MONITORING SYSTEM FOR MONITORING HASH-BASED DIGITAL SIGNATURES

Monitoring system for monitoring hash-based digital signatures comprising a requesting device (23, 33), a monitoring apparatus (22, 32, 40) and a signing device (21, 31), wherein the monitoring apparatus (22, 32, 40) is configured to- receive a signature request data structure comprising data to be digitally signed from the requesting device (23, 33),- store the data to be signed,- receive a hash-based digital signature (HBsig) generated for the data to be signed by a stateful hash-based cryptographic function of the signing device (21, 31),- extract a one-time signature (OTsig) from the received hash-based digital signature (HBsig),- determine a one-time public key (OTpk) based on the stored data to be signed and the one-time signature (OTsig),- compare the one-time public key (OTpk) with previous one-time public keys determined from hash-based digital signatures previously received from the signing device (21, 31), and- if the one-time public key (OTpk) is different to any of previous one-time public keys, store the one-time public key (OTpk) in a data storage unit (25),if the one-time public key (OTpk) is equal to at least one of the previous one-time public keys, output a first warning signal to the requesting device (23, 33).