attack in conjunction with a traffic confirmation attack was launched against the Tor anonymity network for several months in 2014. There are other examples of Sybil attacks run against Tor network users. This includes the 2020 Bitcoin address rewrite attacks. The attacker controlled a quarter of all Tor exit relays and employed SSL stripping to downgrade secure connections and divert funds to the wallet of the threat actor known as BTCMITM20. Another notable example is the 2017–2021 attack run by threat actor KAX17. This entity controlled over 900 malicious servers, primarily middle points, in an attempt to deanonymize Tor users. Prevention Known approaches to Sybil attack prevention include identity validation, social trust graph algorithms, or economic costs, personhood validation, and application-specific defenses. Identity validation Validation techniques can be used to prevent Sybil attacks and dismiss masquerading hostile entities. A local entity may accept a remote identity based on a central authority which ensures a one-to-one correspondence between an identity and an entity and may even provide a reverse lookup. An identity may be validated either directly or indirectly. In direct validation the local entity queries the central authority to validate the remote identities. In indirect validation the local entity relies on already-accepted identities which in turn vouch for the validity of the remote identity in question. Practical network applications and services often use a variety of identity proxies to achieve limited Sybil attack resistance, such as telephone number verification, credit card verification, or even based on the IP address of a client. These methods have the limitations that it is usually possible to obtain multiple such identity proxies at some cost – or even to obtain many at low cost through techniques such as SMS spoofing or IP address spoofing. Use of such identity proxies can also exclude those without ready access to the required identity proxy: e.g., those without their own mobile phone or credit card, or users located behind carrier-grade network address translation who share their IP addresses with many others. Identity-based validation techniques generally provide accountability at the expense of anonymity, which can be an undesirable tradeoff especially in online forums that wish to permit censorship-free information exchange and open discussion of sensitive topics. A validation authority can attempt to preserve users' anonymity by refusing to perform reverse lookups, but this approach makes the validation authority a prime target for attack. Protocols using threshold cryptography can potentially