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> Chapter 14 - IPsec VPN > IPsec VPN concepts > Encryption


Encryption mathematically transforms data to appear as meaningless random numbers. The original data is called plaintext and the encrypted data is called ciphertext. The opposite process, called decryption, performs the inverse operation to recover the original plaintext from the ciphertext.

The process by which the plaintext is transformed to ciphertext and back again is called an algorithm. All algorithms use a small piece of information, a key, in the arithmetic process of converted plaintext to ciphertext, or vice-versa. IPsec uses symmetrical algorithms, in which the same key is used to both encrypt and decrypt the data. The security of an encryption algorithm is determined by the length of the key that it uses. FortiGate IPsec VPNs offer the following encryption algorithms, in descending order of security:

AES256 A 128-bit block algorithm that uses a 256-bit key.
AES192 A 128-bit block algorithm that uses a 192-bit key.
AES128 A 128-bit block algorithm that uses a 128-bit key.
3DES Triple-DES, in which plain text is DES-encrypted three times by three keys.
DES Digital Encryption Standard, a 64-bit block algorithm that uses a 56-bit key

The default encryption algorithms provided on FortiGate units make recovery of encrypted data almost impossible without the proper encryption keys.

There is a human factor in the security of encryption. The key must be kept secret, known only to the sender and receiver of the messages. Also, the key must not be something that unauthorized parties might easily guess, such as the sender’s name, birthday or simple sequence such as 123456.

IPsec overheads

The FortiGate sets an IPsec tunnel Maximum Transmission Unit (MTU) of 1436 for 3DES/SHA1 and an MTU of 1412 for AES128/SHA1, as seen with diag vpn tunnel list. This indicates that the FortiGate allocates 64 bytes of overhead for 3DES/SHA1 and 88 bytes for AES128/SHA1, which is the difference if you subtract this MTU from a typical ethernet MTU of 1500 bytes.

During the encryption process, AES/DES operates using a specific size of data which is block size. If data is smaller than that, it will be padded for the operation. MD5/SHA-1 HMAC also operates using a specific block size.

The following table describes the potential maximum overhead for each IPsec encryption:

IPsec Transform Set IPsec Overhead (Max. bytes)
ESP-AES (256, 192, or 128),ESP-SHA-HMAC, or MD5 73
ESP-AES (256, 192, or 128) 61
ESP-(DES or 3DES), ESP-SHA-HMAC, or MD5 57
ESP-Null, ESP-SHA-HMAC, or MD5 45