A kernel key of asymmetric type acts as a handle to an asymmetric key
as used for public-key cryptography. The key material itself may be
held inside the kernel or it may be held in hardware with operations
being offloaded. This prevents direct user access to the
Keys may be any asymmetric type (RSA, ECDSA, ...) and may have both
private and public components present or just the public component.
Asymmetric keys can be made use of by both the kernel and userspace.
The kernel can make use of them for module signature verification and
kexec image verification for example. Userspace is provided with a
set of keyctl(KEYCTL_PKEY_*) calls for querying and using the key.
These are wrapped by libkeyutils as functions named keyctl_pkey_*().
An asymmetric-type key can be loaded by the keyctl utility using a
command line like:
openssl x509 -in key.x509 -outform DER |
keyctl padd asymmetric foo @s
The asymmetric-type key can be viewed as a container that comprises
of a number of components:
The asymmetric key parsers attempt to identify the content of
the payload blob and extract useful data from it with which to
instantiate the key. The parser is only used when adding,
instantiating or updating a key and isn't thereafter
associated with the key.
Available parsers include ones that can deal with DER-encoded
X.509, DER-encoded PKCS#8 and DER-encoded TPM-wrapped blobs.
Public and private keys
These are the cryptographic components of the key pair. The
public half should always be available, but the private half
might not be. What operations are available can be queried,
as can the size of the key. The key material may or may not
actually reside in the kernel.
In addition to the normal key description (which can be
generated by the parser), a number of supplementary
identifiers may be available that can be searched for. These
may be obtained, for example, by hashing the public key
material or from the subjectKeyIdentifier in an X.509
Identifier-based searches are selected by passing as the
description to keyctl_search() a string constructed of hex
characters prefixed with either "id:" or "ex:". The "id:"
prefix indicates that a partial tail match is permissible
whereas "ex:" requires an exact match on the full string. The
hex characters indicate the data to match.
This is the driver inside the kernel that accesses the key
material and performs operations on it. It might be entirely
software-based or it may offload the operations to a hardware
key store, such as a TPM.
Note that expiry times from the payload are ignored as these patches
may be used during boot before the system clock is set.
The asymmetric key parsers can handle keys in a number of forms:
X.509 DER-encoded X.509 certificates can be accepted. Two
identifiers are constructed: one from from the certificate
issuer and serial number and the other from the
subjectKeyIdentifier, if present. If left blank, the key
description will be filled in from the subject field plus
either the subjectKeyIdentifier or the serialNumber. Only the
public key is filled in and only the encrypt and verify
operations are supported.
The signature on the X.509 certificate may be checked by the
keyring it is being added to and it may also be rejected if
the key is blacklisted.
PKCS#8 Unencrypted DER-encoded PKCS#8 key data containers can be
accepted. Currently no identifiers are constructed. The
private key and the public key are loaded from the PKCS#8
blobs. Encrypted PKCS#8 is not currently supported.
DER-encoded TPM-wrapped TSS key blobs can be accepted.
Currently no identifiers are constructed. The public key is
extracted from the blob but the private key is expected to be
resident in the TPM. Encryption and signature verification is
done in software, but decryption and signing are offloaded to
the TPM so as not to expose the private key.
This parser only supports TPM-1.2 wrappings and enc=pkcs1
encoding type. It also uses a hard-coded null SRK password;
password-protected SRKs are not yet supported.
In addition to the standard keyutils library functions, such as
keyctl_update(), there are five calls specific to the asymmetric key
type (though they are open to being used by other key types also):
The query function can be used to retrieve information about an
asymmetric key, such as the key size, the amount of space required by
buffers for the other operations and which operations are actually
The other operations form two pairs: encrypt/decrypt and
create/verify signature. Not all of these operations will
necessarily be available; typically, encrypt and verify only require
the public key to be available whereas decrypt and sign require the
private key as well.
All of these operations take an information string parameter that
supplies additional information such as encoding type/form and the
password(s) needed to unlock/unwrap the key. This takes the form of
a comma-separated list of "key[=value]" pairs, the exact set of which
depends on the subtype driver used by a particular key.
Available parameters include:
The encoding type for use in an encrypted blob or a signature.
An example might be "enc=pkcs1".
The name of the hash algorithm that was used to digest the
data to be signed. Note that this is only used to construct
any encoding that is used in a signature. The data to be
signed or verified must have been parsed by the caller and the
hash passed to keyctl_pkey_sign() or keyctl_pkey_verify()
beforehand. An example might be "hash=sha256".
Note that not all parameters are used by all subtypes.
An additional keyutils function, keyctl_restrict_keyring(), can be
used to gate a keyring so that a new key can only be added to the
affected keyring if (a) it's an asymmetric key, (b) it's validly
signed by a key in some appropriate keyring and (c) it's not
Where <signing-key> is the ID of a key or a ring of keys that act as
the authority to permit a new key to be added to the keyring. The
chain flag indicates that keys that have been added to the keyring
may also be used to verify new keys. Authorising keys must them‐
selves be asymmetric-type keys that can be used to do a signature
verification on the key being added.
Note that there are various system keyrings visible to the root user
that may permit additional keys to be added. These are typically
gated by keys that already exist, preventing unauthorised keys from
being used for such things as module verification.
When the attempt is made to add a key to the kernel, a hash of the
public key is checked against the blacklist. This is a system
keyring named .blacklist and contains keys of type blacklist. If the
blacklist contains a key whose description matches the hash of the
new key, that new key will be rejected with error EKEYREJECTED.
The blacklist keyring may be loaded from multiple sources, including
a list compiled into the kernel and the UEFI dbx variable. Further
hashes may also be blacklisted by the administrator. Note that
blacklisting is not retroactive, so an asymmetric key that is already
on the system cannot be blacklisted by adding a matching blacklist
This page is part of the keyutils (key management utilities) project.
Information about the project can be found at [unknown -- if you
know, please contact firstname.lastname@example.org] If you have a bug report for
this manual page, send it to email@example.com. This page was
obtained from the project's upstream Git repository
on 2020-07-14. (At that time, the date of the most recent commit
that was found in the repository was 2020-07-07.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
Linux 8 Nov 2018 ASYMMETRIC-KEY(7)