Microsoft is donating “PhotoDNA” technology to National Center for Missing & Exploited Children, that has potential to make a drastic difference in fight against child pornography online to find hidden copies of the worst images of child sexual exploitation. The basis for PhotoDNA is a technology called “robust hashing,” which calculates particular characteristics of a given digital image — its digital fingerprint or “hash value” — to match it to other copies of that same image. “Like human beings, every photo is a little different,” Ernie Allen says. “The problem was that the signature was extremely fragile — the tiniest change to image and signature would be completely different,” Farid says. “The PhotoDNA technology extends signature to make it robust and reliable, so even if you change image a little bit, we can still find it.” Once NCMEC assigns PhotoDNA signatures to known images of abuse, those signatures can be shared with online service providers, who can match them against hashes of photos on their own services, find copies of the same photos and remove them. Also, by identifying previously “invisible” copies of identified photos, law enforcement may get new leads to help track down the perpetrators.