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authorDamien Miller <>1999-10-27 13:42:43 +1000
committerDamien Miller <>1999-10-27 13:42:43 +1000
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+.\" -*- nroff -*-
+.\" $Id: RFC.nroff,v 1.1 1999/10/27 03:42:43 damien Exp $
+.\" 10.0i
+.po 0
+.ll 7.2i 7.2i LL 7.2i LT 7.2i
+.ds LF Ylonen
+.ds RF FORMFEED[Page %]
+.ds CF
+.ds LH Internet-Draft
+.ds RH 15 November 1995
+.ds CH SSH (Secure Shell) Remote Login Protocol
+.hy 0 0
+Network Working Group T. Ylonen
+Internet-Draft Helsinki University of Technology
+draft-ylonen-ssh-protocol-00.txt 15 November 1995
+Expires: 15 May 1996
+ 3
+The SSH (Secure Shell) Remote Login Protocol
+.ti 0
+Status of This Memo
+This document is an Internet-Draft. Internet-Drafts are working
+documents of the Internet Engineering Task Force (IETF), its areas,
+and its working groups. Note that other groups may also distribute
+working documents as Internet-Drafts.
+Internet-Drafts are draft documents valid for a maximum of six
+months and may be updated, replaced, or obsoleted by other docu-
+ments at any time. It is inappropriate to use Internet-Drafts as
+reference material or to cite them other than as ``work in pro-
+To learn the current status of any Internet-Draft, please check the
+``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow
+Directories on (Africa), (Europe), (Pacific Rim), (US East Coast), or (US West Coast).
+The distribution of this memo is unlimited.
+.ti 0
+SSH (Secure Shell) is a program to log into another computer over a
+network, to execute commands in a remote machine, and to move files
+from one machine to another. It provides strong authentication and
+secure communications over insecure networks. Its features include
+the following:
+.IP o
+Closes several security holes (e.g., IP, routing, and DNS spoofing).
+New authentication methods: .rhosts together with RSA [RSA] based host
+authentication, and pure RSA authentication.
+.IP o
+All communications are automatically and transparently encrypted.
+Encryption is also used to protect integrity.
+.IP o
+X11 connection forwarding provides secure X11 sessions.
+.IP o
+Arbitrary TCP/IP ports can be redirected over the encrypted channel
+in both directions.
+.IP o
+Client RSA-authenticates the server machine in the beginning of every
+connection to prevent trojan horses (by routing or DNS spoofing) and
+man-in-the-middle attacks, and the server RSA-authenticates the client
+machine before accepting .rhosts or /etc/hosts.equiv authentication
+(to prevent DNS, routing, or IP spoofing).
+.IP o
+An authentication agent, running in the user's local workstation or
+laptop, can be used to hold the user's RSA authentication keys.
+The goal has been to make the software as easy to use as possible for
+ordinary users. The protocol has been designed to be as secure as
+possible while making it possible to create implementations that
+are easy to use and install. The sample implementation has a number
+of convenient features that are not described in this document as they
+are not relevant for the protocol.
+.ti 0
+Overview of the Protocol
+The software consists of a server program running on a server machine,
+and a client program running on a client machine (plus a few auxiliary
+programs). The machines are connected by an insecure IP [RFC0791]
+network (that can be monitored, tampered with, and spoofed by hostile
+A connection is always initiated by the client side. The server
+listens on a specific port waiting for connections. Many clients may
+connect to the same server machine.
+The client and the server are connected via a TCP/IP [RFC0793] socket
+that is used for bidirectional communication. Other types of
+transport can be used but are currently not defined.
+When the client connects the server, the server accepts the connection
+and responds by sending back its version identification string. The
+client parses the server's identification, and sends its own
+identification. The purpose of the identification strings is to
+validate that the connection was to the correct port, declare the
+protocol version number used, and to declare the software version used
+on each side (for debugging purposes). The identification strings are
+human-readable. If either side fails to understand or support the
+other side's version, it closes the connection.
+After the protocol identification phase, both sides switch to a packet
+based binary protocol. The server starts by sending its host key
+(every host has an RSA key used to authenticate the host), server key
+(an RSA key regenerated every hour), and other information to the
+client. The client then generates a 256 bit session key, encrypts it
+using both RSA keys (see below for details), and sends the encrypted
+session key and selected cipher type to the server. Both sides then
+turn on encryption using the selected algorithm and key. The server
+sends an encrypted confirmation message to the client.
+The client then authenticates itself using any of a number of
+authentication methods. The currently supported authentication
+methods are .rhosts or /etc/hosts.equiv authentication (disabled by
+default), the same with RSA-based host authentication, RSA
+authentication, and password authentication.
+After successful authentication, the client makes a number of requests
+to prepare for the session. Typical requests include allocating a
+pseudo tty, starting X11 [X11] or TCP/IP port forwarding, starting
+authentication agent forwarding, and executing the shell or a command.
+When a shell or command is executed, the connection enters interactive
+session mode. In this mode, data is passed in both directions,
+new forwarded connections may be opened, etc. The interactive session
+normally terminates when the server sends the exit status of the
+program to the client.
+The protocol makes several reservations for future extensibility.
+First of all, the initial protocol identification messages include the
+protocol version number. Second, the first packet by both sides
+includes a protocol flags field, which can be used to agree on
+extensions in a compatible manner. Third, the authentication and
+session preparation phases work so that the client sends requests to
+the server, and the server responds with success or failure. If the
+client sends a request that the server does not support, the server
+simply returns failure for it. This permits compatible addition of
+new authentication methods and preparation operations. The
+interactive session phase, on the other hand, works asynchronously and
+does not permit the use of any extensions (because there is no easy
+and reliable way to signal rejection to the other side and problems
+would be hard to debug). Any compatible extensions to this phase must
+be agreed upon during any of the earlier phases.
+.ti 0
+The Binary Packet Protocol
+After the protocol identification strings, both sides only send
+specially formatted packets. The packet layout is as follows:
+.IP o
+Packet length: 32 bit unsigned integer, coded as four 8-bit bytes, msb
+first. Gives the length of the packet, not including the length field
+and padding. The maximum length of a packet (not including the length
+field and padding) is 262144 bytes.
+.IP o
+Padding: 1-8 bytes of random data (or zeroes if not encrypting). The
+amount of padding is (8 - (length % 8)) bytes (where % stands for the
+modulo operator). The rationale for always having some random padding
+at the beginning of each packet is to make known plaintext attacks
+more difficult.
+.IP o
+Packet type: 8-bit unsigned byte. The value 255 is reserved for
+future extension.
+.IP o
+Data: binary data bytes, depending on the packet type. The number of
+data bytes is the "length" field minus 5.
+.IP o
+Check bytes: 32-bit crc, four 8-bit bytes, msb first. The crc is the
+Cyclic Redundancy Check, with the polynomial 0xedb88320, of the
+Padding, Packet type, and Data fields. The crc is computed before
+any encryption.
+The packet, except for the length field, may be encrypted using any of
+a number of algorithms. The length of the encrypted part (Padding +
+Type + Data + Check) is always a multiple of 8 bytes. Typically the
+cipher is used in a chained mode, with all packets chained together as
+if it was a single data stream (the length field is never included in
+the encryption process). Details of encryption are described below.
+When the session starts, encryption is turned off. Encryption is
+enabled after the client has sent the session key. The encryption
+algorithm to use is selected by the client.
+.ti 0
+Packet Compression
+If compression is supported (it is an optional feature, see
+SSH_CMSG_REQUEST_COMPRESSION below), the packet type and data fields
+of the packet are compressed using the gzip deflate algorithm [GZIP].
+If compression is in effect, the packet length field indicates the
+length of the compressed data, plus 4 for the crc. The amount of
+padding is computed from the compressed data, so that the amount of
+data to be encrypted becomes a multiple of 8 bytes.
+When compressing, the packets (type + data portions) in each direction
+are compressed as if they formed a continuous data stream, with only the
+current compression block flushed between packets. This corresponds
+to the GNU ZLIB library Z_PARTIAL_FLUSH option. The compression
+dictionary is not flushed between packets. The two directions are
+compressed independently of each other.
+.ti 0
+Packet Encryption
+The protocol supports several encryption methods. During session
+initialization, the server sends a bitmask of all encryption methods
+that it supports, and the client selects one of these methods. The
+client also generates a 256-bit random session key (32 8-bit bytes) and
+sends it to the server.
+The encryption methods supported by the current implementation, and
+their codes are:
+l r l.
+SSH_CIPHER_NONE 0 No encryption
+SSH_CIPHER_3DES 3 Triple-DES in CBC mode
+SSH_CIPHER_TSS 4 An experimental stream cipher
+All implementations are required to support SSH_CIPHER_DES and
+SSH_CIPHER_NONE is recommended. Support for SSH_CIPHER_TSS is
+optional (and it is not described in this document). Other ciphers
+may be added at a later time; support for them is optional.
+For encryption, the encrypted portion of the packet is considered a
+linear byte stream. The length of the stream is always a multiple of
+8. The encrypted portions of consecutive packets (in the same
+direction) are encrypted as if they were a continuous buffer (that is,
+any initialization vectors are passed from the previous packet to the
+next packet). Data in each direction is encrypted independently.
+The key is taken from the first 8 bytes of the session key. The least
+significant bit of each byte is ignored. This results in 56 bits of
+key data. DES [DES] is used in CBC mode. The iv (initialization vector) is
+initialized to all zeroes.
+The variant of triple-DES used here works as follows: there are three
+independent DES-CBC ciphers, with independent initialization vectors.
+The data (the whole encrypted data stream) is first encrypted with the
+first cipher, then decrypted with the second cipher, and finally
+encrypted with the third cipher. All these operations are performed
+in CBC mode.
+The key for the first cipher is taken from the first 8 bytes of the
+session key; the key for the next cipher from the next 8 bytes, and
+the key for the third cipher from the following 8 bytes. All three
+initialization vectors are initialized to zero.
+(Note: the variant of 3DES used here differs from some other
+The key is taken from the first 16 bytes of the session key. IDEA
+[IDEA] is used in CFB mode. The initialization vector is initialized
+to all zeroes.
+All 32 bytes of the session key are used as the key.
+There is no reference available for the TSS algorithm; it is currently
+only documented in the sample implementation source code. The
+security of this cipher is unknown (but it is quite fast). The cipher
+is basically a stream cipher that uses MD5 as a random number
+generator and takes feedback from the data.
+The first 16 bytes of the session key are used as the key for the
+server to client direction. The remaining 16 bytes are used as the
+key for the client to server direction. This gives independent
+128-bit keys for each direction.
+This algorithm is the alleged RC4 cipher posted to the Usenet in 1995.
+It is widely believed to be equivalent with the original RSADSI RC4
+cipher. This is a very fast algorithm.
+.ti 0
+Data Type Encodings
+The Data field of each packet contains data encoded as described in
+this section. There may be several data items; each item is coded as
+described here, and their representations are concatenated together
+(without any alignment or padding).
+Each data type is stored as follows:
+.IP "8-bit byte"
+The byte is stored directly as a single byte.
+.IP "32-bit unsigned integer"
+Stored in 4 bytes, msb first.
+.IP "Arbitrary length binary string"
+First 4 bytes are the length of the string, msb first (not including
+the length itself). The following "length" bytes are the string
+value. There are no terminating null characters.
+.IP "Multiple-precision integer"
+First 2 bytes are the number of bits in the integer, msb first (for
+example, the value 0x00012345 would have 17 bits). The value zero has
+zero bits. It is permissible that the number of bits be larger than the
+real number of bits.
+The number of bits is followed by (bits + 7) / 8 bytes of binary data,
+msb first, giving the value of the integer.
+.ti 0
+TCP/IP Port Number and Other Options
+The server listens for connections on TCP/IP port 22.
+The client may connect the server from any port. However, if the
+client wishes to use any form of .rhosts or /etc/hosts.equiv
+authentication, it must connect from a privileged port (less than
+For the IP Type of Service field [RFC0791], it is recommended that
+interactive sessions (those having a user terminal or forwarding X11
+connections) use the IPTOS_LOWDELAY, and non-interactive connections
+It is recommended that keepalives are used, because otherwise programs
+on the server may never notice if the other end of the connection is
+.ti 0
+Protocol Version Identification
+After the socket is opened, the server sends an identification string,
+which is of the form
+"SSH-<protocolmajor>.<protocolminor>-<version>\\n", where
+<protocolmajor> and <protocolminor> are integers and specify the
+protocol version number (not software distribution version).
+<version> is server side software version string (max 40 characters);
+it is not interpreted by the remote side but may be useful for
+The client parses the server's string, and sends a corresponding
+string with its own information in response. If the server has lower
+version number, and the client contains special code to emulate it,
+the client responds with the lower number; otherwise it responds with
+its own number. The server then compares the version number the
+client sent with its own, and determines whether they can work
+together. The server either disconnects, or sends the first packet
+using the binary packet protocol and both sides start working
+according to the lower of the protocol versions.
+By convention, changes which keep the protocol compatible with
+previous versions keep the same major protocol version; changes that
+are not compatible increment the major version (which will hopefully
+never happen). The version described in this document is 1.3.
+The client will
+.ti 0
+Key Exchange and Server Host Authentication
+The first message sent by the server using the packet protocol is
+SSH_SMSG_PUBLIC_KEY. It declares the server's host key, server public
+key, supported ciphers, supported authentication methods, and flags
+for protocol extensions. It also contains a 64-bit random number
+(cookie) that must be returned in the client's reply (to make IP
+spoofing more difficult). No encryption is used for this message.
+Both sides compute a session id as follows. The modulus of the server
+key is interpreted as a byte string (without explicit length field,
+with minimum length able to hold the whole value), most significant
+byte first. This string is concatenated with the server host key
+interpreted the same way. Additionally, the cookie is concatenated
+with this. Both sides compute MD5 of the resulting string. The
+resulting 16 bytes (128 bits) are stored by both parties and are
+called the session id.
+The client responds with a SSH_CMSG_SESSION_KEY message, which
+contains the selected cipher type, a copy of the 64-bit cookie sent by
+the server, client's protocol flags, and a session key encrypted
+with both the server's host key and server key. No encryption is used
+for this message.
+The session key is 32 8-bit bytes (a total of 256 random bits
+generated by the client). The client first xors the 16 bytes of the
+session id with the first 16 bytes of the session key. The resulting
+string is then encrypted using the smaller key (one with smaller
+modulus), and the result is then encrypted using the other key. The
+number of bits in the public modulus of the two keys must differ by at
+least 128 bits.
+At each encryption step, a multiple-precision integer is constructed
+from the data to be encrypted as follows (the integer is here
+interpreted as a sequence of bytes, msb first; the number of bytes is
+the number of bytes needed to represent the modulus).
+The most significant byte (which is only partial as the value must be
+less than the public modulus, which is never a power of two) is zero.
+The next byte contains the value 2 (which stands for public-key
+encrypted data in the PKCS standard [PKCS#1]). Then, there are
+non-zero random bytes to fill any unused space, a zero byte, and the
+data to be encrypted in the least significant bytes, the last byte of
+the data in the least significant byte.
+This algorithm is used twice. First, it is used to encrypt the 32
+random bytes generated by the client to be used as the session key
+(xored by the session id). This value is converted to an integer as
+described above, and encrypted with RSA using the key with the smaller
+modulus. The resulting integer is converted to a byte stream, msb
+first. This byte stream is padded and encrypted identically using the
+key with the larger modulus.
+After the client has sent the session key, it starts to use the
+selected algorithm and key for decrypting any received packets, and
+for encrypting any sent packets. Separate ciphers are used for
+different directions (that is, both directions have separate
+initialization vectors or other state for the ciphers).
+When the server has received the session key message, and has turned
+on encryption, it sends a SSH_SMSG_SUCCESS message to the client.
+The recommended size of the host key is 1024 bits, and 768 bits for
+the server key. The minimum size is 512 bits for the smaller key.
+.ti 0
+Declaring the User Name
+The client then sends a SSH_CMSG_USER message to the server. This
+message specifies the user name to log in as.
+The server validates that such a user exists, checks whether
+authentication is needed, and responds with either SSH_SMSG_SUCCESS or
+SSH_SMSG_FAILURE. SSH_SMSG_SUCCESS indicates that no authentication
+is needed for this user (no password), and authentication phase has
+now been completed. SSH_SMSG_FAILURE indicates that authentication is
+needed (or the user does not exist).
+If the user does not exist, it is recommended that this returns
+failure, but the server keeps reading messages from the client, and
+responds to any messages (except SSH_MSG_DISCONNECT, SSH_MSG_IGNORE,
+and SSH_MSG_DEBUG) with SSH_SMSG_FAILURE. This way the client cannot
+be certain whether the user exists.
+.ti 0
+Authentication Phase
+Provided the server didn't immediately accept the login, an
+authentication exchange begins. The client sends messages to the
+server requesting different types of authentication in arbitrary order as
+many times as desired (however, the server may close the connection
+after a timeout). The server always responds with SSH_SMSG_SUCCESS if
+it has accepted the authentication, and with SSH_SMSG_FAILURE if it has
+denied authentication with the requested method or it does not
+recognize the message. Some authentication methods cause an exchange
+of further messages before the final result is sent. The
+authentication phase ends when the server responds with success.
+The recommended value for the authentication timeout (timeout before
+disconnecting if no successful authentication has been made) is 5
+The following authentication methods are currently supported:
+l r l.
+SSH_AUTH_RHOSTS 1 .rhosts or /etc/hosts.equiv
+SSH_AUTH_RSA 2 pure RSA authentication
+SSH_AUTH_PASSWORD 3 password authentication
+SSH_AUTH_RHOSTS_RSA 4 .rhosts with RSA host authentication
+This is the authentication method used by rlogin and rsh [RFC1282].
+The client sends SSH_CMSG_AUTH_RHOSTS with the client-side user name
+as an argument.
+The server checks whether to permit authentication. On UNIX systems,
+this is usually done by checking /etc/hosts.equiv, and .rhosts in the
+user's home directory. The connection must come from a privileged
+It is recommended that the server checks that there are no IP options
+(such as source routing) specified for the socket before accepting
+this type of authentication. The client host name should be
+reverse-mapped and then forward mapped to ensure that it has the
+proper IP-address.
+This authentication method trusts the remote host (root on the remote
+host can pretend to be any other user on that host), the name
+services, and partially the network: anyone who can see packets coming
+out from the server machine can do IP-spoofing and pretend to be any
+machine; however, the protocol prevents blind IP-spoofing (which used
+to be possible with rlogin).
+Many sites probably want to disable this authentication method because
+of the fundamental insecurity of conventional .rhosts or
+/etc/hosts.equiv authentication when faced with spoofing. It is
+recommended that this method not be supported by the server by
+In addition to conventional .rhosts and hosts.equiv authentication,
+this method additionally requires that the client host be
+authenticated using RSA.
+The client sends SSH_CMSG_AUTH_RHOSTS_RSA specifying the client-side
+user name, and the public host key of the client host.
+The server first checks if normal .rhosts or /etc/hosts.equiv
+authentication would be accepted, and if not, responds with
+SSH_SMSG_FAILURE. Otherwise, it checks whether it knows the host key
+for the client machine (using the same name for the host that was used
+for checking the .rhosts and /etc/hosts.equiv files). If it does not
+know the RSA key for the client, access is denied and SSH_SMSG_FAILURE
+is sent.
+If the server knows the host key of the client machine, it verifies
+that the given host key matches that known for the client. If not,
+access is denied and SSH_SMSG_FAILURE is sent.
+The server then sends a SSH_SMSG_AUTH_RSA_CHALLENGE message containing
+an encrypted challenge for the client. The challenge is 32 8-bit
+random bytes (256 bits). When encrypted, the highest (partial) byte
+is left as zero, the next byte contains the value 2, the following are
+non-zero random bytes, followed by a zero byte, and the challenge put
+in the remaining bytes. This is then encrypted using RSA with the
+client host's public key. (The padding and encryption algorithm is
+the same as that used for the session key.)
+The client decrypts the challenge using its private host key,
+concatenates this with the session id, and computes an MD5 checksum
+of the resulting 48 bytes. The MD5 output is returned as 16 bytes in
+a SSH_CMSG_AUTH_RSA_RESPONSE message. (MD5 is used to deter chosen
+plaintext attacks against RSA; the session id binds it to a specific
+The server verifies that the MD5 of the decrypted challenge returned by
+the client matches that of the original value, and sends SSH_SMSG_SUCCESS if
+so. Otherwise it sends SSH_SMSG_FAILURE and refuses the
+authentication attempt.
+This authentication method trusts the client side machine in that root
+on that machine can pretend to be any user on that machine.
+Additionally, it trusts the client host key. The name and/or IP
+address of the client host is only used to select the public host key.
+The same host name is used when scanning .rhosts or /etc/hosts.equiv
+and when selecting the host key. It would in principle be possible to
+eliminate the host name entirely and substitute it directly by the
+host key. IP and/or DNS [RFC1034] spoofing can only be used
+to pretend to be a host for which the attacker has the private host
+The idea behind RSA authentication is that the server recognizes the
+public key offered by the client, generates a random challenge, and
+encrypts the challenge with the public key. The client must then
+prove that it has the corresponding private key by decrypting the
+The client sends SSH_CMSG_AUTH_RSA with public key modulus (n) as an
+The server may respond immediately with SSH_SMSG_FAILURE if it does
+not permit authentication with this key. Otherwise it generates a
+challenge, encrypts it using the user's public key (stored on the
+server and identified using the modulus), and sends
+SSH_SMSG_AUTH_RSA_CHALLENGE with the challenge (mp-int) as an
+The challenge is 32 8-bit random bytes (256 bits). When encrypted,
+the highest (partial) byte is left as zero, the next byte contains the
+value 2, the following are non-zero random bytes, followed by a zero
+byte, and the challenge put in the remaining bytes. This is then
+encrypted with the public key. (The padding and encryption algorithm
+is the same as that used for the session key.)
+The client decrypts the challenge using its private key, concatenates
+it with the session id, and computes an MD5 checksum of the resulting
+48 bytes. The MD5 output is returned as 16 bytes in a
+SSH_CMSG_AUTH_RSA_RESPONSE message. (Note that the MD5 is necessary
+to avoid chosen plaintext attacks against RSA; the session id binds it
+to a specific session.)
+The server verifies that the MD5 of the decrypted challenge returned
+by the client matches that of the original value, and sends
+SSH_SMSG_SUCCESS if so. Otherwise it sends SSH_SMSG_FAILURE and
+refuses the authentication attempt.
+This authentication method does not trust the remote host, the
+network, name services, or anything else. Authentication is based
+solely on the possession of the private identification keys. Anyone
+in possession of the private keys can log in, but nobody else.
+The server may have additional requirements for a successful
+authentiation. For example, to limit damage due to a compromised RSA
+key, a server might restrict access to a limited set of hosts.
+The client sends a SSH_CMSG_AUTH_PASSWORD message with the plain text
+password. (Note that even though the password is plain text inside
+the message, it is normally encrypted by the packet mechanism.)
+The server verifies the password, and sends SSH_SMSG_SUCCESS if
+authentication was accepted and SSH_SMSG_FAILURE otherwise.
+Note that the password is read from the user by the client; the user
+never interacts with a login program.
+This authentication method does not trust the remote host, the
+network, name services or anything else. Authentication is based
+solely on the possession of the password. Anyone in possession of the
+password can log in, but nobody else.
+.ti 0
+Preparatory Operations
+After successful authentication, the server waits for a request from
+the client, processes the request, and responds with SSH_SMSG_SUCCESS
+whenever a request has been successfully processed. If it receives a
+message that it does not recognize or it fails to honor a request, it
+returns SSH_SMSG_FAILURE. It is expected that new message types might
+be added to this phase in future.
+The following messages are currently defined for this phase.
+Requests that compression be enabled for this session. A
+gzip-compatible compression level (1-9) is passed as an argument.
+Requests that a pseudo terminal device be allocated for this session.
+The user terminal type and terminal modes are supplied as arguments.
+Requests forwarding of X11 connections from the remote machine to the
+local machine over the secure channel. Causes an internet-domain
+socket to be allocated and the DISPLAY variable to be set on the server.
+X11 authentication data is automatically passed to the server, and the
+client may implement spoofing of authentication data for added
+security. The authentication data is passed as arguments.
+Requests forwarding of a TCP/IP port on the server host over the
+secure channel. What happens is that whenever a connection is made to
+the port on the server, a connection will be made from the client end
+to the specified host/port. Any user can forward unprivileged ports;
+only the root can forward privileged ports (as determined by
+authentication done earlier).
+Requests forwarding of the connection to the authentication agent.
+Starts a shell (command interpreter) for the user, and moves into
+interactive session mode.
+Executes the given command (actually "<shell> -c <command>" or
+equivalent) for the user, and moves into interactive session mode.
+.ti 0
+Interactive Session and Exchange of Data
+During the interactive session, any data written by the shell or
+command running on the server machine is forwarded to stdin or
+stderr on the client machine, and any input available from stdin on
+the client machine is forwarded to the program on the server machine.
+All exchange is asynchronous; either side can send at any time, and
+there are no acknowledgements (TCP/IP already provides reliable
+transport, and the packet protocol protects against tampering or IP
+When the client receives EOF from its standard input, it will send
+SSH_CMSG_EOF; however, this in no way terminates the exchange. The
+exchange terminates and interactive mode is left when the server sends
+SSH_SMSG_EXITSTATUS to indicate that the client program has
+terminated. Alternatively, either side may disconnect at any time by
+sending SSH_MSG_DISCONNECT or closing the connection.
+The server may send any of the following messages:
+Data written to stdout by the program running on the server. The data
+is passed as a string argument. The client writes this data to
+Data written to stderr by the program running on the server. The data
+is passed as a string argument. The client writes this data to
+stderr. (Note that if the program is running on a tty, it is not
+possible to separate stdout and stderr data, and all data will be sent
+as stdout data.)
+Indicates that the shell or command has exited. Exit status is passed
+as an integer argument. This message causes termination of the
+interactive session.
+Indicates that someone on the server side is requesting a connection
+to the authentication agent. The server-side channel number is passed
+as an argument. The client must respond with either
+Indicates that a connection has been made to the X11 socket on the
+server side and should be forwarded to the real X server. An integer
+argument indicates the channel number allocated for this connection on
+the server side. The client should send back either
+the same server side channel number.
+Indicates that a connection has been made to a port on the server side
+for which forwarding has been requested. Arguments are server side
+channel number, host name to connect to, and port to connect to. The
+client should send back either
+the same server side channel number.
+This is sent by the server to indicate that it has opened a connection
+as requested in a previous message. The first argument indicates the
+client side channel number, and the second argument is the channel number
+that the server has allocated for this connection.
+This is sent by the server to indicate that it failed to open a
+connection as requested in a previous message. The client-side
+channel number is passed as an argument. The client will close the
+descriptor associated with the channel and free the channel.
+This packet contains data for a channel from the server. The first
+argument is the client-side channel number, and the second argument (a
+string) is the data.
+This is sent by the server to indicate that whoever was in the other
+end of the channel has closed it. The argument is the client side channel
+number. The client will let all buffered data in the channel to
+drain, and when ready, will close the socket, free the channel, and
+send the server a SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message for the
+This is send by the server to indicate that a channel previously
+closed by the client has now been closed on the server side as well.
+The argument indicates the client channel number. The client frees
+the channel.
+The client may send any of the following messages:
+This is data to be sent as input to the program running on the server.
+The data is passed as a string.
+Indicates that the client has encountered EOF while reading standard
+input. The server will allow any buffered input data to drain, and
+will then close the input to the program.
+Indicates that window size on the client has been changed. The server
+updates the window size of the tty and causes SIGWINCH to be sent to
+the program. The new window size is passed as four integer arguments:
+row, col, xpixel, ypixel.
+Indicates that a connection has been made to a port on the client side
+for which forwarding has been requested. Arguments are client side
+channel number, host name to connect to, and port to connect to. The
+server should send back either SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
+SSH_MSG_CHANNEL_OPEN_FAILURE with the same client side channel number.
+This is sent by the client to indicate that it has opened a connection
+as requested in a previous message. The first argument indicates the
+server side channel number, and the second argument is the channel
+number that the client has allocated for this connection.
+This is sent by the client to indicate that it failed to open a
+connection as requested in a previous message. The server side
+channel number is passed as an argument. The server will close the
+descriptor associated with the channel and free the channel.
+This packet contains data for a channel from the client. The first
+argument is the server side channel number, and the second argument (a
+string) is the data.
+This is sent by the client to indicate that whoever was in the other
+end of the channel has closed it. The argument is the server channel
+number. The server will allow buffered data to drain, and when ready,
+will close the socket, free the channel, and send the client a
+SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message for the channel.
+This is send by the client to indicate that a channel previously
+closed by the server has now been closed on the client side as well.
+The argument indicates the server channel number. The server frees
+the channel.
+Any unsupported messages during interactive mode cause the connection
+to be terminated with SSH_MSG_DISCONNECT and an error message.
+Compatible protocol upgrades should agree about any extensions during
+the preparation phase or earlier.
+.ti 0
+Termination of the Connection
+Normal termination of the connection is always initiated by the server
+by sending SSH_SMSG_EXITSTATUS after the program has exited. The
+client responds to this message by sending SSH_CMSG_EXIT_CONFIRMATION
+and closes the socket; the server then closes the socket. There are
+two purposes for the confirmation: some systems may lose previously
+sent data when the socket is closed, and closing the client side first
+causes any TCP/IP TIME_WAIT [RFC0793] waits to occur on the client side, not
+consuming server resources.
+If the program terminates due to a signal, the server will send
+SSH_MSG_DISCONNECT with an appropriate message. If the connection is
+closed, all file descriptors to the program will be closed and the
+server will exit. If the program runs on a tty, the kernel sends it
+the SIGHUP signal when the pty master side is closed.
+.ti 0
+Protocol Flags
+Both the server and the client pass 32 bits of protocol flags to the
+other side. The flags are intended for compatible protocol extension;
+the server first announces which added capabilities it supports, and
+the client then sends the capabilities that it supports.
+The following flags are currently defined (the values are bit masks):
+This flag can only be sent by the client. It indicates that the X11
+forwarding requests it sends will include the screen number.
+If both sides specify this flag, SSH_SMSG_X11_OPEN and
+SSH_MSG_PORT_OPEN messages will contain an additional field containing
+a description of the host at the other end of the connection.
+.ti 0
+Detailed Description of Packet Types and Formats
+The supported packet types and the corresponding message numbers are
+given in the following table. Messages with _MSG_ in their name may
+be sent by either side. Messages with _CMSG_ are only sent by the
+client, and messages with _SMSG_ only by the server.
+A packet may contain additional data after the arguments specified
+below. Any such data should be ignored by the receiver. However, it
+is recommended that no such data be stored without good reason. (This
+helps build compatible extensions.)
+This code is reserved. This message type is never sent.
+l l.
+string Cause of disconnection
+This message may be sent by either party at any time. It causes the
+immediate disconnection of the connection. The message is intended to
+be displayed to a human, and describes the reason for disconnection.
+l l.
+8 bytes anti_spoofing_cookie
+32-bit int server_key_bits
+mp-int server_key_public_exponent
+mp-int server_key_public_modulus
+32-bit int host_key_bits
+mp-int host_key_public_exponent
+mp-int host_key_public_modulus
+32-bit int protocol_flags
+32-bit int supported_ciphers_mask
+32-bit int supported_authentications_mask
+Sent as the first message by the server. This message gives the
+server's host key, server key, protocol flags (intended for compatible
+protocol extension), supported_ciphers_mask (which is the
+bitwise or of (1 << cipher_number), where << is the left shift
+operator, for all supported ciphers), and
+supported_authentications_mask (which is the bitwise or of (1 <<
+authentication_type) for all supported authentication types). The
+anti_spoofing_cookie is 64 random bytes, and must be sent back
+verbatim by the client in its reply. It is used to make IP-spoofing
+more difficult (encryption and host keys are the real defense against
+l l.
+1 byte cipher_type (must be one of the supported values)
+8 bytes anti_spoofing_cookie (must match data sent by the server)
+mp-int double-encrypted session key
+32-bit int protocol_flags
+Sent by the client as the first message in the session. Selects the
+cipher to use, and sends the encrypted session key to the server. The
+anti_spoofing_cookie must be the same bytes that were sent by the
+server. Protocol_flags is intended for negotiating compatible
+protocol extensions.
+l l.
+string user login name on server
+Sent by the client to begin authentication. Specifies the user name
+on the server to log in as. The server responds with SSH_SMSG_SUCCESS
+if no authentication is needed for this user, or SSH_SMSG_FAILURE if
+authentication is needed (or the user does not exist). [Note to the
+implementator: the user name is of arbitrary size. The implementation
+must be careful not to overflow internal buffers.]
+l l.
+string client-side user name
+Requests authentication using /etc/hosts.equiv and .rhosts (or
+equivalent mechanisms). This authentication method is normally
+disabled in the server because it is not secure (but this is the
+method used by rsh and rlogin). The server responds with
+SSH_SMSG_SUCCESS if authentication was successful, and
+SSH_SMSG_FAILURE if access was not granted. The server should check
+that the client side port number is less than 1024 (a privileged
+port), and immediately reject authentication if it is not. Supporting
+this authentication method is optional. This method should normally
+not be enabled in the server because it is not safe. (However, not
+enabling this only helps if rlogind and rshd are disabled.)
+l l.
+mp-int identity_public_modulus
+Requests authentication using pure RSA authentication. The server
+checks if the given key is permitted to log in, and if so, responds
+with SSH_SMSG_AUTH_RSA_CHALLENGE. Otherwise, it responds with
+SSH_SMSG_FAILURE. The client often tries several different keys in
+sequence until one supported by the server is found. Authentication
+is accepted if the client gives the correct response to the challenge.
+The server is free to add other criteria for authentication, such as a
+requirement that the connection must come from a certain host. Such
+additions are not visible at the protocol level. Supporting this
+authentication method is optional but recommended.
+l l.
+mp-int encrypted challenge
+Presents an RSA authentication challenge to the client. The challenge
+is a 256-bit random value encrypted as described elsewhere in this
+document. The client must decrypt the challenge using the RSA private
+key, compute MD5 of the challenge plus session id, and send back the
+resulting 16 bytes using SSH_CMSG_AUTH_RSA_RESPONSE.
+l l.
+16 bytes MD5 of decrypted challenge
+This message is sent by the client in response to an RSA challenge.
+The MD5 checksum is returned instead of the decrypted challenge to
+deter known-plaintext attacks against the RSA key. The server
+responds to this message with either SSH_SMSG_SUCCESS or
+l l.
+string plain text password
+Requests password authentication using the given password. Note that
+even though the password is plain text inside the packet, the whole
+packet is normally encrypted by the packet layer. It would not be
+possible for the client to perform password encryption/hashing,
+because it cannot know which kind of encryption/hashing, if any, the
+server uses. The server responds to this message with
+l l.
+string TERM environment variable value (e.g. vt100)
+32-bit int terminal height, rows (e.g., 24)
+32-bit int terminal width, columns (e.g., 80)
+32-bit int terminal width, pixels (0 if no graphics) (e.g., 480)
+32-bit int terminal height, pixels (0 if no graphics) (e.g., 640)
+n bytes tty modes encoded in binary
+Requests a pseudo-terminal to be allocated for this command. This
+message can be used regardless of whether the session will later
+execute the shell or a command. If a pty has been requested with this
+message, the shell or command will run on a pty. Otherwise it will
+communicate with the server using pipes, sockets or some other similar
+The terminal type gives the type of the user's terminal. In the UNIX
+environment it is passed to the shell or command in the TERM
+environment variable.
+The width and height values give the initial size of the user's
+terminal or window. All values can be zero if not supported by the
+operating system. The server will pass these values to the kernel if
+Terminal modes are encoded into a byte stream in a portable format.
+The exact format is described later in this document.
+The server responds to the request with either SSH_SMSG_SUCCESS or
+SSH_SMSG_FAILURE. If the server does not have the concept of pseudo
+terminals, it should return success if it is possible to execute a
+shell or a command so that it looks to the client as if it was running
+on a pseudo terminal.
+l l.
+32-bit int terminal height, rows
+32-bit int terminal width, columns
+32-bit int terminal width, pixels
+32-bit int terminal height, pixels
+This message can only be sent by the client during the interactive
+session. This indicates that the size of the user's window has
+changed, and provides the new size. The server will update the
+kernel's notion of the window size, and a SIGWINCH signal or
+equivalent will be sent to the shell or command (if supported by the
+operating system).
+(no arguments)
+Starts a shell (command interpreter), and enters interactive session
+l l.
+string command to execute
+Starts executing the given command, and enters interactive session
+mode. On UNIX, the command is run as "<shell> -c <command>", where
+<shell> is the user's login shell.
+(no arguments)
+This message is sent by the server in response to the session key, a
+successful authentication request, and a successfully completed
+preparatory operation.
+(no arguments)
+This message is sent by the server in response to a failed
+authentication operation to indicate that the user has not yet been
+successfully authenticated, and in response to a failed preparatory
+operation. This is also sent in response to an authentication or
+preparatory operation request that is not recognized or supported.
+l l.
+string data
+Delivers data from the client to be supplied as input to the shell or
+program running on the server side. This message can only be used in
+the interactive session mode. No acknowledgement is sent for this
+l l.
+string data
+Delivers data from the server that was read from the standard output of
+the shell or program running on the server side. This message can
+only be used in the interactive session mode. No acknowledgement is
+sent for this message.
+l l.
+string data
+Delivers data from the server that was read from the standard error of
+the shell or program running on the server side. This message can
+only be used in the interactive session mode. No acknowledgement is
+sent for this message.
+(no arguments)
+This message is sent by the client to indicate that EOF has been
+reached on the input. Upon receiving this message, and after all
+buffered input data has been sent to the shell or program, the server
+will close the input file descriptor to the program. This message can
+only be used in the interactive session mode. No acknowledgement is
+sent for this message.
+l l.
+32-bit int exit status of the command
+Returns the exit status of the shell or program after it has exited.
+The client should respond with SSH_CMSG_EXIT_CONFIRMATION when it has
+received this message. This will be the last message sent by the
+server. If the program being executed dies with a signal instead of
+exiting normally, the server should terminate the session with
+SSH_MSG_DISCONNECT (which can be used to pass a human-readable string
+indicating that the program died due to a signal) instead of using
+this message.
+l l.
+32-bit int remote_channel
+32-bit int local_channel
+This is sent in response to any channel open request if the channel
+has been successfully opened. Remote_channel is the channel number
+received in the initial open request; local_channel is the channel
+number the side sending this message has allocated for the channel.
+Data can be transmitted on the channel after this message.
+l l.
+32-bit int remote_channel
+This message indicates that an earlier channel open request by the
+other side has failed or has been denied. Remote_channel is the
+channel number given in the original request.
+l l.
+32-bit int remote_channel
+string data
+Data is transmitted in a channel in these messages. A channel is
+bidirectional, and both sides can send these messages. There is no
+acknowledgement for these messages. It is possible that either side
+receives these messages after it has sent SSH_MSG_CHANNEL_CLOSE for
+the channel. These messages cannot be received after the party has
+l l.
+32-bit int remote_channel
+When a channel is closed at one end of the connection, that side sends
+this message. Upon receiving this message, the channel should be
+closed. When this message is received, if the channel is already
+closed (the receiving side has sent this message for the same channel
+earlier), the channel is freed and no further action is taken;
+otherwise the channel is freed and SSH_MSG_CHANNEL_CLOSE_CONFIRMATION
+is sent in response. (It is possible that the channel is closed
+simultaneously at both ends.)
+l l.
+32-bit int remote_channel
+This message is sent in response to SSH_MSG_CHANNEL_CLOSE unless the
+channel was already closed. When this message is sent or received,
+the channel is freed.
+.IP "26 (OBSOLETED; was unix-domain X11 forwarding)
+l l.
+32-bit int local_channel
+string originator_string (see below)
+This message can be sent by the server during the interactive session
+mode to indicate that a client has connected the fake X server.
+Local_channel is the channel number that the server has allocated for
+the connection. The client should try to open a connection to the
+real X server, and respond with SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
+The field originator_string is present if both sides
+specified SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags. It
+contains a description of the host originating the connection.
+l l.
+32-bit int server_port
+string host_to_connect
+32-bit int port_to_connect
+Sent by the client in the preparatory phase, this message requests
+that server_port on the server machine be forwarded over the secure
+channel to the client machine, and from there to the specified host
+and port. The server should start listening on the port, and send
+SSH_MSG_PORT_OPEN whenever a connection is made to it. Supporting
+this message is optional, and the server is free to reject any forward
+request. For example, it is highly recommended that unless the user
+has been authenticated as root, forwarding any privileged port numbers
+(below 1024) is denied.
+l l.
+32-bit int local_channel
+string host_name
+32-bit int port
+string originator_string (see below)
+Sent by either party in interactive session mode, this message
+indicates that a connection has been opened to a forwarded TCP/IP
+port. Local_channel is the channel number that the sending party has
+allocated for the connection. Host_name is the host the connection
+should be be forwarded to, and the port is the port on that host to
+connect. The receiving party should open the connection, and respond
+SSH_MSG_CHANNEL_OPEN_FAILURE. It is recommended that the receiving
+side check the host_name and port for validity to avoid compromising
+local security by compromised remote side software. Particularly, it
+is recommended that the client permit connections only to those ports
+for which it has requested forwarding with SSH_CMSG_PORT_FORWARD_REQUEST.
+The field originator_string is present if both sides
+specified SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags. It
+contains a description of the host originating the connection.
+(no arguments)
+Requests that the connection to the authentication agent be forwarded
+over the secure channel. The method used by clients to contact the
+authentication agent within each machine is implementation and machine
+dependent. If the server accepts this request, it should arrange that
+any clients run from this session will actually contact the server
+program when they try to contact the authentication agent. The server
+should then send a SSH_SMSG_AGENT_OPEN to open a channel to the agent,
+and the client should forward the connection to the real
+authentication agent. Supporting this message is optional.
+l l.
+32-bit int local_channel
+Sent by the server in interactive session mode, this message requests
+opening a channel to the authentication agent. The client should open
+a channel, and respond with either SSH_MSG_CHANNEL_OPEN_CONFIRMATION
+l l.
+string data
+Either party may send this message at any time. This message, and the
+argument string, is silently ignored. This message might be used in
+some implementations to make traffic analysis more difficult. This
+message is not currently sent by the implementation, but all
+implementations are required to recognize and ignore it.
+(no arguments)
+Sent by the client in response to SSH_SMSG_EXITSTATUS. This is the
+last message sent by the client.
+l l.
+string x11_authentication_protocol
+string x11_authentication_data
+32-bit int screen number (if SSH_PROTOFLAG_SCREEN_NUMBER)
+Sent by the client during the preparatory phase, this message requests
+that the server create a fake X11 display and set the DISPLAY
+environment variable accordingly. An internet-domain display is
+preferable. The given authentication protocol and the associated data
+should be recorded by the server so that it is used as authentication
+on connections (e.g., in .Xauthority). The authentication protocol
+must be one of the supported X11 authentication protocols, e.g.,
+"MIT-MAGIC-COOKIE-1". Authentication data must be a lowercase hex
+string of even length. Its interpretation is protocol dependent.
+The data is in a format that can be used with e.g. the xauth program.
+Supporting this message is optional.
+The client is permitted (and recommended) to generate fake
+authentication information and send fake information to the server.
+This way, a corrupt server will not have access to the user's terminal
+after the connection has terminated. The correct authorization codes
+will also not be left hanging around in files on the server (many
+users keep the same X session for months, thus protecting the
+authorization data becomes important).
+X11 authentication spoofing works by initially sending fake (random)
+authentication data to the server, and interpreting the first packet
+sent by the X11 client after the connection has been opened. The
+first packet contains the client's authentication. If the packet
+contains the correct fake data, it is replaced by the client by the
+correct authentication data, and then sent to the X server.
+l l.
+string clint-side user name
+32-bit int client_host_key_bits
+mp-int client_host_key_public_exponent
+mp-int client_host_key_public_modulus
+Requests authentication using /etc/hosts.equiv and .rhosts (or
+equivalent) together with RSA host authentication. The server should
+check that the client side port number is less than 1024 (a privileged
+port), and immediately reject authentication if it is not. The server
+client must respond to the challenge with the proper
+SSH_CMSG_AUTH_RSA_RESPONSE. The server then responds with success if
+access was granted, or failure if the client gave a wrong response.
+Supporting this authentication method is optional but recommended in
+most environments.
+l l.
+string debugging message sent to the other side
+This message may be sent by either party at any time. It is used to
+send debugging messages that may be informative to the user in
+solving various problems. For example, if authentication fails
+because of some configuration error (e.g., incorrect permissions for
+some file), it can be very helpful for the user to make the cause of
+failure available. On the other hand, one should not make too much
+information available for security reasons. It is recommended that
+the client provides an option to display the debugging information
+sent by the sender (the user probably does not want to see it by default).
+The server can log debugging data sent by the client (if any). Either
+party is free to ignore any received debugging data. Every
+implementation must be able to receive this message, but no
+implementation is required to send these.
+l l.
+32-bit int gzip compression level (1-9)
+This message can be sent by the client in the preparatory operations
+phase. The server responds with SSH_SMSG_FAILURE if it does not
+support compression or does not want to compress; it responds with
+SSH_SMSG_SUCCESS if it accepted the compression request. In the
+latter case the response to this packet will still be uncompressed,
+but all further packets in either direction will be compressed by gzip.
+.ti 0
+Encoding of Terminal Modes
+Terminal modes (as passed in SSH_CMSG_REQUEST_PTY) are encoded into a
+byte stream. It is intended that the coding be portable across
+different environments.
+The tty mode description is a stream of bytes. The stream consists of
+opcode-argument pairs. It is terminated by opcode TTY_OP_END (0).
+Opcodes 1-127 have one-byte arguments. Opcodes 128-159 have 32-bit
+integer arguments (stored msb first). Opcodes 160-255 are not yet
+defined, and cause parsing to stop (they should only be used after any
+other data).
+The client puts in the stream any modes it knows about, and the server
+ignores any modes it does not know about. This allows some degree of
+machine-independence, at least between systems that use a POSIX-like
+[POSIX] tty interface. The protocol can support other systems as
+well, but the client may need to fill reasonable values for a number
+of parameters so the server pty gets set to a reasonable mode (the
+server leaves all unspecified mode bits in their default values, and
+only some combinations make sense).
+The following opcodes have been defined. The naming of opcodes mostly
+follows the POSIX terminal mode flags.
+Indicates end of options.
+.IP "1 VINTR"
+Interrupt character; 255 if none. Similarly for the other characters.
+Not all of these characters are supported on all systems.
+.IP "2 VQUIT"
+The quit character (sends SIGQUIT signal on UNIX systems).
+Erase the character to left of the cursor.
+.IP "4 VKILL"
+Kill the current input line.
+.IP "5 VEOF "
+End-of-file character (sends EOF from the terminal).
+.IP "6 VEOL "
+End-of-line character in addition to carriage return and/or linefeed.
+.IP "7 VEOL2"
+Additional end-of-line character.
+Continues paused output (normally ^Q).
+.IP "9 VSTOP"
+Pauses output (^S).
+.IP "10 VSUSP"
+Suspends the current program.
+.IP "11 VDSUSP"
+Another suspend character.
+Reprints the current input line.
+Erases a word left of cursor.
+.IP "14 VLNEXT"
+More special input characters; these are probably not supported on
+most systems.
+.IP "15 VFLUSH"
+.IP "16 VSWTCH"
+.IP "30 IGNPAR"
+The ignore parity flag. The next byte should be 0 if this flag is not
+set, and 1 if it is set.
+.IP "31 PARMRK"
+More flags. The exact definitions can be found in the POSIX standard.
+.IP "32 INPCK"
+.IP "33 ISTRIP"
+.IP "34 INLCR"
+.IP "35 IGNCR"
+.IP "36 ICRNL"
+.IP "37 IUCLC"
+.IP "38 IXON"
+.IP "39 IXANY"
+.IP "40 IXOFF"
+.IP "50 ISIG"
+.IP "51 ICANON"
+.IP "52 XCASE"
+.IP "53 ECHO"
+.IP "54 ECHOE"
+.IP "55 ECHOK"
+.IP "56 ECHONL"
+.IP "57 NOFLSH"
+.IP "58 TOSTOP"
+.IP "59 IEXTEN"
+.IP "61 ECHOKE"
+.IP "62 PENDIN"
+.IP "70 OPOST"
+.IP "71 OLCUC"
+.IP "72 ONLCR"
+.IP "73 OCRNL"
+.IP "74 ONOCR"
+.IP "75 ONLRET"
+.IP "90 CS7"
+.IP "91 CS8"
+.IP "92 PARENB"
+.IP "93 PARODD"
+Specifies the input baud rate in bits per second.
+Specifies the output baud rate in bits per second.
+.ti 0
+The Authentication Agent Protocol
+The authentication agent is a program that can be used to hold RSA
+authentication keys for the user (in future, it might hold data for
+other authentication types as well). An authorized program can send
+requests to the agent to generate a proper response to an RSA
+challenge. How the connection is made to the agent (or its
+representative) inside a host and how access control is done inside a
+host is implementation-dependent; however, how it is forwarded and how
+one interacts with it is specified in this protocol. The connection
+to the agent is normally automatically forwarded over the secure
+A program that wishes to use the agent first opens a connection to its
+local representative (typically, the agent itself or an SSH server).
+It then writes a request to the connection, and waits for response.
+It is recommended that at least five minutes of timeout are provided
+waiting for the agent to respond to an authentication challenge (this
+gives sufficient time for the user to cut-and-paste the challenge to a
+separate machine, perform the computation there, and cut-and-paste the
+result back if so desired).
+Messages sent to and by the agent are in the following format:
+l l.
+4 bytes Length, msb first. Does not include length itself.
+1 byte Packet type. The value 255 is reserved for future extensions.
+data Any data, depending on packet type. Encoding as in the ssh packet
+The following message types are currently defined:
+(no arguments)
+Requests the agent to send a list of all RSA keys for which it can
+answer a challenge.
+l l.
+32-bit int howmany
+howmany times:
+32-bit int bits
+mp-int public exponent
+mp-int public modulus
+string comment
+The agent sends this message in response to the to
+SSH_AGENTC_REQUEST_RSA_IDENTITIES. The answer lists all RSA keys for
+which the agent can answer a challenge. The comment field is intended
+to help identify each key; it may be printed by an application to
+indicate which key is being used. If the agent is not holding any
+keys, howmany will be zero.
+l l.
+32-bit int bits
+mp-int public exponent
+mp-int public modulus
+mp-int challenge
+16 bytes session_id
+32-bit int response_type
+Requests RSA decryption of random challenge to authenticate the other
+side. The challenge will be decrypted with the RSA private key
+corresponding to the given public key.
+The decrypted challenge must contain a zero in the highest (partial)
+byte, 2 in the next byte, followed by non-zero random bytes, a zero
+byte, and then the real challenge value in the lowermost bytes. The
+real challenge must be 32 8-bit bytes (256 bits).
+Response_type indicates the format of the response to be returned.
+Currently the only supported value is 1, which means to compute MD5 of
+the real challenge plus session id, and return the resulting 16 bytes
+l l.
+16 bytes MD5 of decrypted challenge
+Answers an RSA authentication challenge. The response is 16 bytes:
+the MD5 checksum of the 32-byte challenge.
+(no arguments)
+This message is sent whenever the agent fails to answer a request
+properly. For example, if the agent cannot answer a challenge (e.g.,
+no longer has the proper key), it can respond with this. The agent
+also responds with this message if it receives a message it does not
+(no arguments)
+This message is sent by the agent as a response to certain requests
+that do not otherwise cause a message be sent. Currently, this is
+only sent in response to SSH_AGENTC_ADD_RSA_IDENTITY and
+l l.
+32-bit int bits
+mp-int public modulus
+mp-int public exponent
+mp-int private exponent
+mp-int multiplicative inverse of p mod q
+mp-int p
+mp-int q
+string comment
+Registers an RSA key with the agent. After this request, the agent can
+use this RSA key to answer requests. The agent responds with
+l l.
+32-bit int bits
+mp-int public exponent
+mp-int public modulus
+Removes an RSA key from the agent. The agent will no longer accept
+challenges for this key and will not list it as a supported identity.
+The agent responds with SSH_AGENT_SUCCESS or SSH_AGENT_FAILURE.
+If the agent receives a message that it does not understand, it
+responds with SSH_AGENT_FAILURE. This permits compatible future
+It is possible that several clients have a connection open to the
+authentication agent simultaneously. Each client will use a separate
+connection (thus, any SSH connection can have multiple agent
+connections active simultaneously).
+.ti 0
+.IP "[DES] "
+FIPS PUB 46-1: Data Encryption Standard. National Bureau of
+Standards, January 1988. FIPS PUB 81: DES Modes of Operation.
+National Bureau of Standards, December 1980. Bruce Schneier: Applied
+Cryptography. John Wiley & Sons, 1994. J. Seberry and J. Pieprzyk:
+Cryptography: An Introduction to Computer Security. Prentice-Hall,
+.IP "[GZIP] "
+The GNU GZIP program; available for anonymous ftp at
+Please let me know if you know a paper describing the algorithm.
+.IP "[IDEA] "
+Xuejia Lai: On the Design and Security of Block Ciphers, ETH Series in
+Information Processing, vol. 1, Hartung-Gorre Verlag, Konstanz,
+Switzerland, 1992. Bruce Schneier: Applied Cryptography, John Wiley &
+Sons, 1994. See also the following patents: PCT/CH91/00117, EP 0 482
+154 B1, US Pat. 5,214,703.
+.IP [PKCS#1]
+PKCS #1: RSA Encryption Standard. Version 1.5, RSA Laboratories,
+November 1993. Available for anonymous ftp at
+Portable Operating System Interface (POSIX) - Part 1: Application
+Program Interface (API) [C language], ISO/IEC 9945-1, IEEE Std 1003.1,
+.IP [RFC0791]
+J. Postel: Internet Protocol, RFC 791, USC/ISI, September 1981.
+.IP [RFC0793]
+J. Postel: Transmission Control Protocol, RFC 793, USC/ISI, September
+.IP [RFC1034]
+P. Mockapetris: Domain Names - Concepts and Facilities, RFC 1034,
+USC/ISI, November 1987.
+.IP [RFC1282]
+B. Kantor: BSD Rlogin, RFC 1258, UCSD, December 1991.
+.IP "[RSA] "
+Bruce Schneier: Applied Cryptography. John Wiley & Sons, 1994. See
+also R. Rivest, A. Shamir, and L. M. Adleman: Cryptographic
+Communications System and Method. US Patent 4,405,829, 1983.
+.IP "[X11] "
+R. Scheifler: X Window System Protocol, X Consortium Standard, Version
+11, Release 6. Massachusetts Institute of Technology, Laboratory of
+Computer Science, 1994.
+.ti 0
+Security Considerations
+This protocol deals with the very issue of user authentication and
+First of all, as an implementation issue, the server program will have
+to run as root (or equivalent) on the server machine. This is because
+the server program will need be able to change to an arbitrary user
+id. The server must also be able to create a privileged TCP/IP port.
+The client program will need to run as root if any variant of .rhosts
+authentication is to be used. This is because the client program will
+need to create a privileged port. The client host key is also usually
+stored in a file which is readable by root only. The client needs the
+host key in .rhosts authentication only. Root privileges can be
+dropped as soon as the privileged port has been created and the host
+key has been read.
+The SSH protocol offers major security advantages over existing telnet
+and rlogin protocols.
+.IP o
+IP spoofing is restricted to closing a connection (by encryption, host
+keys, and the special random cookie). If encryption is not used, IP
+spoofing is possible for those who can hear packets going out from the
+.IP o
+DNS spoofing is made ineffective (by host keys).
+.IP o
+Routing spoofing is made ineffective (by host keys).
+.IP o
+All data is encrypted with strong algorithms to make eavesdropping as
+difficult as possible. This includes encrypting any authentication
+information such as passwords. The information for decrypting session
+keys is destroyed every hour.
+.IP o
+Strong authentication methods: .rhosts combined with RSA host
+authentication, and pure RSA authentication.
+.IP o
+X11 connections and arbitrary TCP/IP ports can be forwarded securely.
+.IP o
+Man-in-the-middle attacks are deterred by using the server host key to
+encrypt the session key.
+.IP o
+Trojan horses to catch a password by routing manipulation are deterred
+by checking that the host key of the server machine matches that
+stored on the client host.
+The security of SSH against man-in-the-middle attacks and the security
+of the new form of .rhosts authentication, as well as server host
+validation, depends on the integrity of the host key and the files
+containing known host keys.
+The host key is normally stored in a root-readable file. If the host
+key is compromised, it permits attackers to use IP, DNS and routing
+spoofing as with current rlogin and rsh. It should never be any worse
+than the current situation.
+The files containing known host keys are not sensitive. However, if an
+attacker gets to modify the known host key files, it has the same
+consequences as a compromised host key, because the attacker can then
+change the recorded host key.
+The security improvements obtained by this protocol for X11 are of
+particular significance. Previously, there has been no way to protect
+data communicated between an X server and a client running on a remote
+machine. By creating a fake display on the server, and forwarding all
+X11 requests over the secure channel, SSH can be used to run any X11
+applications securely without any cooperation with the vendors of the
+X server or the application.
+Finally, the security of this program relies on the strength of the
+underlying cryptographic algorithms. The RSA algorithm is used for
+authentication key exchange. It is widely believed to be secure. Of
+the algorithms used to encrypt the session, DES has a rather small key
+these days, probably permitting governments and organized criminals to
+break it in very short time with specialized hardware. 3DES is
+probably safe (but slower). IDEA is widely believed to be secure.
+People have varying degrees of confidence in the other algorithms.
+This program is not secure if used with no encryption at all.
+.ti 0
+Additional Information
+Additional information (especially on the implementation and mailing
+lists) is available via WWW at
+Comments should be sent to Tatu Ylonen <> or the SSH
+Mailing List <>.
+.ti 0
+Author's Address
+Tatu Ylonen
+Helsinki University of Technology
+Otakaari 1
+FIN-02150 Espoo, Finland
+Phone: +358-0-451-3374
+Fax: +358-0-451-3293