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How small can a detached PGP signature be? The only size I get is 287 bytes for a 2048-bit key and 543 bytes for a 4096-bit key, regardless of the size of the data being signed. However, on this post from 2004, the author says he can get a 65-byte detached signature. How is that possible, and is there still a way to do it?
H. C. Barton
Posted 2017-10-31T14:51:28.990
Reputation: 98
You can't get smaller than the key size using RSA. Some additional overhead for metadata and headers will be added, I recently provided a similar discussion in How to use GnuPG to get the smallest possible output (symmetric encryption)? which mostly also applies to signatures. – Jens Erat – 2017-10-31T17:24:47.550
Can El Gamal encrypt something to be smaller than the key size? – H. C. Barton – 2017-11-01T02:35:26.117
I got a digital signature down to 96 bytes. But when I'm encrypting, is it not possible to shrink the output without using symmetric encryption? – H. C. Barton – 2017-11-01T14:04:59.517
What do you mean? OpenPGP is a hybrid cryptosystem, which always encrypts data with symmetric encryption and only uses public/private key cryptography to encrypt the symmetric session key. – Jens Erat – 2017-11-01T15:19:43.813
So you're saying that the reason PGP encryption of 1 byte is big (>500 bytes) is because it has to embed a public key? – H. C. Barton – 2017-11-01T19:03:21.957
No, it does not embed a public key (but usually it includes a reference to a public key, the key's fingerprint). It embed a cipher block for symmetric encryption. I'd recommend reading the Wikipedia article on hybrid cryptosystems.
– Jens Erat – 2017-11-05T08:46:04.827