Key safety vs Backup : History of a bad day (key-restoration problem)

[Robert J. Hansen]

Just to head a question off at the pass...

Sven Radde wrote:
>> Q1: I have the public key (0x26A2F0AE if it's of any use), i know the
>> secret key passphrase perfectly. Is there any way i could re-compute /
>> restore / whatever the secret part using this information ?
> 
> No. The passphrase is in no way connected to the actual private key
> material. It is only used to encrypt that key material.

I said "yes, it's possible, but totally impractical".  Sven has said
"no, it's not".  As is often the case when talking about crypto, both
answers are totally correct.

Sven is answering the question of "can I recover the private key,
knowing the passphrase".  He's right.  The passphrase doesn't help you here.

I'm answering the question of "can I recover the private key, knowing
the public key".  I'm right.  The task moves from categorically
impossible to theoretically possible.  However, the practical
difficulties are presently insurmountable.