The World's First Digitally Signed International Agreement
theatlantic.com
theatlantic.com
The link discussed President Obama's use of an Autopen to sign a bill. I'm not entirely sure how that is comparable to Digital Signatures.
From http://www.washingtontimes.com/news/2013/jan/3/obama-uses-au...:
> The OLC said in 2005 that Article 1, Section 7 of the Constitution allows the president to use the autopen to sign legislation, stating “the president need not personally perform the physical act of affixing his signature to a bill to sign it.”