Is it possible that at least one of them had an exploit in them?
Making all of the transactions/addresses sent during that period vulnerable to attack?
Hi,
To answer the question in the title : since 0.10, bitcoin-core uses its own implementation of secp256k1 :
https://github.com/bitcoin/bitcoin/tree/v0.10.0/src/secp256k1 (BIP66). It previously used OpenSSL.
To answer the question in the post :
Is it possible that at least one of them had an exploit in them?
A bug in OpenSSL which could have caused a consensus failure in Bitcoin is what accelerated the move to using libsecp.
Making all of the transactions/addresses sent during that period vulnerable to attack?
How ?