Predictable Serial Number Attack - Countermeasures

Countermeasures

A common approach to prevent predictable serial number attacks is to use a cryptographic hash function such as SHA-2 to generate the actual serial numbers. Internally, the issuing organization creates a (pseudo-)random nonce as a salt for generating the serial numbers, and keeps it secret. The issuer increments their internal serial number and appends it to the salt, and the computed message digest is used to create the actual serial number. The issuer does have to take care to prevent collisions between existing values so as not to wrongly issue two identical serial numbers.