No, that's absolutely wrong. The Sacrament of Penance, like any other Sacrament, cannot be effected without the form. Here are some relevant decrees of the Church:
The holy synod doth furthermore teach, that the form of the sacrament of penance, wherein its force principally consists, is placed in those words of the minister, I absolve thee, &c: to which words indeed certain prayers are, according to the custom of holy Church, laudably joined, which nevertheless by no means regard the essence of that form, neither are they neces sary for the administration of the sacrament itself. But the acts of the penitent himself, to wit, contrition, confession and satisfaction, are as it were the matter of this sacrament. (Trent, Session XIV, Decree on Penance, cap. iii)But, as regards the minister of this sacrament, the holy Synod declares all these doctrines to be false, and utterly alien from the truth of the Gospel, which perniciously extend the ministry of the keys to any others soever besides bishops and priests ... For neither would faith without penance bestow any remission of sins; nor would he be otherwise than most careless of his own salvation, who, knowing that a priest but absolved him in jest, should not care fully seek for another who would act in earnest. (Trent, Session XIV, Decree on Penance, cap. iv)
Every Sacrament consists of two things, matter, which is called the element, and form, which is commonly called the word ... In our Sacraments, on the contrary, the form is so definite that any, even a casual deviation from it renders the Sacrament null. (Roman Catechism, On the Sacraments)
There is a longstanding principle of sacramental theology known as "supplied jurisdiction."
"Supplied jurisdiction" refers to exactly that: jurisdiction. It does not supply for a defect in the form:
Can. 144 §1 In common error, whether of fact or of law, and in positive and probable doubt, whether of law or of fact, the Church supplies executive power of governance for both the external and the internal forum.
Even if you confessed to an impostor who had never been ordained, your sins would be absoved through the Church's Power of the Keys.
Only if you have perfect contrition. If you don't, then those sins are not absolved until you either make an act of perfect contrition, confess to a real priest, or receive Holy Communion (which forgives even mortal sins of which the recipient is unaware).
Yes, you are right about the question of form -- I stand corrected and thank you for the citation.
But your point about jurisdiction still troubles me. I have heard of cases (some through hearsay, some actually reported in news accounts) of fraudulent priests hearing confessions, and I was always told that the absolutions were "validated" through the jurisdiction supplied by Canon Law. And these were definitely cases of men who had never taken Holy Orders, not simply priests who lacked jurisdiction.
Can you (or anyone else) shed some light on this point?