asymmetric-key(7) — Linux manual page


ASYMMETRIC-KEY(7)      Asymmetric Kernel Key Type      ASYMMETRIC-KEY(7)

NAME         top

       asymmetric - Kernel key type for holding asymmetric keys

OVERVIEW         top

       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 cryptographic material.

       Keys may be any asymmetric type (RSA, ECDSA, ...) and may have
       both private and public components present or just the public

       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

DESCRIPTION         top

       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.

PARSERS         top

       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.

       TPM-Wrapped keys
              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.

USERSPACE API         top

       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


       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.

       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

       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

       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 blacklisted.

            keyctl_restrict_keyring(keyring, "asymmetric",

       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 themselves 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

BLACKLISTING         top

       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 entry later.

VERSIONS         top

       The asymmetric key type first appeared in v3.7 of the Linux
       kernel, the restriction function in v4.11 and the public key
       operations in v4.20.

SEE ALSO         top

       keyctl(1), add_key(2), keyctl(3), keyctl_pkey_encrypt(3),
       keyctl_pkey_query(3), keyctl_pkey_sign(3), keyrings(7),

COLOPHON         top

       This page is part of the keyutils (key management utilities)
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       on 2023-12-22.  (At that time, the date of the most recent commit
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Linux                          8 Nov 2018              ASYMMETRIC-KEY(7)

Pages that refer to this page: keyctl_pkey_query(3)