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Why are mail attachments bigger than the original file?

  • Type: Knowledgebase
  • Date changed: one year ago
Solution #00007528


Applies to all Barracuda Spam And Email Security Gateway


MIME (Multipurpose Internet Mail Extensions) specification, defined in RFC 2045, lists "base64" as one of several binary-to-text encoding schemes. MIME's base64 encoding is based on that of the RFC 1421 version of privacy enhanced mail (PEM), it uses the same 64-character alphabet and encoding mechanism as PEM, and uses the "=" symbol for output padding in the same way.

MIME does not specify a fixed length for base64-encoded lines, but it does specify a maximum length of 76 characters. Additionally it specifies that any extra-alphabetic characters must be ignored by a compliant decoder, although most implementations use a CR/LF newline pair to delimit encoded lines.

Thus, the actual length of MIME-compliant base64-encoded binary data is usually about 137% of the original data length, though for very short messages the overhead can be a lot higher because of the overhead of the headers. Very roughly, the final size of base64-encoded binary data is equal to 1.37 times the original data size + 814 bytes (for headers).