hashed?

forgive me if I sound ignorant, but this sentence doesn't make sense to 
me:

"This encoded information is cryptographically hashed with a secret key 
to form the sequence number of the SYN-ACK and sent to the client."

Shouldn't it read something like "encrypted with a secret key", rather 
than "cryptographically hashed with a secret key"? I was thinking if it 
was hashed, the kernel wouldn't be able to decode it when the ack comes 
in.