MD5 generates 128bit message digest through the input of message of
arbitrary length. Compared with SHA-1: it has fast calculating speed, but
the security strength is slightly lower.
SHA-1 generates 160bit message digest through the 64 power bit message
with the input length of less than 2.
Compared with MD5: it has lower calculating speed, but the security
strength is higher.
Encryption algorithm:
The encryption algorithm is achieved mainly through the symmetric key system. It uses the same
key to encrypt and decrypt data.
IPSec supports three encryption algorithms:
Use a 64bit key to encrypt a 64bit plaintext block
Use three 64bit DES keys (192bit key) to encrypt the plaintext
Use AES algorithm with the key length of 128bit, 192bit or 256bit to
encrypt the plaintext
The securities of three encryption algorithms in descending order are AES, 3DES and
DES. The encryption algorithm of high security can achieve complex mechanism, but
the arithmetic speed is slow. For ordinary safety requirements, DES algorithm can
meet requirements.
(3) Way of Negotiation
SA can be established through the following two ways of negotiation:
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