6.4 General public-key related Functions

A couple of utility functions are available to retrieve the length of the key, map algorithm identifiers and perform sanity checks:

Function: const char * gcry_pk_algo_name (int algo)

Map the public key algorithm id algo to a string representation of the algorithm name. For unknown algorithms this functions returns the string "?". This function should not be used to test for the availability of an algorithm.

Function: int gcry_pk_map_name (const char *name)

Map the algorithm name to a public key algorithm Id. Returns 0 if the algorithm name is not known.

Function: int gcry_pk_test_algo (int algo)

Return 0 if the public key algorithm algo is available for use. Note that this is implemented as a macro.

Function: unsigned int gcry_pk_get_nbits (gcry_sexp_t key)

Return what is commonly referred as the key length for the given public or private in key.

Function: unsigned char * gcry_pk_get_keygrip (gcry_sexp_t key, unsigned char *array)

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.

Function: gcry_error_t gcry_pk_testkey (gcry_sexp_t 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.

Function: gcry_error_t gcry_pk_algo_info (int algo, int what, void *buffer, size_t *nbytes)

Depending on the value of what return various information about the public key algorithm with the id algo. Note that the function returns -1 on error and the actual error code must be retrieved using the function gcry_errno. The currently defined values for what are:

GCRYCTL_TEST_ALGO:

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 flags:

GCRY_PK_USAGE_SIGN

Algorithm is usable for signing.

GCRY_PK_USAGE_ENCR

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.

GCRYCTL_GET_ALGO_USAGE:

Return the usage flags for the given algorithm. 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.

GCRYCTL_GET_ALGO_NPKEY

Return the number of elements the public key for algorithm algo consist of. Return 0 for an unknown algorithm.

GCRYCTL_GET_ALGO_NSKEY

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.

GCRYCTL_GET_ALGO_NSIGN

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.

GCRYCTL_GET_ALGO_NENC

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 NULL.

Function: gcry_error_t gcry_pk_ctl (int cmd, void *buffer, size_t buflen)

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:

GCRYCTL_DISABLE_ALGO

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:

Function: gcry_error_t gcry_pk_genkey (gcry_sexp_t *r_key, gcry_sexp_t parms)

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:

nbits

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 above example.

curve name

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.

rsa-use-e value

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:

‘0’

Use a secure and fast value. This is currently the number 41.

‘1’

Use a value as required by some crypto policies. This is currently the number 65537.

‘2’

Reserved

‘> 2’

Use the given value.

If this parameter is not used, Libgcrypt uses for historic reasons 65537.

qbits n

This is only meanigful for DSA keys. If it is given the DSA key is generated with a Q parameyer of size n bits. If it is not given or zero Q is deduced from NBITS in this way:

‘512 <= N <= 1024’

Q = 160

‘N = 2048’

Q = 224

‘N = 3072’

Q = 256

‘N = 7680’

Q = 384

‘N = 15360’

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.

domain list

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))))

nbits and qbits may not be specified because they are derived from the domain parameters.

derive-parms list

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))))

flags flaglist

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)))

transient-key

use-x931

use-fips186

use-fips186-2

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 interfaced 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:

Function: gcry_error_t gcry_pubkey_get_sexp (gcry_sexp_t *r_sexp, int mode, gcry_ctx_t ctx)

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:

0

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.

GCRY_PK_GET_PUBKEY

Return the public key even if the context has the private key parameter.

GCRY_PK_GET_SECKEY

Return the private key or the error GPG_ERR_NO_SECKEY if it is not possible.

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.