SLSK

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.

Signature
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.

Remove personalization
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 plaintext.

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.