Tag cleanup and tag wikis before public beta?

Question

I think it would useful to have some cleanup in the existing tags, as well as descriptions (e.g. tag wiki excerpts) for those we want to retain here. (For duplicate or ill-named tags, see The big tag naming discussion.)

(Of course, with public beta coming, there will be more tags created, but we want to set the right examples before that.)

Now (in private beta) the reputation needed to vote to approve/reject tag wiki edits is 750, which only Thomas Pornin has (and it needs more than one vote, if I remember right). In public beta, the reputation need will be even more.
Thus we will need the help of staff moderators (as there are no community moderators right now).

Here is a list of now existing tags, with some categorization by me.

Algorithms, protocols, programs

• rsa, des, aes, playfair, ripemd, sha1, bcrypt, blowfish, ecdsa, (one-time-pad?)
• ssl, https
• gnupg, bitcoin

I think for these there is not much to be done, and not much confusion potential. They can wait a bit for a description.

Algorithm families

• hash, block-cipher, substitution-cipher, stream-cipher, (signature ?)

I think these could really benefit from a good tag wiki excerpt which makes clear what these tags are for. Please make an answer for individual tags (or add your comment to an existing one for this tag), proposing a description.

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What can we do with an algorithm?

• encryption, signature, authentication, random, oblivious-transfer

What is used in an algorithm?

• s-boxes, permutation, xor

How can we attack/break an algorithm?

• cryptanalysis
• side-channel-attacks, timing-attacks
• algebraic-attack, differential (differential-cryptanalyis)

mathematical bases

• hermitian-curves, factorization, algebraic-geometry, elliptic-curves, coding-theory

Other

• passwords, salt, keys, public-private-key-pair, key-size
• asymmetric
• reduction
• statistical-test, test-vectors
• implementation
• two-party-computation, multiparty-computation,
• protocol-idea (I'm guilty of this)
• key-derivation
• post-quantum-cryptography
• nist
• file-encryption
• function-evaluation
• algorithm

Maybe not so useful:

• resources, classroom, literature,
• usability, user-interface
• mobile, cloning

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There might be some tags which will only be useful for off-topic posts - these should be removed. Others might be of doubtful use, or would be better a tag synonym.

Some might be easily confused, where a tag wiki excerpt will help. Please propose these in one answer for each tag.

Please add your comments and answers.

Answer 1

hash

A cryptographic hash algorithm is a function which takes a variable size input and produces a fixed size output. The algorithm makes it difficult to predict the output for a given input, find two inputs with the same output, or reconstruct the input from the output.

Taken from the security.se tag wiki.

Answer 2

encryption

Encryption is the process of transforming plaintext using a cipher to make it unreadable to anyone except those possessing the key.

(Taken from the security.se tag wiki, which in turn adapted the first sentence of the Wikipedia article).