Asymmetric cryptography based on public-private key pairs is nowadays used in many applications, notably for authentication and digital signature. It is very frequently used together with digital certificates which link public keys to real-world identities. In a system using digital certificates, one core issue is the question of whether a certificate is valid. This issue has been treated extensively in the literature. In this paper, we are interested in an issue which has received less attention: the question of whether a certificate
valid. This question is of particular importance for digitally signed documents which have to be verified a long time after their signature. We present and analyze a new algorithm to validate certificates “in the past”, with the goals of it being simple to present, simple to implement and to fall back on standard algorithms with current time settings.