110 lines
5.3 KiB
Text
110 lines
5.3 KiB
Text
t_* stuff is from the srp 1.7.1 dist
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bn_* stuff is from openssl 0.9.6
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(The 7 in libtinysrp's version number reflects the srp version.)
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Licensing and copyright for srp and openssl are as indicated in the relevant
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source files. Everything else here is GPL, including the tinysrp protocol.
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Changelog since initial release:
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0.7.4 more robust terminal modes in t_getpass
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a potential buffer overflow in tinysrp
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0.7.5 uninitialized pointer bug in tconf
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Changes from the base srp and openssl distributions:
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I've removed everything that's not needed for client/server operations, and
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all the bn_* stuff that's only used for prime generation has been moved to
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t_conf.c, which isn't part of the library anymore. Also, all the routines
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used for passphrase file maintenance have been moved to tphrase.c.
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The library has been optimized (a bit) for space instead of speed. Since
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authentication is usually only done once, this isn't a big problem. Modern
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CPUs are plenty fast for this task, and even 100 MHz CPUs are fine. If you
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really need the speed, get the regular distributions.
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Note that if the server sends the client a prime that the client doesn't
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know about, the client MUST test for primality. Since this is pretty
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expensive, and takes 30 seconds on a 100 MHz machine, and uses lots of code,
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I've removed that ability from the client. So only KNOWN primes can be
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used. You can still generate new ones with tconf, but you have to install
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them in the table of known primes (pre_params) in t_getconf.c that's common
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to the client and server, and recompile. The configuration file is gone.
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The default prime (the last entry in the table) is 1024 bits; there are
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others with more bits but they will be correspondingly slower.
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The default tpasswd file (which is an ascii file that may be editted with a
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regular text editor) contains two users: moo (passphrase "glub glub") and
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"new user" (passphrase "this is a test"). Passphrases may be added or
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changed with tphrase; you can also change the user's prime. To delete a
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user, edit the tpasswd file and remove that line. The tpasswd file's
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default name is DEFAULT_PASSWD in t_pwd.h. Note that you can't change a
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user's username by editting the file: the username is encoded in the
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verifier. If you change a username you must set a new passphrase with
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tphrase.
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Here is an example session, using the supplied srvtest and clitest. First,
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start both programs in different windows, and enter the user names. Normally,
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the client would send the username to the server. Server lines are marked
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with S>, client lines with C>.
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S> % srvtest
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S> Enter username: moo
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S> index (to client): 5
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S> salt (to client): 19AI0Hc9jEkdFc
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C> % clitest
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C> Enter username: moo
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C> Enter index (from server): 5
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C> Enter salt (from server): 19AI0Hc9jEkdFc
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The server reports the index and salt values used for that user. They
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are sent over the network to the client. (Simulate this by cutting and
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pasting from one window to the other.)
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C> A (to server): 5wCDXRxLIv/zLazYfKupV/OY3BlhTZuJ71wVgI0HcL1kSJEpkMuWF.xEz/BV2wlJl7vk5Eoz9KMS1ccnaatsVP5D6CBm7UA.yVB59EQFN0dNBirvX29NAFdtdMsMppo5tHRy987XjJWrWSLpeibq6emr.gP8nYyX75GQqSiMY1j
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C> Enter password:
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S> Enter A (from client): 5wCDXRxLIv/zLazYfKupV/OY3BlhTZuJ71wVgI0HcL1kSJEpkMuWF.xEz/BV2wlJl7vk5Eoz9KMS1ccnaatsVP5D6CBm7UA.yVB59EQFN0dNBirvX29NAFdtdMsMppo5tHRy987XjJWrWSLpeibq6emr.gP8nYyX75GQqSiMY1j
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Now the client calculates A and sends it to the server, and while the
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server is munching on that, the client gets the password from the user.
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S> B (to client): 9dcCpulxQAbaDXI0NHWY6B.QH6B9fsoXs/x/5SCNBNJm/6H6bYfbVrwNmdquhLZjYMvpcgGc2mBYqL77RNfw1kVQo17//GfsByECBIjRnrAn02ffX9Y/llJcfscAQiii0hyZhJf9PT5wE7pC7WUjIgSqckIZ0JLNDbSr7fJcrgw
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S> Session key: ebbcf3a45c968defdcfff6e144ad8d4f5412167c9716e79cbf7cacfe18257947ad46fa5d6418a1fd
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The server now calculates B and sends it to the client. The session key
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is not sent -- it is a shared secret that can be used for encryption.
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C> Enter B (from server): 9dcCpulxQAbaDXI0NHWY6B.QH6B9fsoXs/x/5SCNBNJm/6H6bYfbVrwNmdquhLZjYMvpcgGc2mBYqL77RNfw1kVQo17//GfsByECBIjRnrAn02ffX9Y/llJcfscAQiii0hyZhJf9PT5wE7pC7WUjIgSqckIZ0JLNDbSr7fJcrgw
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C> Session key: ebbcf3a45c968defdcfff6e144ad8d4f5412167c9716e79cbf7cacfe18257947ad46fa5d6418a1fd
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C> Response (to server): b9ea99094a176c4be28eb469982066cc7146d180
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The client uses the B value to calculate its own copy of the shared secret
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session key, and sends a response to the server proving that it does know
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the correct key.
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S> Enter response (from client): b9ea99094a176c4be28eb469982066cc7146d180
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S> Authentication successful.
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S> Response (to client): cd46c839ccad2d0c76f3ca1905ae8ceda8d1c1dc
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The server authenticates the client. (You're in!)
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C> Enter server response: cd46c839ccad2d0c76f3ca1905ae8ceda8d1c1dc
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C> Server authentication successful.
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The client authenticates the server (prevents server spoofing in the case
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where the session key isn't used to encrypt the channel -- a spoofed server
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might just respond with random values and _pretend_ to authenticate the
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client; but the spoofed server won't know the session key and this check
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catches that).
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Final note:
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Remember that many breaches of security involve buggy software, such as
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servers susceptible to buffer overflow exploits that totally bypass any
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passphrase, secure or not. If an attacker roots your client, or the server,
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no form of authentication will work. Consider MAC-based schemes if this
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worries you.
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