Examples:

RSA key pairs can be used for signing or encryption.
EC key pairs can be used for signing or key agreement.
EC key pairs can used for encryption using the EC Integrated Encryption Scheme. In this case, the EC key pair is being used for key agreement as part of the algorithm.
Symmetric keys can be used for encryption and authentication.

Do:

Don’t:

Don’t use the same key for different types of data / different security domains.
The same key should not be used with different parameters. For example, an RSA key should not be used to create RSA/SHA256 and RSA/SHA512 signatures.

Reasons:

Key cycling time frames may differ for different types of keys.
Revocation: It may be desirable to be able to revoke the signing key pair or encryption key pair, without affecting the other key pair.
Non-repudiation and Key Escrow: Only the owner of a signing private key should be able to access a signing private key. However, it may be desirable to escrow encryption private keys.
Algorithm Compromise: In the unlikely event that one algorithm or implementation becomes compromised, using a single key pair for two usages would compromise both usages.
Lack of Security Proofs: Individual algorithms have security proofs written for them to show that they are secure. However, generally, two separate algorithms do not have a joint security proof showing that using the same key for two algorithms at the same time is secure.
Known issues: RSA

Resources

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