Basics: The Key Usage Certificate Extension

Certificate extensions were introduced with version 3 of the X.509 standard. The Key Usage extension is an optional certificate extension that can be used in the RFC 5280 is defined and is used to limit the allowed uses for a key.

This is a simple bitmask.

KeyUsage ::= BIT STRING {
  digitalSignature (0),
  nonRepudiation (1),
  keyEncipherment (2),
  dataEncipherment (3),
  keyAgreement (4),
  keyCertSign (5),
  cRLSign (6),
  encipherOnly (7),
  decipherOnly (8)
}

However, the representation of the bit mask is usually in hexadecimal form.

Bit	Hex	Description	Label
8	0x0	decipherOnly	Decrypt only, in conjunction with keyAgreement
7	0x1	encipherOnly	Encrypt only, in conjunction with keyAgreement
6	0x2	cRLSign	Signing blacklists
5	0x4	keyCertSign	Signing certificates
4	0x8	keyAgreement	Used, for example, for data encryption with the Diffie-Hellman method
3	0x10	dataEncipherment	Data encryption, directly with the key contained in the certificate
2	0x20	keyEncipherment	Key encryption, i.e. when a symmetric key is used for data encryption and this is encrypted with the key contained in the certificate
1	0x40	nonRepudiation	Non-repudiation
0	0x80	digitalSignature	Digital signature

Typical examples of common use cases for certificates

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Certification Authority Certificates

Certification authority certificates must have a key usage extension according to RRC 5280. This should be marked as critical. The extension will typically include keyCertSign and keyCrlSign. The Microsoft Certificate Authority will also include digitalSignature, but this should not be done according to RFC 5280.

There is the possibility, configure the Key Usage extension differently when installing a certificate authority.

TLS web server certificates

At TLS web server certificates it depends on which type of key is used (see RFC 5246 and RFC 4492).

Key algorithm	Value
RSA	Signature and Encryption
ECDSA	Signature
ECDH	Signature and Encryption

S/MIME certificates

With Secure/Multipurpose Internet Mail Extensions (S/MIME), two operations are basically possible with regard to e-mail messages:

• Signing sent messages or verifying the signatures of received messages
• Encrypt messages sent or decrypt messages received

When configuring the certificates, you have the choice of using a hybrid certificate (which supports both operations) or separate certificates for both operations.

Type of certificate	Key Usage Extension
Encrypt/Decrypt only	keyEncipherment (0x20)
Sign/verify only	digitalSignature (0x80)
both operations (hybrid certificate)	digitalSignature, keyEncipherment (0xA0)

Calculate with the Key Usage extension

A very simple way to calculate a key usage is possible with Windows PowerShell.

An example: digitalSignature (0x80) and keyEncipherment (0x20) together result in 0xA0.

"{0:x}" -f (0x80 -bor 0x20)

Related links

External sources

• Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile / Key Usage (Internet Engineering Task Force)
• All About Certificate Extensions (Mozilla)