Encryption - Public and private keys, digital signature (message digests)


In Runtime
Hi, I was wondering about public and private keys with digital signatures

I thought that a private key can decrypt ciphered text that has been ciphered by a public key only, but a public key cannot decrypt text that has been encypted by a private?? Right?

That also implies that public keys cant decrypt public key messages? Only the private key right?

So my textbook says for creating digital signature:

when the message digest has been created (hashed), it is encrypted by the sender's private key, but then goes onto say that it's decrypted by the reciever's private key

The internet page here http://www.youdzone.com/signature.html says:

"decrypts the signature (using Bob's public key) changing it back into a message digest"

to get the message digest, but at the top of the page it says

"Either one of Bob's two keys can encrypt data, and the other key can decrypt that data."

so how can the public key be used to decrypt :\

Both ways don't make sense :\
Can someone tell me how it works?

Thanks :)


Fully Optimized
I don't know enough info on Hashes or the private keys to be able to answer your question. Although I will tell you that I have definitely peaked my interest. I have tomorrow off and then the weekend. RESEARCH! WOOT!