Tiny asymmetric cipher implementation to validate download

Question

To allow a small C++ application to update itself at clients connected over the internet, I am in need of a mechanism that validates the download based on a public key. Algorithms such as DSA or RSA seem to be able to do this nicely.

However, looking at well-known available libraries for this (Crypto++, LibTomCrypt) they all end up making my binary > 500k larger, while it seems to me such logic can be implemented in a couple of k. Are there any libraries that implement RSA/DSA hash verification in a, say, <20k footprint?

Answer 1

Since I found no libraries that fitted my specific need, I whipped up my own library for this: http://github.com/paiq/dsa_verify. The current implementation has a ~50k footprint of program memory, mainly due to the included bignum math lib, but future versions may be stripped even more.

Answer 2

Do you really need ciphers ? Generally, to validate a download, you can use a hash function like MD5 or SHA. Maybe you can find a small library using these.

Either way, you can try the openssl library. However the .a on my machine is about 400K and 250K stripped.