DS1961S
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Compute Next Secret [33h]
Some applications may require a higher level of security than can be achieved by a single, directly written
secret. For additional security the DS1961S can compute a new secret based on the current secret, the
contents of a selected memory page, and a partial secret that consists of all data in the scratchpad. To
install a computed secret the master issues the compute next secret command, which activates the 512-bit
SHA-1 engine, provided that the secret is not write-protected. Table 1 shows how the various data
components involved enter the SHA engine and how a portion of the SHA result is loaded into the
secret's memory location. The SHA computation algorithm itself is explained later in this document. The
compute next secret command can be applied as often as desired to increase the level of security. The bus
master does not need to know the device’s current secret in order to successfully compute a new one and
then overwrite the existing secret.
Table 1. SHA-1 INPUT DATA FOR COMPUTE NEXT SECRET COMMAND
M0[31:24] = (SS + 0) M0[23:16] = (SS + 1) M0[15:8] = (SS + 2) M0[7:0] = (SS + 3)
M1[31:24] = (PP + 0) M1[23:16] = (PP + 1) M1[15:8] = (PP + 2) M1[7:0] = (PP + 3)
M2[31:24] = (PP + 4) M2[23:16] = (PP + 5) M2[15:8] = (PP + 6) M2[7:0] = (PP + 7)
M3[31:24] = (PP + 8) M3[23:16] = (PP + 9) M3[15:8] = (PP + 10) M3[7:0] = (PP + 11)
M4[31:24] = (PP + 12) M4[23:16] = (PP + 13) M4[15:8] = (PP + 14) M4[7:0] = (PP + 15)
M5[31:24] = (PP + 16) M5[23:16] = (PP + 17) M5[15:8] = (PP + 18) M5[7:0] = (PP + 19)
M6[31:24] = (PP + 20) M6[23:16] = (PP + 21) M6[15:8] = (PP + 22) M6[7:0] = (PP + 23)
M7[31:24] = (PP + 24) M7[23:16] = (PP + 25) M7[15:8] = (PP + 26) M7[7:0] = (PP + 27)
M8[31:24] = (PP + 28) M8[23:16] = (PP + 29) M8[15:8] = (PP + 30) M8[7:0] = (PP + 31)
M9[31:24] = FFh M9[23:16] = FFh M9[15:8] = FFh M9[7:0] = FFh
M10[31:24] = MPX M10[23:16] = (SP + 1) M10[15:8] = (SP + 2) M10[7:0] = (SP + 3)
M11[31:24] = (SP + 4) M11[23:16] = (SP + 5) M11[15:8] = (SP + 6) M11[7:0] = (SP + 7)
M12[31:24] = (SS + 4) M12[23:16] = (SS + 5) M12[15:8] = (SS + 6) M12[7:0] = (SS + 7)
M13[31:24] = FFh M13[23:16] = FFh M13[15:8] = FFh M13[7:0] = 80h
M14[31:24] = 00h M14[23:16] = 00h M14[15:8] = 00h M14[7:0] = 00h
M15[31:24] = 00h M15[23:16] = 00h M15[15:8] = 01h M15[7:0] = B8h
RESULT OF COMPUTE NEXT SECRET
(SS + 0) := E[7:0] (SS + 1) := E[15:8] (SS + 2) := E[23:16] (SS + 3) := E[31:24]
(SS + 4) := D[7:0] (SS + 5) := D[15:8] (SS + 6) := D[23:16] (SS + 7) := D[31:24]
Legend
Mt Input Buffer of SHA Engine
0 £ t £ 15; 32-Bit Words
(SS + N) Byte N of Secret; Secret Begins at Address 0080h
(See Memory Map)
(PP + N) Byte N of Memory Page; Memory Pages Begin at
0000h, 0020h, 0040h and 0060h (See Memory Map)
(SP + N) Byte N of Scratchpad
MPX
MPX[7] = 0; MPX[6] = 0; MPX[5:0] = (SP + 0)[5:0]
D, E
32-Bit Words, Portions of the 160-Bit SHA Result
After issuing the compute next secret command the master must provide a 2-byte target address to select
the memory page that contributes 256 bits of the SHA input data. After receiving the target addresses
