The term key in the context of encryption
In the context discussed here, a key is a generated bit combination with which a ciphertext is generated from a plaintext and with which the plaintext is recovered from the ciphertext on the receiving side. The key is generated by means of mathematical procedures or random procedures involving prime numbers. The key length is decisive for the security of the transmitted data.
Depending on the encryption method, it can be a public or private key. The public key is the publicly known part of an asymmetric key pair. It is contained in the certificate and can be used by all participants of the security infrastructure (PKI). Only by using the private key can the key holder decrypt the data. The private key, on the other hand, is the secret part of the asymmetric key pair.
The term key in the context of databases
A key is used to uniquely identify a record (tuple) in a relational database - each relation has at least one key. A key is an attribute, or even a minimal conjunction ofattributes, whose values can then be used to uniquely identify tuples. As a consequence, such a key may appear only once in a relation. Since a set of tuples differs at least in the key value, this conversely means that a tuple may only appear once in a relation. Basically, the principle of minimality applies to the formation of keys - i.e. a key must be kept as short as possible. To distinguish keys from other attributes, they are marked in a relation by underlining the key attribute(s)
From the point of view of a table, the columns form a key of the relation if the complete tuple can be uniquely determined with the help of the values of some of the columns of a relation. There is generally a functional dependency between the key of a relation and all other columns of this relation. A functional dependency exists if the values of fields can be uniquely traced back to other fields in a data record. Relational databases often allow tables to be created without keys. However, this is not sufficient for practical relevance, since as soon as identical data records occur in a table, it is then no longer possible to identify them. In the sense of the relational database schema, a tuple cannot belong to a relation more than once
For the treatment of relational database schemas, the following definitions have been formed in connection with the term key:
primary key . Each relation has a primary key that uniquely identifies the tuples in the table. To insert a tuple into a relation, at least the primary key value must be specified. Further attribute values of the tuple do not necessarily have to be known for the creation of the data record, since they are represented by so-called null values. A null value in this context means something like "non-existent" and has no relationship with a numeric 0 value. A primary key should be stable over the life cycle of a table. If a table has no unique attributes - also called data fields - an additional unique ID numbercan be assigned as a primary key, for example
All attributes or combinations of attributes are called key candidates. The values of these attributes must in turn be unique.In
addition, a relation can have any number of additional keys, whereby the primary key and any additional key are mutually associated.
This refers to an attribute in a relation that creates a relationship to a key field of another relation. If a table does not have any unique attributes - also called data fields - a unique ID number can be assigned as a primary key, for example.Index . An index is a relational database tool used to find records more efficiently in search functions. Indexes are created in addition to the primary key to speed up access to the data. Indexes can be created for any column - it makes sense to use them for records that are searched for most often. However, when using multiple indexes, for example when filling a table, a lot of time is needed to update the indexes. In this case, it is more advantageous to update the indexes only after the table has been filled.
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