Rijndael is a symmetric block cipher that was developed by two Belgian cryptographers, Vincent Rijmen and Joan Daemen, and its design is based on an earlier block cipher named Square. Both the block length and key length in Rijndael are variable, with possible block lengths of 128, 192, or 256 bits and key lengths of the same three values (both lengths can be extended further in multiples of 32 bits). Like DES and the International Data Encryption Algorithm (IDEA), Rijndael uses a series of rounds to transform plaintext blocks into ciphertext, with more rounds used for bigger block sizes and larger keys. Each round combines substitutions, rotations, XOR operations, and mixing of columns in the state table.
The design and operation of Rijndael is freely available and has been described by its authors in detail in the book The Design of Rijndael, published by Springer-Verlag. Publishing the operation of an encryption algorithm like this generally enhances its security rather than diminishes it by exposing it to peer review so that cryptanalysts everywhere can try to devise methods for cracking it.