfupdate
DEBUG: Copying /Users/pokey/Library/Preferences/com.apple.dt.Xcode.plist to /opt/local/var/macports/home/Library/Preferences
DEBUG: MacPorts sources location: /opt/local/var/macports/sources/rsync.macports.org/release/tarballs
--->  Updating MacPorts base sources using rsync
receiving file list ... done

sent 36 bytes  received 69 bytes  210.00 bytes/sec
total size is 27013120  speedup is 257267.81
receiving file list ... done

sent 36 bytes  received 76 bytes  224.00 bytes/sec
total size is 512  speedup is 4.57
DEBUG: failed verification with key /opt/local/share/macports/macports-pubkey.pem
DEBUG: openssl output: Verification Failure
child process exited abnormally
DEBUG: Failed to verify signature for MacPorts source!
    while executing
"macports::selfupdate [array get global_options] base_updated"
Error: Failed to verify signature for MacPorts source!
To report a bug, follow the instructions in the guide:
    http://guide.macports.org/#project.tickets
Error: /opt/local/bin/port: port selfupdate failed: Failed to verify signature for MacPorts source!

[ryandesign (Ryan Carsten Schmidt)]

Duplicate of #49233.