|0x4||0x4||Offset to code|
|0x8||0x4||Size of plaintext version string, 0 on 0.931, 0x10 on other|
|0xC||0x4||Size of unknown block, only seen as 0|
|0x14||0x2||AES key revision, possible values 0 to 5|
|0x16||0x2||Public key revision, possible values 0 to 15|
|0x20||0x20||sha256 hash of decrypted body|
|0x40||0x10||Version in ASCII, not present on 0.931|
|0x50 (0x40 on 0.931)||0x90||Zero|
|0xE0 (0xD0 on 0.931)||Until Data||Encrypted Header|
At offset 0xE0 there is a 0x1E0 sized buffer that is speculated to be an encrypted header. For any given firmware version, secure_kernel.enc/second_loader.enc and secure_kernel.enp/second_loader.enp share the first 0xC0 bytes. For non-retail PUPs, each SLSK share the first 0xC0 bytes as observed in 0.931, 0.995, 1.000.41. Similarly in retail PUPs, each SLSK also share the first 0xC0 bytes as observed in 1.05 and 3.60. However, the bytes differ from retail and non-retail SLSK.
There is likely a 0xC0 sized "common" header that is shared by every firmware and by both secure_kernel and second_loader but different between retail and non-retail builds. Then there is likely a 0x20 byte section that is unique per SLSK (maybe contains version, size, load offset, etc). Then a 0x100 byte RSA-2048 signature of the header.
The last 0x340 bytes of each SLSK is not personalized. For both secure_kernel and second_loader, both the enc and enp variants share the last 0x340 bytes (although they differ from each other and across firmwares). This is likely the signature and might also contain certificates.
Reading through brom code, it appears last 0x340 bytes are not used in any way.
Bootrom enc loading process
First, personalization layer is removed. It uses AES-128-CBC with a derived key and decrypts data at ENC+0xE0 (or ENC+0xD0 if there's no plaintext version) for size of code_size+0x1E0.
There are two possible paths to derive the key used to remove personalization. Normally, the key is derived using keyslot 0x206. There's however an alternative path, triggered in secret debug mode, when instead the keyslot 0x207 is used with a different seed.
Once personalization is removed, the source keys are locked down. Keyslots 0x9, 0x206, 0x207 are locked down completely (leaving only 0xA0 protection). However, keyslot 0x8 allows encryption, this lets update manager SM add personalization layer during update without having to derive the keys itself.
Header RSA check
A key is derived from keyslot 0x344 and put into keyslot 0x20. This key is then immediately used to calculate HMAC-SHA256 over enc header, excluding the RSA sig (typically 0x00 to 0x1C0).
2 bytes are read from keyring slot 0x603 is read. This is the bitmask of allowed RSA public keys (0xFFFF on 1.692). If the mask is zero, a hardcoded RSA modulus is used. Otherwise, it checks enc rsa revision against the mask and if it's allowed, it gets the modulus from keyring RSA storage starting at keyslot 0x700.
The signature is typically located at 0x1C0 and is 0x100 bytes. After calculating powmod, it checks the padding and compares previously calculated HMAC-SHA256 against the contents.
Finally, it protects keyslots 0x700 to 0x77F to disable f00d reading out the modulus. However, only reading is protected, it should still be possible to encrypt/decrypt using these keyslots.
Metadata decryption and code verification
Using keyslot 0x208+aes_key_revision and metadata buffer (0xE0 offset for 0x20 bytes) the code decryption key is derived and put into keyslot 10. Then, 5 more keys are derived in the same way, using seed data [0x100; 0x19F]. These 5 keys are put into keyslots 11, 12, 13, 14, 15.
Keyslots 0x208, 0x209, 0x20A, 0x20B, 0x20C, 0x20D (all possible AES key revision keys) are protected.
Data at [0x1A0; 0x1C0) is decrypted using keyslot 10. This is HMAC-SHA256 of the code segment. HMAC-SHA256 is calculated over the code segment using keyslot 0x20, then keyslot 0x20 is protected. Finally, the calculated hmac is compared to the decrypted one.
Protecting the keys
Some keys are protected, depending on bit flags buffer located right after plaintext version string (so, at offset 0x50). However, on the latest 3.68 enc it is all zeroes so no keys should be protected by this function (?)
Code is decrypted using key 10 and a hard coded IV. Then, the key is protected.
The remainder (0x1C000 - code_sz) after the decrypted code is cleared with dmac. Dmac regs are also cleared.