+(defun mml1991-epg-find-usable-key (keys usage)
+ (catch 'found
+ (while keys
+ (let ((pointer (epg-key-sub-key-list (car keys))))
+ (while pointer
+ (if (and (memq usage (epg-sub-key-capability (car pointer)))
+ (not (memq 'disabled (epg-sub-key-capability (car pointer))))
+ (not (memq (epg-sub-key-validity (car pointer))
+ '(revoked expired))))
+ (throw 'found (car keys)))
+ (setq pointer (cdr pointer))))
+ (setq keys (cdr keys)))))
+
+;; XXX: since gpg --list-secret-keys does not return validity of each
+;; key, `mml1991-epg-find-usable-key' defined above is not enough for
+;; secret keys. The function `mml1991-epg-find-usable-secret-key'
+;; below looks at appropriate public keys to check usability.
+(defun mml1991-epg-find-usable-secret-key (context name usage)
+ (let ((secret-keys (epg-list-keys context name t))
+ secret-key)
+ (while (and (not secret-key) secret-keys)
+ (if (mml1991-epg-find-usable-key
+ (epg-list-keys context (epg-sub-key-fingerprint
+ (car (epg-key-sub-key-list
+ (car secret-keys)))))
+ usage)
+ (setq secret-key (car secret-keys)
+ secret-keys nil)
+ (setq secret-keys (cdr secret-keys))))
+ secret-key))
+