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6.3. Encryption

6.3. Encryption​

An encryption algorithm and a key will be negotiated during the key exchange. When encryption is in effect, the packet length, padding length, payload, and padding fields of each packet MUST be encrypted with the given algorithm.

The encrypted data in all packets sent in one direction SHOULD be considered a single data stream. For example, initialization vectors SHOULD be passed from the end of one packet to the beginning of the next packet. All ciphers SHOULD use keys with an effective key length of 128 bits or more.

The ciphers in each direction MUST run independently of each other. Implementations MUST allow the algorithm for each direction to be independently selected, if multiple algorithms are allowed by local policy. In practice however, it is RECOMMENDED that the same algorithm be used in both directions.

The following ciphers are currently defined:

3des-cbc         REQUIRED          three-key 3DES in CBC mode
blowfish-cbc     OPTIONAL          Blowfish in CBC mode
twofish256-cbc   OPTIONAL          Twofish in CBC mode, with a 256-bit key
twofish-cbc      OPTIONAL          alias for "twofish256-cbc" (retained for historical reasons)
twofish192-cbc   OPTIONAL          Twofish with a 192-bit key
twofish128-cbc   OPTIONAL          Twofish with a 128-bit key
aes256-cbc       OPTIONAL          AES in CBC mode, with a 256-bit key
aes192-cbc       OPTIONAL          AES with a 192-bit key
aes128-cbc       RECOMMENDED       AES with a 128-bit key
serpent256-cbc   OPTIONAL          Serpent in CBC mode, with a 256-bit key
serpent192-cbc   OPTIONAL          Serpent with a 192-bit key
serpent128-cbc   OPTIONAL          Serpent with a 128-bit key
arcfour          OPTIONAL          the ARCFOUR stream cipher with a 128-bit key
idea-cbc         OPTIONAL          IDEA in CBC mode
cast128-cbc      OPTIONAL          CAST-128 in CBC mode
none             OPTIONAL          no encryption; NOT RECOMMENDED

Detailed specifications for each cipher can be found in the referenced standard documents.

Additional methods may be defined as specified in [SSH-ARCH] and in [SSH-NUMBERS].

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