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copied!<blockquote> <p>First off, I would like to ask if any of you know of an encryption algorithm that uses a key to encrypt the data, but no key to decrypt the data.</p> </blockquote> <p>No. There are algorithms that use a different key to decrypt than to encrypt, but a keyless method would rely on secrecy of the algorithm, generally regarded as a poor idea.</p> <blockquote> <p>My final question is, say you have access to the plain text data before it is encrypted, the key used to encrypt the plain text data, and the resulting encrypted data, would figuring out which algorithm used to encrypt the data be feasible?</p> </blockquote> <p>Most likely yes, especially given the key. A good crypto algorithm relies on the secrecy of the key, and the key alone. See <a href="http://en.wikipedia.org/wiki/Kerckhoffs%27_principle" rel="nofollow noreferrer">kerckhoff's principle</a>.</p> <p>Also if a common algorithm is used it would be a simple matter of trial and error, and besides cryptotext often is accompanied by metadata which tells you algorithm details.</p> <p><em>edit: as per comments, you may be thinking of <a href="http://en.wikipedia.org/wiki/Digital_signature" rel="nofollow noreferrer">digital signature</a> (which requires a secret only on the sender side), a <a href="http://en.wikipedia.org/wiki/Cryptographic_hash_function" rel="nofollow noreferrer">hash algorithm</a> (which requires no key but isn't encryption), or a <a href="http://en.wikipedia.org/wiki/Zero_knowledge_proof" rel="nofollow noreferrer">zero-knowledge proof</a> (which can prove knowledge of a secret without revealing it).</em></p>

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