A couple of utility functions are available to retrieve the length of the key, map algorithm identifiers and perform sanity checks:
Map the public key algorithm id algo to a string representation of
the algorithm name. For unknown algorithms this functions returns the
"?". This function should not be used to test for the
availability of an algorithm.
Map the algorithm name to a public key algorithm Id. Returns 0 if the algorithm name is not known.
Return 0 if the public key algorithm algo is available for use. Note that this is implemented as a macro.
Return what is commonly referred as the key length for the given public or private key in key.
Return the so called "keygrip" which is the SHA-1 hash of the public key
parameters expressed in a way depended on the algorithm. array
must either provide space for 20 bytes or be
NULL. In the latter
case a newly allocated array of that size is returned. On success a
pointer to the newly allocated space or to array is returned.
NULL is returned to indicate an error which is most likely an
unknown algorithm or one where a "keygrip" has not yet been defined.
The function accepts public or secret keys in key.
Return zero if the private key key is ‘sane’, an error code otherwise. Note that it is not possible to check the ‘saneness’ of a public key.
Depending on the value of what return various information about
the public key algorithm with the id algo. Note that the
-1 on error and the actual error code must be
retrieved using the function
gcry_errno. The currently defined
values for what are:
Return 0 if the specified algorithm is available for use.
buffer must be
NULL, nbytes may be passed as
NULL or point to a variable with the required usage of the
algorithm. This may be 0 for "don’t care" or the bit-wise OR of these
Algorithm is usable for signing.
Algorithm is usable for encryption.
Unless you need to test for the allowed usage, it is in general better to use the macro gcry_pk_test_algo instead.
Return the usage flags for the given algorithm. For an invalid algorithm return 0. Disabled algorithms are ignored here because we want to know whether the algorithm is at all capable of a certain usage.
Return the number of elements the public key for algorithm algo consist of. Return 0 for an unknown algorithm.
Return the number of elements the private key for algorithm algo consist of. Note that this value is always larger than that of the public key. Return 0 for an unknown algorithm.
Return the number of elements a signature created with the algorithm algo consists of. Return 0 for an unknown algorithm or for an algorithm not capable of creating signatures.
Return the number of elements a encrypted message created with the algorithm algo consists of. Return 0 for an unknown algorithm or for an algorithm not capable of encryption.
Please note that parameters not required should be passed as
This is a general purpose function to perform certain control operations. cmd controls what is to be done. The return value is 0 for success or an error code. Currently supported values for cmd are:
Disable the algorithm given as an algorithm id in buffer.
buffer must point to an
int variable with the algorithm
id and buflen must have the value
sizeof (int). This
function is not thread safe and should thus be used before any other
threads are started.
Libgcrypt also provides a function to generate public key pairs:
This function create a new public key pair using information given in
the S-expression parms and stores the private and the public key
in one new S-expression at the address given by r_key. In case of
an error, r_key is set to
NULL. The return code is 0 for
success or an error code otherwise.
Here is an example for parms to create an 2048 bit RSA key:
(genkey (rsa (nbits 4:2048)))
To create an Elgamal key, substitute "elg" for "rsa" and to create a DSA key use "dsa". Valid ranges for the key length depend on the algorithms; all commonly used key lengths are supported. Currently supported parameters are:
This is always required to specify the length of the key. The
argument is a string with a number in C-notation. The value should be
a multiple of 8. Note that the S-expression syntax requires that a
number is prefixed with its string length; thus the
4: in the
For ECC a named curve may be used instead of giving the number of
requested bits. This allows to request a specific curve to override a
default selection Libgcrypt would have taken if
nbits has been
given. The available names are listed with the description of the ECC
public key parameters.
This is only used with RSA to give a hint for the public exponent. The value will be used as a base to test for a usable exponent. Some values are special:
Use a secure and fast value. This is currently the number 41.
Use a value as required by some crypto policies. This is currently the number 65537.
Use the given value.
If this parameter is not used, Libgcrypt uses for historic reasons 65537. Note that the value must fit into a 32 bit unsigned variable and that the usual C prefixes are considered (e.g. 017 gives 15).
This is only meanigful for DSA keys. If it is given, the DSA key is generated with a Q parameter of size n bits. If it is not given or zero, Q is deduced from nbits in this way:
Q = 160
Q = 224
Q = 256
Q = 384
Q = 512
Note that in this case only the values for N, as given in the table, are allowed. When specifying Q, all values of N in the range 512 to 15680 are valid as long as they are multiples of 8.
This is only meaningful for DLP algorithms. If specified, keys are generated with domain parameters taken from this list. The exact format of this parameter depends on the actual algorithm. It is currently only implemented for DSA using this format:
(genkey (dsa (domain (p p-mpi) (q q-mpi) (g q-mpi))))
qbits may not be specified because they are
derived from the domain parameters.
This is currently only implemented for RSA and DSA keys. It is not
allowed to use this together with a
domain specification. If
given, it is used to derive the keys using the given parameters.
If given for an RSA key, the X9.31 key generation algorithm is used even if libgcrypt is not in FIPS mode. If given for a DSA key, the FIPS 186 algorithm is used even if libgcrypt is not in FIPS mode.
(genkey (rsa (nbits 4:1024) (rsa-use-e 1:3) (derive-parms (Xp1 #1A1916DDB29B4EB7EB6732E128#) (Xp2 #192E8AAC41C576C822D93EA433#) (Xp #D8CD81F035EC57EFE822955149D3BFF70C53520D 769D6D76646C7A792E16EBD89FE6FC5B605A6493 39DFC925A86A4C6D150B71B9EEA02D68885F5009 B98BD984#) (Xq1 #1A5CF72EE770DE50CB09ACCEA9#) (Xq2 #134E4CAA16D2350A21D775C404#) (Xq #CC1092495D867E64065DEE3E7955F2EBC7D47A2D 7C9953388F97DDDC3E1CA19C35CA659EDC2FC325 6D29C2627479C086A699A49C4C9CEE7EF7BD1B34 321DE34A#))))
(genkey (dsa (nbits 4:1024) (derive-parms (seed seed-mpi))))
This is preferred way to define flags. flaglist may contain any number of flags. See above for a specification of these flags.
Here is an example on how to create a key using curve Ed25519 with the ECDSA signature algorithm. Note that the use of ECDSA with that curve is in general not recommended.
(genkey (ecc (flags transient-key)))
These are deprecated ways to set a flag with that name; see above for a description of each flag.
The key pair is returned in a format depending on the algorithm. Both private and public keys are returned in one container and may be accompanied by some miscellaneous information.
Here are two examples: the first for Elgamal and the second for elliptic curve key generation:
(key-data (public-key (elg (p p-mpi) (g g-mpi) (y y-mpi))) (private-key (elg (p p-mpi) (g g-mpi) (y y-mpi) (x x-mpi))) (misc-key-info (pm1-factors n1 n2 ... nn))
(key-data (public-key (ecc (curve Ed25519) (flags eddsa) (q q-value))) (private-key (ecc (curve Ed25519) (flags eddsa) (q q-value) (d d-value))))
As you can see, some of the information is duplicated, but this provides an easy way to extract either the public or the private key. Note that the order of the elements is not defined, e.g. the private key may be stored before the public key. n1 n2 ... nn is a list of prime numbers used to composite p-mpi; this is in general not a very useful information and only available if the key generation algorithm provides them.
Future versions of Libgcrypt will have extended versions of the public key interface which will take an additional context to allow for pre-computations, special operations, and other optimization. As a first step a new function is introduced to help using the ECC algorithms in new ways:
Return an S-expression representing the context ctx. Depending
on the state of that context, the S-expression may either be a public
key, a private key or any other object used with public key
operations. On success 0 is returned and a new S-expression is stored
at r_sexp; on error an error code is returned and
NULL is stored
at r_sexp. mode must be one of:
Decide what to return depending on the context. For example if the private key parameter is available, a private key is returned; if not, a public key is returned.
Return the public key even if the context has the private key parameter.
Return the private key or the error
GPG_ERR_NO_SECKEY if it is
As of now this function supports only certain ECC operations because a context object is right now only defined for ECC. Over time this function will be extended to cover more algorithms.