How is xxencode different from uuencode?

They share the same structure (begin header, lines of up to 45 bytes packed 3-into-4, end trailer) but use different alphabets. UUEncode maps 6-bit values onto a range that includes many punctuation marks, while xxencode uses only plus, minus, digits and letters.

Why prefer the XX alphabet?

Some early mail and EBCDIC gateways rewrote or stripped punctuation characters, which silently corrupted uuencoded blocks. The XX alphabet avoids all punctuation except plus and minus, so the encoded data passed through those gateways intact.

What is the line-length character?

Each data line begins with one alphabet character whose value is the number of original bytes that line represents, from 1 to 45. The decoder reads it to know how many bytes to emit, which is how trailing padding is discarded correctly.

Can xxencode handle binary, not just text?

Yes, the algorithm encodes raw bytes. This tool treats your input as UTF-8 text for convenience, but the same 3-byte-to-4-character packing works on any byte stream.

Is anything uploaded?

No. Encoding and decoding run entirely in your browser. No data is sent to a server.

What is the XXEncode / XXDecode?

Free xxencode and xxdecode tool. Encodes binary data with the punctuation-free XX alphabet (digits, letters, plus and minus) that survives strict mail gateways, using the same begin header and length-prefixed lines as uuencode. Client-side. It runs free in your browser on Gera Tools, with nothing uploaded.

XXEncode / XXDecode — Gera Tools

Name: XXEncode / XXDecode
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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XXEncode is a close cousin of uuencode, built to solve the same problem of moving binary through 7-bit text channels, but with an alphabet chosen to survive even hostile gateways. This tool encodes text into an xxencoded block and decodes blocks back to the original bytes, all in your browser.

How it works

XXEncode borrows uuencode’s exact framing: a begin 644 filename header, body lines of up to 45 source bytes, and an end trailer. For each three input bytes it forms four 6-bit values and looks each one up in the 64-character XX alphabet, which is +, -, the digits, and the upper- and lower-case letters. Every line starts with one alphabet character whose index equals the number of original bytes that line carries.

The crucial difference from uuencode is the alphabet. UUEncode’s mapping lands on many punctuation marks that some legacy mail systems silently rewrote. XXEncode deliberately uses only alphanumerics plus + and -, so a block could pass through those gateways without corruption.

The XX alphabet compared to UUEncode

Both formats pack three bytes into four characters using the same arithmetic, but the lookup table they use differs:

Position	XX alphabet character	UUEncode character
0	`+`	space (ASCII 32)
1	`-`	`!`
2–11	`0`–`9`	`"` through `)`
12–37	`A`–`Z`	`*` through `9` and more
38–63	`a`–`z`	continued punctuation range

UUEncode’s mapping starts at ASCII 32 (space) and runs through 95, which sweeps up characters like backtick, tilde, and many punctuation marks. Some EBCDIC gateways and early email relays rewrote those, silently breaking the block. XX’s tighter alphabet sidesteps the problem: +, -, digits, and letters all have stable representations in 7-bit ASCII across every gateway that mattered.

Anatomy of an xxencoded block

begin 644 example.txt
hJDDE5MDIFKSDLDM5AKLD
h+ABCD12345-abcde
`
end

begin 644 example.txt — the header. 644 is the Unix file permission in octal; example.txt is the decoded file name.
Each body line starts with a length character whose XX value is the number of original bytes on that line (1–45). The backtick line (`, value 0) signals the end of data before the end trailer.
end closes the block.

The decoder expects the complete framing. Text before begin or after end is ignored, which makes it safe to decode blocks cut from an email where surrounding message text is included.

When to use xxencode

XXEncode is a legacy format from the 1990s Usenet era. If you are:

Recovering old Usenet archives or mailing-list archives — these frequently contain xxencoded attachments, particularly from gateways that stripped or corrupted UUEncode blocks.
Working with a transport that still mangles punctuation — some industrial or embedded email relays still have this property.

For anything new, use MIME base64 (or just SMTP MIME attachments). XXEncode and uuencode are not interchangeable — they look similar but the alphabets differ, so decoding an xxencoded block with a uudecode tool will produce garbage. Make sure you identify which format your block uses before choosing a decoder.