Take the 2-minute tour ×
Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's 100% free, no registration required.

How secure is libgcrypt's Elgamal implementation of encryption (how different it is from textbook Elgamal), and how can I tweak padding and other preprocessing actions?

For example, I know that usign RSA I can use this S-expression to the function gcry_pk_encrypt:

(data
  (flags pkcs1)
  (value BLOCK))

And I can use oaep or pss instead of "pkcs1". But what do I use with Elgamal? I can encrypt and decrypt using Elgamal, using

(data
  (value BLOCK))

but then I have no idea if raw Elgamal was used or if it's actually DHAES or some padded-Elgamal variant.

What is the proper way to use Elgamal in libgcrypt, and what options can I pass? Or would it be better to switch to RSA?

edit: I think that what I wanted to ask is "what is the intended use of Elgamal as implemented in libgcrypt?"

share|improve this question
2  
Unless you have a specific need for Elgamal, I'd suggest using RSA. –  mikeazo Jul 23 '12 at 1:57

1 Answer 1

up vote 3 down vote accepted

I took the world's shortest look at the code, so please don't hold me to any of this. But it seems that the implementation is textbook. You provide a block of data, it gets encoded to an integer in the range 1,...,p-1 and then encrypted using standard Elgamal without padding.

There's does not seem** to be an encoding to the subgroup of quadratic residues, and this implementation is /not/ CCA secure. These probably aren't going to kill you if you're encrypting a large random key, but they're definitely not the right way to encrypt.

Rather than hacking Elgamal, I'd say you're better off encrypting with RSA-OAEP.

In the long term, libgcrypt should implement a secure KEM/DEM for Elgamal encryption. DHAES would be a great example. Alternatively the authors should give a proper (non-textbook) Elgamal implementation with a group encoding and protections against adaptive-chosen ciphertext attacks.

(Elgamal seems to be the forgotten primitive. pycrypto has the exact same problems. Nobody loves Elgamal, which is too bad because it's a great encryption scheme.)

** A footnote: I didn't look deeply into the encoding function (below), so I admit I could be wrong about this point. But I don't see anything obvious. Here's the code:

init_encoding_ctx (&ctx, PUBKEY_OP_ENCRYPT, gcry_pk_get_nbits (s_pkey)); rc = sexp_data_to_mpi (s_data, &data, &ctx);

share|improve this answer
    
Thank you! This was really useful. –  Jay Jul 30 '12 at 10:54

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.