What is the purpose of using separate key pairs for signing and encryption?
Why do I need to use separate public key pairs for signing and encryption and not use the same key pair with RSA for example? Is there any security problem with using the same key?
Answer
The reason for using separate key pairs for signing and encryption is to spread the risk: If someone recovers the private encryption key, he/she can decrypt documents that were encrypted using the public encryption key but can’t use it to also sign documents and vice versa.
Another reason could be a legal reason:
In several countries, a digital signature has a status somewhat like that of a traditional pen and paper signature, like in the EU digital signature legislation. Generally, these provisions mean that anything digitally signed legally binds the signer of the document to the terms therein. For that reason, it is often thought best to use separate key pairs for encrypting and signing. Using the encryption key pair, a person can engage in an encrypted conversation (e.g., regarding a real estate transaction), but the encryption does not legally sign every message he sends. Only when both parties come to an agreement do they sign a contract with their signing keys, and only then are they legally bound by the terms of a specific document. After signing, the document can be sent over the encrypted link. If a signing key is lost or compromised, it can be revoked to mitigate any future transactions. If an encryption key is lost, a backup or key escrow should be utilized to continue viewing encrypted content. Signing keys should never be backed up or escrowed.