Email Compatibility

By: Jesse Miller

When PGP is used, at least part of the block to be transmitted is encrypted. If only the signature service is used, then the message digest is encrypted. If the confidentiality service is used, the message plus signature are encrypted. Thus, part or the entire resulting block consists of a stream of arbitrary 8-bit octets.

However, many electronic mail systems only permit the use of blocks consisting of ASCII text. To accommodate this restriction, PGP provides the service of converting the raw 8-bit binary stream to a stream of printable ASCII characters.

The scheme used for this purpose is radix-64 conversion. Each group of three octets of binary data is mapped into four ASCII characters. This format also appends a CRC to detect transmission errors.

The use of radix 64 expands a message by 33%. Fortunately, the session key and signature portions of the message are relatively compact, and the plaintext message has been compressed. In fact, the compression should be more than enough to compensate for the radix-64 expansion. HELD96 reports an average compression ratio of about 2.0 using ZIP. If we ignore the relatively small signature and key components, the typical overall effect of compression and expansion of a file of length X would be 1.33*0.5*x=0.665*x. Thus, there is still an overall compression of about one-third.

Radix-64 algorithm is that it blindly converts the input stream to radix-64 format regardless of content, even if the input happens to be ASCII text. Thus, a message is signed but not encrypted and the conversion is applied to the entire block, the output will be unreadable to the casual observer, which provides a certain level of confidentiality. As an option, PGP can be configured to convert to radix -64 formats only the signature portion of signed plaintext messages. This enables the human recipient to read the message without using PGP. PGP would still have to be used to verify the signature.

On transmission, if it is required, a signature is generated using a hash code of the uncompressed plaintext. Then the plain text, plus signature if present, is compressed. If confidentiality is required, the block is encrypted and prep ended with the public key encrypted symmetric encryption key. The entire block is converted to radix-64 format.

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The incoming block is first converted back from radix-64 format to binary. If the message is encrypted it recovers the session key and decrypts the message.

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