Weaknesses in MD4 were demonstrated by Den Boer and Bosselaers in a paper published in 1991. The first full-round MD4 collision attack was found by Hans Dobbertin in 1995, which took only seconds to carry out at that time. In August 2004, Wang et al. found a very efficient collision attack, alongside attacks on later hash function designs in the MD4/MD5/SHA-1/RIPEMD family. This result was improved later by Sasaki et al., and generating a collision is now as cheap as verifying it (a few microseconds).