Uuencoding (Unix-to-Unix encoding) is a binary-to-text encoding that was used to transmit binary files over UUCP and early email systems in the 1980s and 1990s. It has been largely superseded by Base64 MIME encoding.

How does Uuencoding work?

Uuencoding groups input bytes into 3-byte groups and encodes each as 4 printable characters in the range ASCII 32–95. Each line starts with a length character and is 60 encoded characters long.

Uuencoder / Uudecoder — Free Online Tool

Uuencoding (Unix-to-Unix encoding) is one of the earliest binary-to-text encoding schemes, developed in the late 1970s for transmitting binary files between UNIX systems over the UUCP network. It was widely used in the 1980s and 1990s for sharing files over email and Usenet newsgroups before MIME encoding became the standard.

How Uuencoding Works

Uuencoding takes 3 bytes of input and encodes them as 4 printable ASCII characters in the range 32 (space) to 95 (_). Each line of output is 60 encoded characters long (representing 45 bytes of input) and is prefixed by a length character. The encoded block starts with a "begin" header (including file permissions and filename) and ends with "end".

Uuencoding vs Base64

Uuencoding uses ASCII 32–95; Base64 uses A-Z, a-z, 0-9, +, /
Both encode 3 bytes as 4 characters (~33% overhead)
Base64 is the modern standard; Uuencoding is a historic legacy format
Uuencoding includes file metadata (name and permissions) in the header

Frequently Asked Questions

Is Uuencoding still used?

Rarely. Uuencoding has been superseded by Base64 MIME encoding for email and HTTP. You may encounter it when working with very old systems, legacy Usenet archives, or certain embedded hardware protocols.

What does the M at the start of Uuencoded lines mean?

The first character of each Uuencoded line is the length character. M (ASCII 77 = 32 + 45) means the line encodes 45 bytes — the standard full-line length.

Uuencoder / Uudecoder online

Encode text with Unix-to-Unix encoding and decode Uuencoded data — runs in your browser

How Uuencoding Works

Uuencoding vs Base64

Frequently Asked Questions

Is Uuencoding still used?

What does the M at the start of Uuencoded lines mean?