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.\" Copyright (C) 2018 Red Hat, Inc. All Rights Reserved.
.\" Written by David Howells (dhowells@redhat.com)
.\"
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.TH ASYMMETRIC-KEY 7 "8 Nov 2018" Linux "Asymmetric Kernel Key Type"
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.SH NAME
asymmetric \- Kernel key type for holding asymmetric keys
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.SH OVERVIEW
.PP
A kernel key of
.B 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 cryptographic material.
.PP
Keys may be any asymmetric type (RSA, ECDSA, ...) and may have both private and
public components present or just the public component.
.PP
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
.BR keyctl ( KEYCTL_PKEY_* )
calls for querying and using the key.  These are wrapped by
.B libkeyutils
as functions named
.BR keyctl_pkey_*() .
.PP
An asymmetric-type key can be loaded by the keyctl utility using a command
line like:
.PP
.in +4n
.EX
openssl x509 -in key.x509 -outform DER |
keyctl padd asymmetric foo @s
.EE
.in
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.SH DESCRIPTION
.PP
The asymmetric-type key can be viewed as a container that comprises of a
number of components:
.TP
Parsers
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.
.IP
Available parsers include ones that can deal with
.RB "DER-encoded " X.509 ", DER-encoded " PKCS#8 " and DER-encoded " TPM "-wrapped blobs."
.TP
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.
.TP
Identifiers
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 certificate.
.IP
Identifier-based searches are selected by passing as the description to
.BR 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.
.TP
Subtype
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
.BR TPM .
.PP
Note that expiry times from the payload are ignored as these patches may be
used during boot before the system clock is set.
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.SH PARSERS
.PP
The asymmetric key parsers can handle keys in a number of forms:
.TP
\fBX.509\fP
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.
.IP
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.
.TP
\fBPKCS#8\fP
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.
.TP
\fBTPM-Wrapped keys\fP
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.
.IP
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.
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.SH USERSPACE API
.PP
In addition to the standard keyutils library functions, such as
.IR keyctl_update (),
there are five calls specific to the asymmetric key type (though they are open
to being used by other key types also):
.PP
.RS
\fIkeyctl_pkey_query\fP()
.br
\fIkeyctl_pkey_encrypt\fP()
.br
\fIkeyctl_pkey_decrypt\fP()
.br
\fIkeyctl_pkey_sign\fP()
.br
\fIkeyctl_pkey_verify\fP()
.RE
.PP
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 supported.
.PP
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.
.PP
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.
.PP
Available parameters include:
.TP
enc=<type>
The encoding type for use in an encrypted blob or a signature.  An example
might be "enc=pkcs1".
.TP
hash=<name>
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 \fIkeyctl_pkey_sign\fP() or
\fIkeyctl_pkey_verify\fP() beforehand.  An example might be "hash=sha256".
.PP
Note that not all parameters are used by all subtypes.
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.SH RESTRICTED KEYRINGS
.PP
An additional keyutils function,
.IR 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 blacklisted.
.PP
.in +4n
.EX
 keyctl_restrict_keyring(keyring, "asymmetric",
                         "key_or_keyring:<signing-key>[:chain]");
.EE
.in
.PP
Where \fI<signing-key>\fP 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 \fIchain\fP flag
indicates that keys that have been added to the keyring may also be used to
verify new keys.  Authorising keys must themselves be asymmetric-type keys that
can be used to do a signature verification on the key being added.
.PP
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.
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.SH BLACKLISTING
.PP
When the attempt is made to add a key to the kernel, a hash of the public key
is checked against the
.BR blacklist.
This is a system keyring named
.B .blacklist
and contains keys of type
.BR blacklist .
If the blacklist contains a key whose description matches the hash of the new
key, that new key will be rejected with error
.BR EKEYREJECTED .
.PP
The blacklist keyring may be loaded from multiple sources, including a list
compiled into the kernel and the UEFI
.B 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 entry later.
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.SH VERSIONS
The
.B asymmetric
key type first appeared in v3.7 of the Linux kernel, the
.B restriction
function in v4.11 and the
.B public key operations
in v4.20.
.SH SEE ALSO
.ad l
.nh
.BR keyctl (1),
.BR add_key (2),
.BR keyctl (3),
.BR keyctl_pkey_encrypt (3),
.BR keyctl_pkey_query (3),
.BR keyctl_pkey_sign (3),
.BR keyrings (7),
.BR keyutils (7)