Since we're being grammar Nazis...

"An" is used for words that start with vowel sounds, not just actual vowel letters. The "R" in "RSA" is pronounced like "are" and so is also a vowel sound beginning. And so this should also be "an".

(Insert something about people in glass houses here, https://github.com/pyca/cryptography/pull/619/files#diff-6c0da570372c7404195000e639716c51R231)