net.schmizz.sshj.transport.mac

Class BaseMAC

java.lang.Object

net.schmizz.sshj.transport.mac.BaseMAC

All Implemented Interfaces:

MAC

Direct Known Subclasses:

HMACMD5, HMACMD596, HMACRIPEMD160, HMACSHA1, HMACSHA196, HMACSHA2256, HMACSHA2512

public class BaseMAC
extends java.lang.Object
implements MAC

Base class for MAC implementations based on the JCE provider.

Constructor Summary

Constructors

Constructor and Description
BaseMAC(java.lang.String algorithm, int bsize, int defbsize)
BaseMAC(java.lang.String algorithm, int bsize, int defbsize, boolean isEtm)

Method Summary

Modifier and Type	Method and Description
byte[]	doFinal()
byte[]	doFinal(byte[] input)
void	doFinal(byte[] buf, int offset)
int	getBlockSize()
void	init(byte[] key)
boolean	isEtm() Indicates that an Encrypt-Then-Mac algorithm was selected.
void	update(byte[] foo)
void	update(byte[] foo, int s, int l)
void	update(long i)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

BaseMAC

public BaseMAC(java.lang.String algorithm,
               int bsize,
               int defbsize)

BaseMAC

public BaseMAC(java.lang.String algorithm,
               int bsize,
               int defbsize,
               boolean isEtm)

Method Detail

doFinal

public byte[] doFinal()

Specified by:

doFinal in interface MAC

doFinal

public byte[] doFinal(byte[] input)

Specified by:

doFinal in interface MAC

doFinal

public void doFinal(byte[] buf,
                    int offset)

Specified by:

doFinal in interface MAC

getBlockSize

public int getBlockSize()

Specified by:

getBlockSize in interface MAC

init

public void init(byte[] key)

Specified by:

init in interface MAC

update

public void update(byte[] foo,
                   int s,
                   int l)

Specified by:

update in interface MAC

update

public void update(byte[] foo)

Specified by:

update in interface MAC

update

public void update(long i)

Specified by:

update in interface MAC

isEtm

public boolean isEtm()

Description copied from interface: MAC

Indicates that an Encrypt-Then-Mac algorithm was selected.

This has the following implementation details. 1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms

OpenSSH supports MAC algorithms, whose names contain "-etm", that perform the calculations in a different order to that defined in RFC 4253. These variants use the so-called "encrypt then MAC" ordering, calculating the MAC over the packet ciphertext rather than the plaintext. This ordering closes a security flaw in the SSH transport protocol, where decryption of unauthenticated ciphertext provided a "decryption oracle" that could, in conjunction with cipher flaws, reveal session plaintext.

Specifically, the "-etm" MAC algorithms modify the transport protocol to calculate the MAC over the packet ciphertext and to send the packet length unencrypted. This is necessary for the transport to obtain the length of the packet and location of the MAC tag so that it may be verified without decrypting unauthenticated data.

As such, the MAC covers:

mac = MAC(key, sequence_number || packet_length || encrypted_packet)

where "packet_length" is encoded as a uint32 and "encrypted_packet" contains:

byte padding_length byte[n1] payload; n1 = packet_length - padding_length - 1 byte[n2] random padding; n2 = padding_length

Specified by:

isEtm in interface MAC

Returns:

Whether the MAC algorithm is an Encrypt-Then-Mac algorithm