The history of cryptography provides evidence that it can be difficult to keep the details of a widely used algorithm secret (see security through obscurity). A key is often easier to protect (it's typically a small piece of information) than an encryption algorithm, and easier to change if compromised.
Symmetric-key algorithms[a] are algorithms for cryptography that use the same cryptographic keys for both encryption of plaintext and decryption of ciphertext. The keys may be identical or there may be a simple transformation to go between the two keys.[1] The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link.[2] This requirement that both parties have access to the secret key is one of the main drawbacks of symmetric key encryption, in comparison to public-key encryption (also known as asymmetric key encryption).[3][4]
Types[edit]
Symmetric-key encryption can use either stream ciphers or block ciphers.[5]
Implementations[edit]
Examples of popular symmetric-key algorithms include Twofish, Serpent, AES (Rijndael), Blowfish, CAST5, Kuznyechik, RC4, DES, 3DES, Skipjack, Safer+/++ (Bluetooth), and IDEA.[6]
Cryptographic primitives based on symmetric ciphers[edit]
Symmetric ciphers are commonly used to achieve other cryptographic primitives than just encryption.[citation needed]
Encrypting a message does not guarantee that this message is not changed while encrypted. Hence often a message authentication code is added to a ciphertext to ensure that changes to the ciphertext will be noted by the receiver. Message authentication codes can be constructed from symmetric ciphers (e.g. CBC-MAC).[citation needed]
However, symmetric ciphers cannot be used for non-repudiation purposes except by involving additional parties.[7] See the ISO/IEC 13888-2 standard.
Another application is to build hash functions from block ciphers. See one-way compression function for descriptions of several such methods.
Construction of symmetric ciphers[edit]
Many modern block ciphers are based on a construction proposed by Horst Feistel. Feistel's construction makes it possible to build invertible functions from other functions that are themselves not invertible.[citation needed]
Security of symmetric ciphers[edit]
Symmetric ciphers have historically been susceptible to known-plaintext attacks, chosen-plaintext attacks, differential cryptanalysis and linear cryptanalysis. Careful construction of the functions for each round can greatly reduce the chances of a successful attack.[citation needed]
Key management[edit]Key establishment[edit]
Symmetric-key algorithms require both the sender and the recipient of a message to have the same secret key.All early cryptographic systems required one of those people to somehow receive a copy of that secret key over a physically secure channel.
Nearly all modern cryptographic systems still use symmetric-key algorithms internally to encrypt the bulk of the messages, but they eliminate the need for a physically secure channel by using DiffieâHellman key exchange or some other public-key protocol to securely come to agreement on a fresh new secret key for each message (forward secrecy).
Key generation[edit]
When used with asymmetric ciphers for key transfer, pseudorandom key generators are nearly always used to generate the symmetric cipher session keys. However, lack of randomness in those generators or in their initialization vectors is disastrous and has led to cryptanalytic breaks in the past. Therefore, it is essential that an implementation use a source of high entropy for its initialization.[8][9][10]
Reciprocal cipher[edit]
A reciprocal cipher is a cipher where, just as one enters the plaintext into the cryptography system to get the ciphertext, one could enter the ciphertext into the same place in the system to get the plaintext. A reciprocal cipher is also sometimes referred as self-reciprocal cipher.
Practically all mechanical cipher machines implement a reciprocal cipher, a mathematical involution on each typed-in letter.Instead of designing two kinds of machines, one for encrypting and one for decrypting, all the machines can be identical and can be set up (keyed) the same way.[11]
Examples of reciprocal ciphers include:
Practically all modern ciphers can be classified as either a stream cipher, most of which use a reciprocol XOR cipher combiner, or a block cipher, most of which use use Feistel cipher or LaiâMassey scheme with a reciprocal transformation in each round.
Notes[edit]
References[edit]Asa Generating Secret Keys Unknown Encryption Algorithm Download
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Symmetric-key_algorithm&oldid=948081123'
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By default, user passwords are encrypted with the crypt_unix algorithm. In the Solaris 9 12/02 release, you can use a stronger encryption algorithm, such as MD5 or Blowfish, by changing the default password encryption algorithm. The next time that your users change their password,the algorithm that you specified encrypts the password.
Note â
The following procedures do not work if you are running a Solaris environment from an earlier release. This functionality works only on machines that are running the Solaris 9 12/02 release and later releases of the Solaris operating environment.
How to Specify an Algorithm for Password Encryption
In this procedure, the BSD-Linux version of the MD5 algorithm is the default encryption algorithm that is used when users change their passwords. This algorithm is suitable for a mixed networkof machines that run the Solaris, BSD, and Linux versions of UNIX. See Table 2â1 for a list of password encryption algorithms and algorithm identifiers.
Asa Generating Secret Keys Unknown Encryption Algorithm FreeExampleâUsing the Blowfish Algorithm for Password EncryptionIn this example, the identifier for the Blowfish algorithm, 2a, is specified as the value for the CRYPT_DEFAULTvariable. The policy.conf entries that control password encryption would look like the following:
This configuration is compatible with BSD systems that use the Blowfish algorithm.
How to Specify a New Password Algorithm for an NIS+ Domain
When users in a NIS+ domain change their passwords, the NIS+ name service consults the algorithms configuration in the /etc/security/policy.conf file on the NIS+ master. The NIS+ master, which is running the rpc.nispasswd daemon, creates the encrypted password.
How to Specify a New Password Algorithm for an NIS Domain
When users in an NIS domain change their passwords, the NIS client consults its local algorithms configuration in the /etc/security/policy.conf file. The NIS client machine encrypts the password.
List Of Encryption AlgorithmsHow to Specify a New Password Algorithm for an LDAP Domain
When the LDAP client is properly configured, the LDAP client can use the new password algorithms. The LDAP client behaves just as a NIS clientbehaves.
The PAM entries in the client's pam.conf file enable the password to be encrypted according to the local algorithms configuration, and enable the password to be authenticated.
When users in the LDAP domain change their passwords, the LDAP client consults its local algorithms configuration in the /etc/security/policy.conf file. The LDAP client machine encrypts the password. The client sends the encrypted password, with a {crypt}tag, to the server. The tag tells the server that the password is already encrypted. The password is then stored, as is, on the server. For authentication, the client retrieves the stored password from the server. The client then compares the stored password with the encrypted version that the clienthas just generated from the user's typed password.
Note â
To take advantage of password policy controls on the LDAP server, use the server_policy option with the pam_authtok_store entries in the pam.conf file. Passwords are then encrypted on the server by using the Sun ONE Directory Server'scryptographic mechanism. For the procedure, see âSetting Up Clients (Task)â in System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP).
How to Install a Password Encryption Module From a Third Party
A third-party password encryption algorithm is typically delivered as part of a software package. When you run the pkgadd command, scripts from the vendor should modify the /etc/security/crypt.conf file. You then modify the /etc/security/policy.conf file to include the new module and its identifier.
In this example, the rot13 algorithm is used if the current password was encrypted with the crypt_rot13 algorithm. New user passwords are encrypted with the crypt_sunmd5 algorithm. This algorithms configuration works on Solaris-only networks.
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