[1] RFC 2440 - OpenPGP Message Format (2015-02-15 14:39:41 +09:00 版) https://tools.ietf.org/html/rfc2440#section-6.2

[2] RFC 4880 - OpenPGP Message Format (2015-04-05 14:41:03 +09:00 版) https://tools.ietf.org/html/rfc4880#section-6.2

[3] PGPメッセージ交換形式

[4] RFC 7681 - Email Exchange of Secondary School Transcripts (2015-10-24 10:57:24 +09:00 版) https://tools.ietf.org/html/rfc7681