PLANET CS-2001 - Manual (Page 249)

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AH ( Authentication Header )
 The Authentication Header guarantees connectionless integrity and data origin
authentication of IP datagrams.
ESP (Encapsulating Security Payload)
 The Encapsulated Security Payload provides confidentiality and integrity
protection to IP datagrams.
DES (Data Encryption Standard)
 The Data Encryption Standard is a NIST standard encryption using 56-bit key.
3DES (Triple-DES)
 Triple DES is a block cipher formed from the Data Encryption Standard (DES)
cipher by using it three times. It can achieve an algorithm up to 168 bits.
AES (Advanced Encryption Standard)
 The Advanced Encryption Standard (AES) is a symmetric key encryption
technique, usually using a 126-bit, 192-bit and 256-bit key. AES is a commonly
seen and adopted nowadays.
NULL Algorithm
 The NULL Algorithm is an instant and convenient alternative for connection. It
is merely a simple replacement for ESP (Encapsulating Security Payload)
without any cryptograph protection.
SHA1 (Secure Hash Algorithm-1)
 The SHA1 is a revision of SHA (Secure Hash Algorithm). It has improved the
shortcomings of SHA. By producing summary hash values, it can achieve an
algorithm up to 160 bits.
MD5 Algorithm
 MD5 (Message Digest Algorithm 5) processes a variable-length message into a
fixed–length output of 128 bits.
GRE/ IPSec
 The GRE (Generic Routing Encapsulation) comes in packet packing function
without any encryption against monitoring and attacking. Normally, the GRE
needs to cooperate with IPSec so as to provide a secure connection.