Data Encoding Tutorials - Herong's Tutorial Examples - v5.21, by Herong Yang
RFC 1521 - MIME (Multipurpose Internet Mail Extensions)
This section provides a summary of 'RFC 1521 - MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies' and how the Base64 encoding algorithm was adopted in this RFC.
As background information, let's take a look at the RFC (Request For comments) paper in which the Base64 encoding algorithm was adopted as part of the MIME standard. In 1993, N. Borenstein and others submitted "RFC 1521 - MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies". In this RFC, N. Borenstein and others proposed to use the Base64 encoding as one of the content transfer encodings.
In this RFC, 2 content transfer encodings were proposed:
Content-Transfer-Encoding 1: Quoted-Printable - This encoding reserves the equal sign "=" as the encoding escape character. Generally, any non-printable character (8-bit octet), or the equal sign "=" must be encoded in the form of "=XX", where XX is the two digit hexadecimal representation of the octet's value. For example, a CRLF (Carriage Return Line Feed) sequence will be encoded as "=0D=0A".
Content-Transfer-Encoding 2: Base64 - This encoding was adopted from "RFC 1421 Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures" as is with one change on the encoded output folding length from 64 characters to 76 characters.
This RFC also suggested that Base64 is preferred encoding over quoted-printable for binary data.
Here is an example of email message with a multi-part content using Quoted-Printable and Base64 encodings for different parts taken from kb.datamotion.com:
MIME-Version: 1.0 X-Mailer: MailBee.NET 188.8.131.528 Subject: test subject To: firstname.lastname@example.org Content-Type: multipart/mixed; boundary="XXXXboundary text" --XXXXboundary text Content-Type: multipart/alternative; boundary="XXXXboundary text" --XXXXboundary text Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable This is the body text of a sample message. --XXXXboundary text Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: quoted-printable <pre>This is the body text of a sample message.</pre> --XXXXboundary text Content-Type: text/plain; name="log_attachment.txt" Content-Disposition: attachment; filename="log_attachment.txt" Content-Transfer-Encoding: base64 TUlNRS1WZXJzaW9uOiAxLjANClgtTWFpbGVyOiBNYWlsQmVlLk5FVCA4LjAuNC40MjgNClN1Ympl Y3Q6IHRlc3Qgc3ViamVjdA0KVG86IGtldmlubUBkYXRhbW90aW9uLmNvbQ0KQ29udGVudC1UeXBl OiBtdWx0aXBhcnQvYWx0ZXJuYXRpdmU7DQoJYm91bmRhcnk9Ii0tLS09X05leHRQYXJ0XzAwMF9B RTZCXzcyNUUwOUFGLjg4QjdGOTM0Ig0KDQoNCi0tLS0tLT1fTmV4dFBhcnRfMDAwX0FFNkJfNzI1 RTA5QUYuODhCN0Y5MzQNCkNvbnRlbnQtVHlwZTogdGV4dC9wbGFpbjsNCgljaGFyc2V0PSJ1dGYt OCINCkNvbnRlbnQtVHJhbnNmZXItRW5jb2Rpbmc6IHF1b3RlZC1wcmludGFibGUNCg0KdGVzdCBi b2R5DQotLS0tLS09X05leHRQYXJ0XzAwMF9BRTZCXzcyNUUwOUFGLjg4QjdGOTM0DQpDb250ZW50 LVR5cGU6IHRleHQvaHRtbDsNCgljaGFyc2V0PSJ1dGYtOCINCkNvbnRlbnQtVHJhbnNmZXItRW5j b2Rpbmc6IHF1b3RlZC1wcmludGFibGUNCg0KPHByZT50ZXN0IGJvZHk8L3ByZT4NCi0tLS0tLT1f TmV4dFBhcnRfMDAwX0FFNkJfNzI1RTA5QUYuODhCN0Y5MzQtLQ0K --XXXXboundary text--
For more information, visit RFC 1521 Website at https://tools.ietf.org/html/rfc1521.
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