To access the classified section of Halo Waypoint either download the Halo Waypoint Xbox 360 app and press X or go to the Classified page of this website.
For lots of background information on these codes you should take a look at the Section 3 website. Section 3 have also created an image of all of the codes they have found so far, which does make entering them much easier.
Enter classified codes using a translator such as this one. Top symbol (looks like lb) is 0, then clockwise from outside to centre is 0-9 then A-Z then .
Requiem Archive I “Raider Spartan-IV armor”
1/4 SPEC-Shoulders "Complete Land Grab in Spartan Ops on Legendary"
1UK3CVYJZM - 500 XP, Raider Left Shoulder, Raider Right Shoulder
2/4 SPEC-Torso "Complete Sniper Alley in Spartan Ops on Legendary"
3VGRZJ1BS6 - 500 XP, Raider Torso
3/4 SPEC-Helmet "Complete The Challenge in Spartan Ops on Legendary"
Q239SF2PGZ - 500 XP, Raider Helmet
4/4 SPEC COMPLETE "Unlock all technical specs for the RAIDER armor"
500 XP, Raider emblem, RAIDER avatar helmet, RAIDER artwork (13 images)
Requiem Archive II “Raider DSTT Spartan-IV armor”
1/4 SPEC-Shoulders "Complete 15 Spartan Ops Chapters on Legendary"
MN7VKCZP.Q - 2500 XP, Raider DSTT Left Shoulder, Raider DSTT Right Shoulder, On Your Shield emblem, Spartan Ops Concept Art (13 images)
2/4 SPEC-Torso "Complete Solo Campaign on Legendary"
WQ6F2Y1M1P - 7500 XP, Raider DSTT Torso, Legendary artwork (13 images)
3/4 SPEC-Helmet "Complete 1000 War Games Matchmaking games"
8PXX1SMIQU - 25000 XP, Raider DSTT Helmet, 1K Club emblem, Operation 1K artwork (12 images)
4/4 SPEC COMPLETE "Unlock all technical specs for the RAIDER DSTT armor variant"
15000 XP, Raider Distort emblem, RAIDER Distort avatar helmet
Section 3 Archive
1/10 Wiseguy "Find all Domain Terminals"
Attention to detail is your speciality
9WE621KN8F - 2000 XP, Wiseguy emblem, Domain Terminals Artwork (12 images)
2/10 ONI "Cracking codes comes natural to you"
T5CE6KV3R8 - 5000 XP, ONI emblem, Halo 4 Concept Art (13 images)
3/10 Day 1 "Where were you that day"
M0DKE.EMQC - 12116 XP, Recruit emblem
4/10 LASO “Complete Campaign on Legendary All Skulls On” (Decrypted by Section 3)
This is no limit to your bravery and skill
EPGROIVIDR - 50000 XP, LASO emblem
5/10 CURIOUS. "How did you find me?"
1K3G7N17LL - 20000 XP
6/10 Majestic “Good eye for detail” (Decrypted by Section 3)
GT.RFIEU6Q - 25000 XP, Majestic Concept Art (11 images)
7/10 Rebirth “Silentium Coda” (Decrypted by Section 3)
73P1SJ66O9 - 20000 XP, Silentium epilogue (40 min)
8/10 Rebirth (Full) “Silentium Coda (Full)” (Decrypted by Section 3)
DLS1B0SXUI - 50000 XP, Overachiever emblem, Silentium epilogue (45 min)
9/10 Castle “Nothing gets past you” (Decrypted by Section 3)
EXBBF.K.J4 - 40000 XP, Castle Concept Art (23 images)
10/10 Bullseye "Excellent exploration skills"
5PZ4VHJ8MG - 30000 XP, Bullseye Concept Art (7 images)
Draetheus V Archive - Halo: Spartan Assault
1/5 Recruit “Unlock Recruit achievement” (Decrypted by Section 3)
Your training has begun.
397.WKKR3Q - 10000 XP
2/5 Prodigy “Unlock Prodigy achievement” (Decrypted by Section 3)
You were meant to be a Spartan.
O.09SS2A2V - 15000 XP
3/5 Able Spartan “Unlock Powered by Mjolnir” (Decrypted by Section 3)
Abilities are no mystery to you.
67706L40XV - 15000 XP
4/5 Flying Colors "Unlock first three parts of Draetheus V Archive"
Your training is complete. For now…
10000 XP, Flying Colors emblem
5/5 Flying Colors "Complete a Halo Spartan Assault mission on Xbox One or Xbox 360"
VGEN5.9RQW - 50000 XP, Spartan Assault Concept Art (? images), Flying Colors emblem
The following information applies to Halo: Anniversary and is listed here for completeness.
Alpha Halo Archive “Terminals on or near Installation 04”
1/11 The Pillar of Autumn "Find Terminal 1"
343GS - 7000 CR, Terminal 1 video
2/11 Halo "Find Terminal 2"
HALO4 - 7000 CR, Terminal 2 video
3/11 The Truth and Reconciliation "Find Terminal 3"
SPARK - 7000 CR, Terminal 3 video
4/11 The Silent Cartographer "Find Terminal 4"
FLOOD - 7000 CR, Terminal 4 video
5/11 Assult on the Control Room "Find Terminal 5"
EARTH - 7000 CR, Terminal 5 video
6/11 343 Guilty Spark "Find Terminal 6"
DEMON - 7000 CR, Terminal 6 video
7/11 The Library "Find Terminal 7"
RINGS - 7000 CR, Terminal 7 video
8/11 Two Betrayals "Find Terminal 8"
ARRAY - 7000 CR, Terminal 8 video
9/11 Keyes "Find Terminal 9"
HUMAN - 7000 CR, Terminal 9 video
10/11 The Maw "Find Terminal 10"
HAVEN - 7000 CR, Terminal 10 video
11/11 Threshold "Unlock all Terminals"
30000 CR, Threshold video