  Quotable signature schemes are digital signature schemes with the additional
property that from the signature for a message, any party can extract
signatures for (allowable) quotes from the message, without knowing the secret
key or interacting with the signer of the original message. Crucially, the
extracted signatures are still signed with the original secret key. We define a
notion of security for quotable signature schemes and construct a concrete
example of a quotable signature scheme, using Merkle trees and classical
digital signature schemes. The scheme is shown to be secure, with respect to
the aforementioned notion of security. Additionally, we prove bounds on the
complexity of the constructed scheme and provide algorithms for signing,
quoting, and verifying. Finally, concrete use cases of quotable signatures are
considered, using them to combat misinformation by bolstering authentic content
on social media. We consider both how quotable signatures can be used, and why
using them could help mitigate the effects of fake news.
