About

System administrators and digital archivists frequently encounter KOI8-R when recovering data from early Unix systems or analyzing pre-2000 Russian internet traffic. This 8-bit character encoding (defined in RFC 1489) assigns the upper 128 bytes to Cyrillic characters and box-drawing symbols. Unlike ISO-8859-5 or Windows-1251, the KOI8-R layout is not alphabetical. The designers arranged Cyrillic letters to map onto phonetically similar ASCII characters if the eighth bit is stripped.

Data corruption often occurs when 8-bit text passes through a 7-bit gateway (such as early email servers). In such cases, the phonetic property of KOI8-R acts as a fail-safe. The text degrades into a readable transliteration rather than random garbage. This tool provides a complete lookup table for debugging encoding errors (mojibake) and verifying byte sequences in hex editors.

Formulas

The core design principle of KOI8-R involves a bitwise relationship between the Cyrillic upper range and the Latin lower range. If the most significant bit (MSB) is dropped, the value correlates to a phonetic equivalent.

C_cyr ∧ 0x7F ≈ C_lat

For example, the Cyrillic "Russian a" occupies a specific index. Applying the mask yields the Latin "a".

Let x = 0xC1 (Cyrillic 'a')
x ∧ 01111111₂ = 0x41 (ASCII 'A')

Reference Data

Decimal	Hex	Binary	Char	Description
193	0xC1	11000001	a	CYRILLIC SMALL LETTER A
194	0xC2	11000010	б	CYRILLIC SMALL LETTER BE
225	0xE1	11100001	A	CYRILLIC CAPITAL LETTER A
226	0xE2	11100010	Б	CYRILLIC CAPITAL LETTER BE
128	0x80	10000000	─	BOX DRAWINGS LIGHT HORIZONTAL
154	0x9A	10011010		NO-BREAK SPACE

Frequently Asked Questions

The ordering prioritizes phonetic mapping over alphabetic sorting. The positions 192 through 255 align with ASCII characters 64 through 127. This ensures that if the eighth bit (128) is subtracted or lost during transmission, the resulting 7-bit code displays a readable Latin approximation (e.g., "Русский" becomes 'Russkij').

KOI8-U is the Ukrainian variation. It is largely identical to KOI8-R but replaces specific box-drawing characters in the 0xA4-0xBF range with Ukrainian letters like Ґ, Є, І, and Ї. Systems configured for KOI8-R may display these Ukrainian characters as graphical lines or symbols.

The mapping effectively inverts the case in many instances. For example, Cyrillic lowercase characters (0xC0-0xDF) map to ASCII uppercase (0x40-0x5F) when the bit is stripped. This is a known side effect of the design choice to align specific phonetic sounds within the limitations of the ASCII table structure.

While UTF-8 has largely replaced single-byte encodings, KOI8-R remains relevant in legacy Russian Unix servers, old IRC logs, mail archives from the 1990s, and certain industrial control systems that were never upgraded to Unicode.

If you open a text file and see a mix of Cyrillic letters and box-drawing characters where you expect plain text, or if you see Latin characters that look like a phonetic transliteration of Russian, you may be viewing a KOI8-R file with a stripped bit. Common mojibake signatures include sequences like "рПЙЧЕФ" (Privet) appearing as nonsense in Windows-1251.