We implemented the AES (Advanced Encryption Standard) block based on its standard. Any analysis of the strengths and weaknesses of AES is performed by the standard's committee.
Studies for the strength of AES encryption key have been documented by the standard's committee in the link below, specifically section 6.2 of the document in the below link says that there's no restriction on the key:
http://csrc.nist.gov/publications/fips/fips197/fips-197.pdfHowever, the document above was published in 2001. Since then, there were theoretical attack scenarios. Other resources:
http://www.quadibloc.com/crypto/co040401.htm
http://en.wikipedia.org/wiki/Advanced_Encryption_StandardThe first page has a section on "Attacks on Rijndael". It seems to imply that keys with all constants is not good.