Symmetric-key algorithms are a class of algorithms for cryptography that use the same cryptographic keys for both encryption of plaintext (sender) and decryption of ciphertext (receiver). The keys may be identical, in practice, they represent a shared secret between two or more parties that can be used to maintain a private information link.
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. This is also known as secret key encryption. In symmetric key cryptography, each end of the conversation must have the same key or they cannot decrypt the message sent to them by the other party.
Symmetric key crypto is very fast but more difficult to manage due to the need to distribute the key in a secure means to all parts needing to decrypt the data. There is no key management built within Symmetric crypto.
PKI provides CIA - Confidentiality (Through encryption) Integrity (By guaranteeing that the message hasn't change in transit) and Authentication (Non-repudiation). Symmetric key crypto provides mostly Confidentiality.
The following answers are incorrect:
- PKI - Public Key Infrastructure: This is the opposite of symmetric key crypto. Each side in PKI has their own private key and public key. What one key encrypt the other one can decrypt. You make use of the receiver public key to communicate securely with a remote user. The receiver will use their matching private key to decrypt the data.
- Diffie-Hellman: Sorry, this is an asymmetric key technique. It is used for key agreement over an insecure network such as the Internet. It allows two parties who has never met to negotiate a secret key over an insecure network while preventing Man-In-The-Middle (MITM) attacks.
- DSS - Digital Signature Standard: Sorry, this is an asymmetric key technique.
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http://en.wikipedia.org/wiki/Symmetric-key_algorithm
