# S/Key

S/Key es un sistema de autenticación OTP (One-Time Password), por lo tanto, permite utilizar contraseñas de un único uso. Estas contraseñas son generadas a partir de una función de hash (md4, md5, sha1 o rmd160) y una clave secreta.
Funcionamiento:

F: función de hash
skey: palabra secreta
Pi: contraseña 1-10

F(skey) = P0
F(P0) = P1
F(P1) = P2
F(P2) = P3
F(P3) = P4
F(P4) = P5
F(P5) = P6
F(P6) = P7
F(P7) = P8
F(P8) = P9
F(P9) = P10

S/Key está soportado en sistemas *NIX y se encarga de la autenticación de algunos protocolos (telnet, ftp, ssh, ...).
El servidor almacena la última contraseña generada (P10). Cuando el usuario intenta acceder al servidor, se le solicita la contraseña P9, si F(P9) = P10, el servidor permite el acceso, almacena P9 y en el siguiente acceso solicitará P8.
En el caso de que un atacante interceptara P9, esta ya no sería válida. En el caso de que quisiera obtener P8 a partir de P9, tendría un gran problema (encontrar la función inversa G(P9)->P8), ya que las contraseñas se generan utilizando funciones de hash de una vía (F(P8)->P9).

Práctica:
local# ssh root@192.168.1.12
root@192.168.1.12's password: 
Last login: Sat Oct  4 23:00:28 2008
OpenBSD 4.3 (GENERIC) #698: Wed Mar 12 11:07:05 MDT 2008

remoto# skeyinit -E
remoto# skeyinit
Reminder - Only use this method if you are directly connected
or have an encrypted channel.  If you are using telnet,
hit return now and use skeyinit -s.
[Adding root with md5]
Enter new secret passphrase: 
Again secret passphrase: 

ID root skey is otp-md5 100 open65334
Next login password: YAM LOWE GIBE HOWL LION MAST

remoto# otp-md5 -n 100 `skeyinfo`  
Reminder - Do not use this program while logged in via telnet.
Enter secret passphrase: 
0: ORGY COST EDGY TINT DEAR BLUM
1: RASH HELL LEAD NEIL HARM ADEN
2: GAG PER EMMA BAND USES GET   
3: SAW BALE WAY KNEW RULE CLUE  
4: GRAD BOWL ALGA TONY BUSS NOW 
5: AVOW HALL VIE MUD YEAR COY   
6: YAP DIAL WEAL DUD PER HAAS   
7: VASE HESS ULAN DORA YAWL EYED
8: GASH SIP KISS WHY HEAL TACT  
9: KONG MARE SHIM CAGE BACK RULE
10: WU BUCK LAME RUDY OLAF COCK  
11: SOFA DRUG LAVA HAY BAIL STAB 
12: GIRL VICE KEY AWE AT RENT    
13: DAY OSLO TUFT MA MANY EASY   
14: MILE RAM DINE HOYT THUG PAD  
15: NIP DEFY DATA LINK FILE TAKE 
16: HAND WEIR HOOK LIED PEA LIAR 
17: RYE WAGE HAYS SELL TIE HOP   
18: GULF TONG AT DRAW MINE CADY  
19: CHEW FANG LYLE SUDS EASY MIT 
20: WARM HASH BERT SALK WEE DEBT 
21: EST MAW LUCK EVEN WEST GARB  
22: ROOF ARTY DOSE GLOW GLIB TOOT
23: TELL ROE SELF NOLL GARB CASK 
24: MEAL MILD FIST TEAM COOT ROAR
25: HECK DIET AID SON HIS RUDE   
26: VISE EVIL FEE TIDY BURT SIRE 
27: MOTH FERN OTT SURE WELD REID 
28: MICE LYNN GANG HOLT ROSA LOOK
29: HO TUNA DESK LIP GUSH SOAR   
30: KEG LIN DIRT TEEM COMA SAIL  
31: EVA ANNA APT IQ TUBE WART    
32: AHOY SOWN RISK GAP DARK BADE 
33: LINK HAL CELL SHE LAVA MUST  
34: GIFT MAE SOD SLIM HAIR HESS  
35: KATE NIT TAB SOCK GLOW CLUE  
36: GO PEN EARL TIP WICK SORT    
37: TIER IFFY ARCH YES YEAR WOK  
38: WIRE WISH THEY LOSE NAN ROBE 
39: MOD GARY EARL TONG PIN OW    
40: BASH GINA JURY ONES DAR GIFT 
41: SLUG LIFE FIT AMRA BROW AIDE 
42: HEBE LYON VOID MORE KNEW FROM
43: ONLY TUN AT BUFF BETA HORN   
44: EDDY ABE UP THAN ANNA JAR    
45: SARA GEL DOWN KARL DIME BOG  
46: BAWL KITE BOSE OILY LION TALL
47: BOYD ALAN TOOT MEAN NICE FAD 
48: SOWN ELM AJAR TUFT WEAL ANTE 
49: SELF FIRM GAP NEWS FULL HESS 
50: TOOK JUNE BOB ROLL ABLE BIEN 
51: ROSA DIN GLUE MISS TEET VEAL 
52: DOG FLUB SIFT LARD ACME BABY 
53: JIM TOUT GOT ROVE SEEK USES  
54: ONLY PUP PEW ALTO LIMB GIVE  
55: BEAU HUT SEA HOC CUT SUD     
56: NOV WRY JOHN AMY PHI NODE    
57: GIST DINE DAB AHEM COIL OVER 
58: KNOB CAP YOU CUTS NAIL SIP   
59: PET WED DAM JIM STIR SCAR    
60: AUK RAT HONK SUN GENE AVON   
61: REAM ROAR VEND LIP DID EGG   
62: LIT DUG WAIT GALA WEAK NON   
63: JOVE JAM LOG DIAL LAWN LILT  
64: BARE CHUB CON LIAR STOW FOUL 
65: TREK ROOM NEWS HAN SKIT LAWS 
66: BARE TUSK JOAN TESS BARE DEN 
67: JUNE YAM LUND PHI ARC DUST   
68: BASE WIFE GLUE OTTO MALE SIR 
69: SOAK FANG ELY FOR PI ROUT    
70: TUCK FIRM BONA HOB TOIL SUB  
71: CERN TIED SAG SLID MAYO SOW  
72: DUNK TOE JAVA GOOD JOVE HERB 
73: HOC DUCT MAE MOE BEAK BOSS   
74: SHE SILT MURK UP GAB WEEK    
75: BELA POP HOLT BET LORD BARN  
76: CALF SWAB DAWN FOAM GUY GWEN 
77: DIP BAT ONCE TREK GLEN MACE  
78: VEIL FUSE VETO SUB CHAD EMIT 
79: TWIN PET CALF GREY KEG HIVE  
80: IO TINA KNOT JAM KNOT FUME   
81: LIEU JURY JUT KONG GAFF GLAD 
82: CAW CLAD TONE DIRT PRO ELSE  
83: KIRK LIST CODA NEWS LOSS STAN
84: GREG TESS QUIT LOW SLAY HIKE 
85: FLEA GRAD GOWN WAGE KANT FEED
86: KALE DEFY NOT OLGA GIBE CURD 
87: HUG ACTS YALE PAY RACY JULY  
88: VISE HUG MOD BAG LUCY CLOD   
89: EMIL IF ALGA FIB FAIL MUCK   
90: BETH LIN KARL AFAR FOOT WEAK 
91: NO RON ULAN MIRE RIDE NOEL   
92: DEAL SHAG DAVY OLGA AIDA MAP 
93: DELL FIRM COKE HEAT BEY KEEN 
94: CHAD PAM DUMB FAR ANTI COOL  
95: LULU VIEW BUCK DULL GLOM BOP 
96: BUOY SAY SICK JOEY ANTE COCA 
97: MORN FLIT FAKE LOOT HULK ECHO
98: NOUN KEEL HAVE CITY YELL RIME
99: ROBE DIVE MOST LAVA MIRE BONN
remoto# exit
local# ssh -l root:skey 192.168.1.12
otp-md5 99 open65334
S/Key Password: ROBE DIVE MOST LAVA MIRE BONN
Last login: Sat Oct  4 23:02:03 2008 from 192.168.1.2
OpenBSD 4.3 (GENERIC) #698: Wed Mar 12 11:07:05 MDT 2008

remoto#

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