A block cipher algorithm maps bit strings of a fixed length to strings of:

1. A block cipher is a family of functions with inverses. 2. An invertible map cannot lose or add bits. 3. A block cipher maps bit strings of a fixed length to bit strings of the same length. 4. So the answer is the same length. _Source: NIST SP 800-175B Rev. 1, 'Guideline for Using Cryptographic Standards in the Federal Government'_