Libgcrypt: How to implement ECIES encryption (ECC + AES)

[Stephan Mueller]

Am Montag, 12. November 2018, 13:24:01 CET schrieb Yann Garcia:

Hi Yann,

> Dear All,
> 
> I want to implement ECIES encryption. To do it, I have to 1) generate ECDH
> secret key (the key agreement) and then 2) derive it using Key Derivation
> Function 2 (KDF2).
> 
> 1) For the key agreement, I need to generate a random number.

You need to have the local private ECDH key and the remote public ECDH key. A 
plain random number does not work as you need to send the local public ECDH 
key to your recipient as otherwise he cannot decrypt it.

> Can you
> indicate me the best way to generate this random number because in the
> help, clause 9.2 Retrieving random numbers, I found 3 different methods and
> I don't know which one is the best for my purpose?
> 
> 2) Is it correct to use the function gcry_kdf_derive() with GCRY_KDF_PBKDF2
> flag to do the Key Derivation Function 2?

Nope, PBKDF is not allowed. Key-based KDFs like SP800-108 or RFC5967 are ok. I 
am not sure libgcrypt offers a native implementation of those.
> 
> 3) Does the libgcrypt provides samples for ECIES encryption?
> 
> Many thanks in advance for your help,
> 
> Best regards,
> Yann Garcia
> Senior Software Engineer
> Microsoft MCAD.net Certified
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Ciao
Stephan