Overview #

The RSA module is essentially a hardware-accelerated modular exponentiation engine. It is specially optimized for RSA applications, so its behavior can be incoherent when RSA’s invariants are broken.

Observed edge cases #

if 2 divides mod, output == 0

Registers #

Old3DS	Name	Address	Width
Yes	RSA_CNT	0x1000B000	0x04
Yes	RSA_?	0x1000B0F0	0x04
Yes	RSA_SLOT0	0x1000B100	0x10
Yes	RSA_SLOT1	0x1000B110	0x10
Yes	RSA_SLOT2	0x1000B120	0x10
Yes	RSA_SLOT3	0x1000B130	0x10
Yes	RSA_EXPFIFO	0x1000B200	0x100 (can handle u32 writes to any aligned position in the FIFO)
Yes	RSA_MOD	0x1000B400	0x100
Yes	RSA_TXT	0x1000B800	0x100

RSA_CNT #

Bit	Description
0	Start (1=Enable/Busy, 0=Idle)
1	Interrupt enable (1=enable, 0=disable)
4-7	Keyslot (Bit6-7 don’t actually affect the keyslot)
8	Endianness (1=Little endian, 0=Big endian). Affects RSA_EXPFIFO, RSA_MOD, and RSA_TXT.
9	Word order (1=Normal order, 0=Reversed order). Affects RSA_MOD and RSA_TXT.

RSA_SLOT #

Name	Offset	Width
RSA_SLOTCNT	0x0	0x4
RSA_SLOTSIZE	0x4	0x4
?	0x8	0x4
?	0xC	0x4

RSA_SLOTCNT #

Bits	Description
0	Key status (1=Key has been set, 0=Key has not been set yet)
1	Key write-protect, this bit is RW. (0 = no protection, 1 = protected)
2	Key read-protect, this bit is RW. (0 = no protection, 1 = protected)
30-3	?
31	Key slot protect. Makes all other bits in this reg read-only until reset

Before writing RSA_EXPFIFO/RSA_MOD, bit0 here should be cleared when bit31 is already clear. Otherwise, the ARM9 will hang when attempting to write to RSA_EXPFIFO.

RSA_SLOTSIZE #

This contains the RSA size for this slot, in words. Normally this is 0x40 for RSA-2048.

RSA_EXPFIFO #

The 0x100-byte private or public exponent is written to this write-only FIFO.

RSA_MOD #

The RSA key modulus for the selected keyslot can be written here. When writing the RSA modulus, the modulus must align with the end of the register area.

Writing to RSA_MOD does not change the exponent written with RSA_EXPFIFO. An attack based on the 🔗 Pohlig-Hellman algorithm exists to “read” the contents of RSA_EXPFIFO as a result (see 3DS System Flaws).

RSA_TXT #

The RSA signature can be written here, and the data read from here is the message. When writing the RSA signature, the signature must be prepended with zeroes until it is a multiple of 8 bytes, and the end of the signature must align with the end of the register area.

The PKCS message padding must be manually checked by software, as hardware will only do raw RSA operations.

Keyslots usage #

Keyslot	Description
0	Arbitrary
1	CXI access desc (following the exheader)
2-3	Initialized by the ARM9 bootrom, but not used by any of the FIRMs. It’s unknown what the ARM9 bootrom uses these for, if anything.

RSA Registers