diff --git a/fusee/fusee-secondary/src/key_derivation.c b/fusee/fusee-secondary/src/key_derivation.c
index 1ff61678c..05ca2bc51 100644
--- a/fusee/fusee-secondary/src/key_derivation.c
+++ b/fusee/fusee-secondary/src/key_derivation.c
@@ -1,4 +1,5 @@
 #include "key_derivation.h"
+#include "masterkey.h"
 #include "se.h"
 #include "exocfg.h"
 #include "fuse.h"
@@ -125,6 +126,9 @@ void derive_nx_keydata(u32 target_firmware) {
         default:
             generic_panic();
     }
+    
+    /* Setup master key revision, derive older master keys for use. */
+    mkey_detect_revision();
 }
 
 /* Sets final keyslot flags, for handover to TZ/Exosphere. Setting these will prevent the BPMP from using the device key or master key. */
diff --git a/fusee/fusee-secondary/src/kip.h b/fusee/fusee-secondary/src/kip.h
new file mode 100644
index 000000000..f1bd8b7fa
--- /dev/null
+++ b/fusee/fusee-secondary/src/kip.h
@@ -0,0 +1,47 @@
+#ifndef FUSEE_KIP_H
+#define FUSEE_KIP_H
+#include "utils.h"
+#include <stdint.h>
+
+/* Fusee definitions for INI1/KIP types. This is mostly taken from hactool. */
+
+#define MAGIC_INI1 0x31494E49
+#define MAGIC_KIP1 0x3150494B
+#define INI1_MAX_KIPS 0x50
+
+typedef struct {
+    uint32_t magic;
+    uint32_t size;
+    uint32_t num_processes;
+    uint32_t _0xC;
+    char kip_data[];
+} ini1_header_t;
+
+typedef struct {
+    uint32_t out_offset;
+    uint32_t out_size;
+    uint32_t compressed_size;
+    uint32_t attribute;
+} kip_section_header_t;
+
+typedef struct {
+    uint32_t magic;
+    char name[0xC];
+    uint64_t title_id;
+    uint32_t process_category;
+    uint8_t main_thread_priority;
+    uint8_t default_core;
+    uint8_t _0x1E;
+    uint8_t flags;
+    kip_section_header_t section_headers[6];
+    uint32_t capabilities[0x20];
+    unsigned char data[];
+} kip1_header_t;
+
+static inline uint64_t kip1_get_size_from_header(kip1_header_t *header) {
+    /* Header + .text + .rodata + .rwdata */
+    return 0x100 + header->section_headers[0].compressed_size + header->section_headers[1].compressed_size + header->section_headers[2].compressed_size;
+}
+
+
+#endif
\ No newline at end of file
diff --git a/fusee/fusee-secondary/src/masterkey.c b/fusee/fusee-secondary/src/masterkey.c
new file mode 100644
index 000000000..031f39dc8
--- /dev/null
+++ b/fusee/fusee-secondary/src/masterkey.c
@@ -0,0 +1,90 @@
+#include <stdbool.h>
+#include <stdint.h>
+#include <string.h>
+
+#include "utils.h"
+#include "masterkey.h"
+#include "se.h"
+
+static unsigned int g_mkey_revision = 0;
+static bool g_determined_mkey_revision = false;
+
+static uint8_t g_old_masterkeys[MASTERKEY_REVISION_MAX][0x10];
+
+/* TODO: Extend with new vectors, as needed. */
+static const uint8_t mkey_vectors[MASTERKEY_REVISION_MAX][0x10] =
+{
+    {0x0C, 0xF0, 0x59, 0xAC, 0x85, 0xF6, 0x26, 0x65, 0xE1, 0xE9, 0x19, 0x55, 0xE6, 0xF2, 0x67, 0x3D}, /* Zeroes encrypted with Master Key 00. */
+    {0x29, 0x4C, 0x04, 0xC8, 0xEB, 0x10, 0xED, 0x9D, 0x51, 0x64, 0x97, 0xFB, 0xF3, 0x4D, 0x50, 0xDD}, /* Master key 00 encrypted with Master key 01. */
+    {0xDE, 0xCF, 0xEB, 0xEB, 0x10, 0xAE, 0x74, 0xD8, 0xAD, 0x7C, 0xF4, 0x9E, 0x62, 0xE0, 0xE8, 0x72}, /* Master key 01 encrypted with Master key 02. */
+    {0x0A, 0x0D, 0xDF, 0x34, 0x22, 0x06, 0x6C, 0xA4, 0xE6, 0xB1, 0xEC, 0x71, 0x85, 0xCA, 0x4E, 0x07}, /* Master key 02 encrypted with Master key 03. */
+    {0x6E, 0x7D, 0x2D, 0xC3, 0x0F, 0x59, 0xC8, 0xFA, 0x87, 0xA8, 0x2E, 0xD5, 0x89, 0x5E, 0xF3, 0xE9}, /* Master key 03 encrypted with Master key 04. */
+};
+
+bool check_mkey_revision(unsigned int revision) {
+    uint8_t final_vector[0x10];
+
+    unsigned int check_keyslot = KEYSLOT_SWITCH_MASTERKEY;
+    if (revision > 0) {
+        /* Generate old master key array. */
+        for (unsigned int i = revision; i > 0; i--) {
+            se_aes_ecb_decrypt_block(check_keyslot, g_old_masterkeys[i-1], 0x10, mkey_vectors[i], 0x10);
+            set_aes_keyslot(KEYSLOT_SWITCH_TEMPKEY, g_old_masterkeys[i-1], 0x10);
+            check_keyslot = KEYSLOT_SWITCH_TEMPKEY;
+        }
+    }
+
+    se_aes_ecb_decrypt_block(check_keyslot, final_vector, 0x10, mkey_vectors[0], 0x10);
+    for (unsigned int i = 0; i < 0x10; i++) {
+        if (final_vector[i] != 0) {
+            return false;
+        }
+    }
+    return true;
+}
+
+void mkey_detect_revision(void) {
+    if (g_determined_mkey_revision) {
+        generic_panic();
+    }
+    
+    for (unsigned int rev = 0; rev < MASTERKEY_REVISION_MAX; rev++) {
+        if (check_mkey_revision(rev)) {
+            g_determined_mkey_revision = true;
+            g_mkey_revision = rev;
+            break;
+        }
+    }
+    
+    /* We must have determined the master key, or we're not running on a Switch. */
+    if (!g_determined_mkey_revision) {
+        /* Panic in bright red. */
+        panic(0x00F00060);
+    }
+}
+
+unsigned int mkey_get_revision(void) {
+    if (!g_determined_mkey_revision) {
+        generic_panic();
+    }
+
+    return g_mkey_revision;
+}
+
+unsigned int mkey_get_keyslot(unsigned int revision) {
+    if (!g_determined_mkey_revision || revision >= MASTERKEY_REVISION_MAX) {
+        generic_panic();
+    }
+
+    if (revision > g_mkey_revision) {
+        generic_panic();
+    }
+
+    if (revision == g_mkey_revision) {
+        return KEYSLOT_SWITCH_MASTERKEY;
+    } else {
+        /* Load into a temp keyslot. */
+        set_aes_keyslot(KEYSLOT_SWITCH_TEMPKEY, g_old_masterkeys[revision], 0x10);
+        return KEYSLOT_SWITCH_TEMPKEY;
+    }
+}
\ No newline at end of file
diff --git a/fusee/fusee-secondary/src/masterkey.h b/fusee/fusee-secondary/src/masterkey.h
new file mode 100644
index 000000000..f6b7d61c4
--- /dev/null
+++ b/fusee/fusee-secondary/src/masterkey.h
@@ -0,0 +1,22 @@
+#ifndef FUSEE_MASTERKEY_H
+#define FUSEE_MASTERKEY_H
+
+/* This is glue code to enable master key support across versions. */
+
+/* TODO: Update to 0x6 on release of new master key. */
+#define MASTERKEY_REVISION_MAX 0x5
+
+#define MASTERKEY_REVISION_100_230     0x00
+#define MASTERKEY_REVISION_300         0x01
+#define MASTERKEY_REVISION_301_302     0x02
+#define MASTERKEY_REVISION_400_410     0x03
+#define MASTERKEY_REVISION_500_CURRENT 0x04
+
+/* This should be called during initialization. */
+void mkey_detect_revision(void);
+
+unsigned int mkey_get_revision(void);
+
+unsigned int mkey_get_keyslot(unsigned int revision);
+
+#endif
\ No newline at end of file
diff --git a/fusee/fusee-secondary/src/package2.c b/fusee/fusee-secondary/src/package2.c
index 15a84fa65..831a3833d 100644
--- a/fusee/fusee-secondary/src/package2.c
+++ b/fusee/fusee-secondary/src/package2.c
@@ -1,47 +1,251 @@
 #include "utils.h"
+#include "masterkey.h"
 #include "package2.h"
+#include "kip.h"
 #include "se.h"
+#include "lib/printk.h"
+
+/* Stage 2 executes from DRAM, so we have tons of space. */
+/* This *greatly* simplifies logic. */
+unsigned char g_patched_package2[PACKAGE2_SIZE_MAX];
+unsigned char g_package2_sections[PACKAGE2_SECTION_MAX][PACKAGE2_SIZE_MAX];
+unsigned char g_package2_work_buffer[PACKAGE2_SIZE_MAX];
+
+package2_header_t *g_patched_package2_header = (package2_header_t *)g_patched_package2; 
 
 void package2_decrypt(void *package2_address);
-void package2_add_thermosphere_section(void *package2_address);
-void package2_patch_kernel(void *package2_address);
-void package2_patch_ini1(void *package2_address);
-void package2_fixup_header_and_section_hashes(void *package2_address);
+void package2_add_thermosphere_section(void);
+void package2_patch_kernel(void);
+void package2_patch_ini1(void);
+void package2_fixup_header_and_section_hashes(void);
 
 void package2_patch(void *package2_address) {
-    /* First things first: Decrypt (TODO: Relocate?) Package2. */
+    /* First things first: Decrypt Package2. */
     package2_decrypt(package2_address);
     
     /* Modify Package2 to add an additional thermosphere section. */
-    package2_add_thermosphere_section(package2_address);
+    package2_add_thermosphere_section();
     
     /* Perform any patches we want to the NX kernel. */
-    package2_patch_kernel(package2_address);
+    package2_patch_kernel();
     
     /* Perform any patches we want to the INI1 (This is where our built-in sysmodules will be added.) */
-    package2_patch_ini1(package2_address);
+    package2_patch_ini1();
     
     /* Fix all necessary data in the header to accomodate for the new patches. */
-    package2_fixup_header_and_section_hashes(package2_address);
+    package2_fixup_header_and_section_hashes();
+    
+    /* Relocate Package2. */
+    memcpy(NX_BOOTLOADER_PACKAGE2_LOAD_ADDRESS, g_patched_package2, PACKAGE2_SIZE_MAX);
 }
 
+static void package2_crypt_ctr(unsigned int master_key_rev, void *dst, size_t dst_size, const void *src, size_t src_size, const void *ctr, size_t ctr_size) {
+    /* Derive package2 key. */
+    const uint8_t package2_key_source[0x10] = {0xFB, 0x8B, 0x6A, 0x9C, 0x79, 0x00, 0xC8, 0x49, 0xEF, 0xD2, 0x4D, 0x85, 0x4D, 0x30, 0xA0, 0xC7};
+    unsigned int keyslot = mkey_get_keyslot(master_key_rev);
+    decrypt_data_into_keyslot(KEYSLOT_SWITCH_PACKAGE2KEY, keyslot, package2_key_source, 0x10);
+
+    /* Perform Encryption. */
+    se_aes_ctr_crypt(KEYSLOT_SWITCH_PACKAGE2KEY, dst, dst_size, src, src_size, ctr, ctr_size);
+}
+
+bool validate_package2_metadata(package2_meta_t *metadata) {
+    if (metadata->magic != MAGIC_PK21) {
+        return false;
+    }
+
+    /* Package2 size, version number is stored XORed in header CTR. */
+    /* Nintendo, what the fuck? */
+    uint32_t package_size = metadata->ctr_dwords[0] ^ metadata->ctr_dwords[2] ^ metadata->ctr_dwords[3];
+    uint8_t header_version = (uint8_t)((metadata->ctr_dwords[1] ^ (metadata->ctr_dwords[1] >> 16) ^ (metadata->ctr_dwords[1] >> 24)) & 0xFF);
+
+    /* Ensure package isn't too big or too small. */
+    if (package_size <= sizeof(package2_header_t) || package_size > PACKAGE2_SIZE_MAX - sizeof(package2_header_t)) {
+        return false;
+    }
+
+    /* Validate that we're working with a header we know how to handle. */
+    if (header_version > MASTERKEY_REVISION_MAX) {
+        return false;
+    }
+
+    /* Require aligned entrypoint. */
+    if (metadata->entrypoint & 3) {
+        return false;
+    }
+
+    /* Validate section size sanity. */
+    if (metadata->section_sizes[0] + metadata->section_sizes[1] + metadata->section_sizes[2] + sizeof(package2_header_t) != package_size) {
+        return false;
+    }
+
+    bool entrypoint_found = false;
+
+    /* Header has space for 4 sections, but only 3 are validated/potentially loaded on hardware. */
+    size_t cur_section_offset = 0;
+    for (unsigned int section = 0; section < PACKAGE2_SECTION_MAX; section++) {
+        /* Validate section size alignment. */
+        if (metadata->section_sizes[section] & 3) {
+            return false;
+        }
+
+        /* Validate section does not overflow. */
+        if (check_32bit_additive_overflow(metadata->section_offsets[section], metadata->section_sizes[section])) {
+            return false;
+        }
+
+        /* Check for entrypoint presence. */
+        uint32_t section_end = metadata->section_offsets[section] + metadata->section_sizes[section];
+        if (metadata->section_offsets[section] <= metadata->entrypoint && metadata->entrypoint < section_end) {
+            entrypoint_found = true;
+        }
+
+        /* Ensure no overlap with later sections. */
+        for (unsigned int later_section = section + 1; later_section < PACKAGE2_SECTION_MAX; later_section++) {
+            uint32_t later_section_end = metadata->section_offsets[later_section] + metadata->section_sizes[later_section];
+            if (overlaps(metadata->section_offsets[section], section_end, metadata->section_offsets[later_section], later_section_end)) {
+                return false;
+            }
+        }
+
+        /* Validate section hashes. */
+        if (metadata->section_sizes[section]) {
+            void *section_data = (void *)((uint8_t *)NX_BOOTLOADER_PACKAGE2_LOAD_ADDRESS + sizeof(package2_header_t) + cur_section_offset);
+            uint8_t calculated_hash[0x20];
+            se_calculate_sha256(calculated_hash, section_data, metadata->section_sizes[section]);
+            if (memcmp(calculated_hash, metadata->section_hashes[section], sizeof(metadata->section_hashes[section])) != 0) {
+                return false;
+            }
+            cur_section_offset += metadata->section_sizes[section];
+        }
+
+    }
+
+    /* Ensure that entrypoint is present in one of our sections. */
+    if (!entrypoint_found) {
+        return false;
+    }
+
+    /* Perform version checks. */
+    /* We will be compatible with all package2s released before current, but not newer ones. */
+    if (metadata->version_max >= PACKAGE2_MINVER_THEORETICAL && metadata->version_min < PACKAGE2_MAXVER_500_CURRENT) {
+        return true;
+    }
+
+    return false;
+}
+
+uint32_t decrypt_and_validate_header(package2_header_t *header, bool is_plaintext) {
+    package2_meta_t metadata;
+
+    /* TODO: Also accept plaintext package2 based on bootconfig. */
+    
+    if (!is_plaintext) {
+        uint32_t mkey_rev;
+
+        /* Try to decrypt for all possible master keys. */
+        for (mkey_rev = 0; mkey_rev <= mkey_get_revision(); mkey_rev++) {
+            package2_crypt_ctr(mkey_rev, &metadata, sizeof(package2_meta_t), &header->metadata, sizeof(package2_meta_t), header->metadata.ctr, sizeof(header->metadata.ctr));
+            /* Copy the ctr (which stores information) into the decrypted metadata. */
+            memcpy(metadata.ctr, header->metadata.ctr, sizeof(header->metadata.ctr));
+            /* See if this is the correct key. */
+            if (validate_package2_metadata(&metadata)) {
+                header->metadata = metadata;
+                return mkey_rev;
+            }
+        }
+
+        /* Ensure we successfully decrypted the header. */
+        if (mkey_rev > mkey_get_revision()) {   
+            panic(0xFAF00003);
+        }
+    } else if (!validate_package2_metadata(&header->metadata)) {
+        panic(0xFAF0003);
+    }
+    return 0;
+}
 
 void package2_decrypt(void *package2_address) {
+    /* TODO: Actually decrypt, and copy sections into the relevant g_package2_sections[n] */
+    memcpy(g_patched_package2, NX_BOOTLOADER_PACKAGE2_LOAD_ADDRESS, PACKAGE2_SIZE_MAX);
+    
+    bool is_package2_plaintext = g_patched_package2_header->signature[0];
+    is_package2_plaintext &= memcmp(g_patched_package2_header->signature, g_patched_package2_header->signature + 1, sizeof(g_patched_package2_header->signature) - 1) == 0;
+    is_package2_plaintext &= g_patched_package2_header->metadata.magic == MAGIC_PK21;
+    
+    uint32_t pk21_mkey_revision = decrypt_and_validate_header(g_patched_package2_header, is_package2_plaintext);
+    
+    size_t cur_section_offset = 0;
+    /* Copy each section to its appropriate location, decrypting if necessary. */
+    for (unsigned int section = 0; section < PACKAGE2_SECTION_MAX; section++) {
+        if (g_patched_package2_header->metadata.section_sizes[section] == 0) {
+            continue;
+        }
+
+        void *src_start = g_patched_package2 + sizeof(package2_header_t) + cur_section_offset;
+        void *dst_start = g_package2_sections[section];
+        size_t size = (size_t)g_patched_package2_header->metadata.section_sizes[section];
+
+        if (is_package2_plaintext&& size != 0) {
+            memcpy(dst_start, src_start, size);
+        } else if (size != 0) {
+            package2_crypt_ctr(pk21_mkey_revision, dst_start, size, src_start, size, g_patched_package2_header->metadata.section_ctrs[section], 0x10);
+        }
+        cur_section_offset += size;
+    }
+    
+    /* Clear the signature, to signal that this is a plaintext, unsigned package2. */
+    memset(g_patched_package2_header->signature, 0, sizeof(g_patched_package2_header->signature));
+}
+
+void package2_add_thermosphere_section(void) {
+    if (g_patched_package2_header->metadata.section_sizes[PACKAGE2_SECTION_UNUSED] != 0) {
+        printk("Error: Package2 has no unused section for Thermosph\xe8re!\n");
+        generic_panic();
+    }
+    
+    /* TODO: Copy thermosphere to g_package2_sections[PACKAGE2_SECTION_UNUSED], update header size. */
+}
+
+void package2_patch_kernel(void) {
+    /* TODO: What kind of patching do we want to try to do here? */
+}
+
+void package2_patch_ini1(void) {
+    ini1_header_t *ini_header = (ini1_header_t *)g_package2_sections[PACKAGE2_SECTION_INI1];
+    if (ini_header->magic != MAGIC_INI1) {
+        printk("Error: INI1 section appears to not contain an INI1!\n");
+        generic_panic();
+    }
+    
     /* TODO */
 }
 
-void package2_add_thermosphere_section(void *package2_address) {
-    /* TODO */
-}
-
-void package2_patch_kernel(void *package2_address) {
-    /* TODO */
-}
-
-void package2_patch_ini1(void *package2_address) {
-    /* TODO */
-}
-
-void package2_fixup_header_and_section_hashes(void *package2_address) {
-    /* TODO */
+void package2_fixup_header_and_section_hashes(void) {
+    size_t cur_section_offset = 0;
+    
+    /* Copy each section to its appropriate location. */
+    for (unsigned int section = 0; section < PACKAGE2_SECTION_MAX; section++) {
+        if (g_patched_package2_header->metadata.section_sizes[section] == 0) {
+            continue;
+        }
+        
+        size_t size = (size_t)g_patched_package2_header->metadata.section_sizes[section];
+        if (sizeof(package2_header_t) + cur_section_offset + size > PACKAGE2_SIZE_MAX) {
+            printk("Error: Patched Package2 is too big!\n");
+            generic_panic();
+        }
+        
+        /* Copy the section into the new package2. */
+        memcpy(g_patched_package2 + sizeof(package2_header_t) + cur_section_offset, g_package2_sections[section], size);
+        /* Fix up the hash. */
+        se_calculate_sha256(g_patched_package2_header->metadata.section_hashes[section], g_package2_sections[section], size);
+        
+        cur_section_offset += size;
+    }
+    
+    /* Fix up the size in XOR'd CTR. */
+    uint32_t package_size = g_patched_package2_header->metadata.ctr_dwords[0] ^ g_patched_package2_header->metadata.ctr_dwords[2] ^ g_patched_package2_header->metadata.ctr_dwords[3];
+    uint32_t new_package_size = sizeof(package2_header_t) + cur_section_offset;
+    g_patched_package2_header->metadata.ctr_dwords[3] ^= (package_size ^ new_package_size);
 }
\ No newline at end of file
diff --git a/fusee/fusee-secondary/src/package2.h b/fusee/fusee-secondary/src/package2.h
index 11c0c367b..c8ab0cf48 100644
--- a/fusee/fusee-secondary/src/package2.h
+++ b/fusee/fusee-secondary/src/package2.h
@@ -5,8 +5,29 @@
 
 #define MAGIC_PK21 (0x31324B50)
 #define PACKAGE2_SIZE_MAX 0x7FC000
+
+#define PACKAGE2_SECTION_KERNEL 0x0
+#define PACKAGE2_SECTION_INI1   0x1
+#define PACKAGE2_SECTION_UNUSED 0x2
 #define PACKAGE2_SECTION_MAX 0x3
 
+#define PACKAGE2_MINVER_THEORETICAL 0x0
+#define PACKAGE2_MAXVER_100 0x2
+#define PACKAGE2_MAXVER_200 0x3
+#define PACKAGE2_MAXVER_300 0x4
+#define PACKAGE2_MAXVER_302 0x5
+#define PACKAGE2_MAXVER_400_410 0x6
+#define PACKAGE2_MAXVER_500_CURRENT 0x7
+
+#define PACKAGE2_MINVER_100 0x3
+#define PACKAGE2_MINVER_200 0x4
+#define PACKAGE2_MINVER_300 0x5
+#define PACKAGE2_MINVER_302 0x6
+#define PACKAGE2_MINVER_400_410 0x7
+#define PACKAGE2_MINVER_500_CURRENT 0x8
+
+#define NX_BOOTLOADER_PACKAGE2_LOAD_ADDRESS ((void *)(0xA9800000ull))
+
 typedef struct {
     union {
         uint8_t ctr[0x10];