The “original” cyrillic, whose encoding was adopted by the LaTeX project as OT2. The fonts are distributed as part of the AMS fonts collection (both in MetaFont format and as Adobe Type 1).

Note that, for fully satisfactory typesetting of languages, the LH font set is necessary. Those fonts are also available in Adobe Type 1 format as part of the CM-super bundle)

The author is The American Mathematical Society.