typedef enum { ... } SilcSKRKeyUsage;

    Indicates the usage of the key.  Keys can be added for different
    reasons and for different purpose to the repository.  SilcSKRKeyUsage
    indicates for what reason the key exists in the repository.  The default
    usage is SILC_SKR_USAGE_ANY and allows any kind of usage for the key.
    If the usage should be limited then specific usage bitmask can be
    specified when adding the key.  When searching keys from the
    repository at least one of the key usage bits must be found in order
    to find the key.

    typedef enum {
      SILC_SKR_USAGE_ANY                   = 0x0000,  /* Any usage */
      SILC_SKR_USAGE_AUTH                  = 0x0001,  /* Signatures/verification */
      SILC_SKR_USAGE_ENC                   = 0x0002,  /* Encryption/decryption */
      SILC_SKR_USAGE_KEY_AGREEMENT         = 0x0004,  /* Key agreement protocol */
      SILC_SKR_USAGE_IDENTIFICATION        = 0x0008,  /* Identifying key owner */
      SILC_SKR_USAGE_SERVICE_AUTHORIZATION = 0x0010,  /* Service authorization */
    
      /* From 0x0100 reserved for private/application use. */
    } SilcSKRKeyUsage;