Crypt/GPG/SubKey.php
public function setCanSign($canSign)
    {
        if ($canSign) {
            $this->_usage |= self::USAGE_SIGN;
        } else {
            $this->_usage &= ~self::USAGE_SIGN;
        }

        return $this;
    }
public function setCanSign($canSign)
    {
        if ($canSign) {
            $this->_usage |= self::USAGE_SIGN;
        } else {
            $this->_usage &= ~self::USAGE_SIGN;
        }

        return $this;
    }
Sets whether or not this sub-key can sign data

@param boolean $canSign true if this sub-key can sign data and false if
it can not.

@return Crypt_GPG_SubKey the current object, for fluent interface.