I think it's relying on libcrypto from openssl, at least that was the case when it came out. E.g. it implements TLS/SSL protocols only, no crypto. Not sure if that's changed and they implemented crypto as well.
From a brief perusal of the source code[0], it still does. It makes this claim on the `README.md` seem rather disingenuous:
> One of the key benefits to s2n is far less code surface, with approximately 6,000 lines of code (compared to OpenSSL’s approximately 500,000 lines).
> One of the key benefits to s2n is far less code surface, with approximately 6,000 lines of code (compared to OpenSSL’s approximately 500,000 lines).
0: https://github.com/awslabs/s2n/blob/master/crypto/s2n_dhe.h#...