Business data flows from one computer system (1) to another (2) and its integrity can be protected by cryptographic means, such as digital signatures. In particular, a source system (1) may use a private key (DSPR) to sign outgoing data, and a destination system (2) may use a public key (DSPU) to verify incoming data. For security purposes all keys should be changed at scheduled times calculated using factors including key lifetime (from which is calculated the key expiry time) and key delivery time. If a key is compromised it needs to be changed at other than the scheduled time, and in general this will result in calculation of a new scheduled key change time. If a DSPR key is delivered to the source system (1) encrypted by a key encryption key (KEK), then change to the KEK key will in general also be needed upon compromise of the DSPR key. A new key changetime calculation can be avoided if another public key/private key pair is pre-generated and the public key part pre-supplied to the destination system (2), where it is stored as a spare. When the existing private key (DSPR) is compromised, the new private key corresponding to the spare is supplied to the source system (1) and can be put into use immediately. In this case the expiry time of the spare public key will be substantially the same as that of the original public key, as it will deemed to have been in use as of delivery, and recalculation of the key changetime will not be required. Preferably a public key in use at the destination system (2) is not revoked immediately upon compromise of the corresponding private key, in order to enable messages signed by that private key to be verified at the destination system, using the corresponding public key, during a predetermined time interval (message latency) after signing. A maximum value for the message latency may be set and used as another factor in the scheduled key change time calculation.