The requested package pear-pear.php.net/crypt_gpg could not be found in any version

Hi,
I'm using Roundcube plugin composer which needs crypt_gpg. When running 
the composer,
I get the following error message:  "The requested package 
pear-pear.php.net/crypt_gpg could not be found in any version, there may 
be a typo in the package name."
eventough it is installed (version 1.4.0). I tried several old versions 
with the same result.
Hope you can help me.
Lukas