Crypt/GPG.php
public function addSignKey($key, $passphrase = null)
    {
        $this->_addKey($this->signKeys, false, true, $key, $passphrase);
        return $this;
    }
public function addSignKey($key, $passphrase = null)
    {
        $this->_addKey($this->signKeys, false, true, $key, $passphrase);
        return $this;
    }
Adds a key to use for signing

@param mixed  $key        the key to use. This may be a key identifier,
user id, fingerprint, {@link Crypt_GPG_Key} or
{@link Crypt_GPG_SubKey}. The key must be able
to sign.
@param string $passphrase optional. The passphrase of the key required
for signing.

@return Crypt_GPG the current object, for fluent interface.

@see Crypt_GPG::sign()
@see Crypt_GPG::signFile()
@see Crypt_GPG::clearSignKeys()
@see Crypt_GPG::_addKey()

@sensitive $passphrase