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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?

DoaJC_Blogger
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  • [You can't get smaller than the key size using RSA](https://crypto.stackexchange.com/q/3505/5122). 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)?](https://superuser.com/q/1244070/102155) which mostly also applies to signatures. – Jens Erat Oct 31 '17 at 17:24
  • Can El Gamal encrypt something to be smaller than the key size? – DoaJC_Blogger Nov 01 '17 at 02:35
  • 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? – DoaJC_Blogger Nov 01 '17 at 14:04
  • 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 Nov 01 '17 at 15:19
  • 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? – DoaJC_Blogger Nov 01 '17 at 19:03
  • 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](https://en.wikipedia.org/wiki/Hybrid_cryptosystem). – Jens Erat Nov 05 '17 at 08:46

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