VeraCrypt
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+;; rdrand.asm - written and placed in public domain by Jeffrey Walton and Uri Blumenthal.
+;; Copyright assigned to the Crypto++ project.
+
+;; This ASM file provides RDSEED to downlevel Microsoft tool chains.
+;; Everything "just works" under Visual Studio. Other platforms will
+;; have to run MASM/MASM-64 and then link to the object files.
+
+;; set ASFLAGS=/nologo /D_M_X86 /W3 /Cx /Zi /safeseh
+;; set ASFLAGS64=/nologo /D_M_X64 /W3 /Cx /Zi
+;; "C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\bin\ml.exe" %ASFLAGS% /Fo rdrand-x86.obj /c rdrand.asm
+;; "C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\bin\amd64\ml64.exe" %ASFLAGS64% /Fo rdrand-x64.obj /c rdrand.asm
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+TITLE MASM_RDSEED_GenerateBlock source file
+SUBTITLE Microsoft specific ASM code to utilize RDSEED for down level Microsoft toolchains
+
+PUBLIC MASM_RDSEED_GenerateBlock
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; C/C++ Function prototypes (both are fastcall)
+;; X86:
+;; extern "C" void __fastcall MASM_RDSEED_GenerateBlock(byte* ptr, size_t size);
+;; X64:
+;; extern "C" void __fastcall MASM_RDSEED_GenerateBlock(byte* ptr, size_t size);
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+IFDEF _M_X86 ;; Set via the command line
+
+.486
+.MODEL FLAT
+
+;; Fastcall calling conventions exports
+ALIAS <@MASM_RDSEED_GenerateBlock@8> = <MASM_RDSEED_GenerateBlock>
+
+ENDIF
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+IFDEF _M_X86 ;; Set via the command line
+
+.CODE
+ALIGN 8
+OPTION PROLOGUE:NONE
+OPTION EPILOGUE:NONE
+
+;; No need for Load_Arguments due to fastcall
+;; ECX (in): arg1, byte* buffer
+;; EDX (in): arg2, size_t bsize
+
+MASM_RDSEED_GenerateBlock PROC ;; arg1:DWORD, arg2:DWORD
+
+ MWSIZE EQU 04h ;; machine word size
+ buffer EQU ecx
+ bsize EQU edx
+
+ ;; Top of While loop
+RDSEED_GenerateBlock_Top:
+
+ ;; Check remaining size
+ cmp bsize, 0
+ je RDSEED_GenerateBlock_Return
+
+RDSEED_Call_EAX:
+ ;; RDSEED is not available prior to VS2012. Just emit
+ ;; the byte codes using DB. This is `rdseed eax`.
+ DB 0Fh, 0C7h, 0F8h
+
+ ;; If CF=1, the number returned by RDSEED is valid.
+ ;; If CF=0, a random number was not available.
+
+ ;; Retry immediately
+ jnc RDSEED_Call_EAX
+
+RDSEED_succeeded:
+
+ cmp bsize, MWSIZE
+ jb RDSEED_Partial_Machine_Word
+
+RDSEED_Full_Machine_Word:
+
+ mov DWORD PTR [buffer], eax
+ add buffer, MWSIZE ;; No need for Intel Core 2 slow workarounds, like
+ sub bsize, MWSIZE ;; `lea buffer,[buffer+MWSIZE]` for faster adds
+
+ ;; Continue
+ jmp RDSEED_GenerateBlock_Top
+
+ ;; 1,2,3 bytes remain
+RDSEED_Partial_Machine_Word:
+
+ ;; Test bit 1 to see if size is at least 2
+ test bsize, 2
+ jz RDSEED_Bit_1_Not_Set
+
+ mov WORD PTR [buffer], ax
+ shr eax, 16
+ add buffer, 2
+
+RDSEED_Bit_1_Not_Set:
+
+ ;; Test bit 0 to see if size is at least 1
+ test bsize, 1
+ jz RDSEED_Bit_0_Not_Set
+
+ mov BYTE PTR [buffer], al
+ ;; shr ax, 8
+ ;; add buffer, 1
+
+RDSEED_Bit_0_Not_Set:
+
+ ;; We've hit all the bits
+
+RDSEED_GenerateBlock_Return:
+
+ ;; Clear artifacts
+ xor eax, eax
+ ret
+
+MASM_RDSEED_GenerateBlock ENDP
+
+ENDIF ;; _M_X86
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+IFDEF _M_X64 ;; Set via the command line
+
+.CODE
+ALIGN 16
+OPTION PROLOGUE:NONE
+OPTION EPILOGUE:NONE
+
+;; No need for Load_Arguments due to fastcall
+;; RCX (in): arg1, byte* buffer
+;; RDX (in): arg2, size_t bsize
+
+MASM_RDSEED_GenerateBlock PROC ;; arg1:QWORD, arg2:QWORD
+
+ MWSIZE EQU 08h ;; machine word size
+ buffer EQU rcx
+ bsize EQU rdx
+
+ ;; Top of While loop
+RDSEED_GenerateBlock_Top:
+
+ ;; Check remaining size
+ cmp bsize, 0
+ je RDSEED_GenerateBlock_Return
+
+RDSEED_Call_RAX:
+ ;; RDSEED is not available prior to VS2012. Just emit
+ ;; the byte codes using DB. This is `rdseed rax`.
+ DB 048h, 0Fh, 0C7h, 0F8h
+
+ ;; If CF=1, the number returned by RDSEED is valid.
+ ;; If CF=0, a random number was not available.
+
+ ;; Retry immediately
+ jnc RDSEED_Call_RAX
+
+RDSEED_succeeded:
+
+ cmp bsize, MWSIZE
+ jb RDSEED_Partial_Machine_Word
+
+RDSEED_Full_Machine_Word:
+
+ mov QWORD PTR [buffer], rax
+ add buffer, MWSIZE
+ sub bsize, MWSIZE
+
+ ;; Continue
+ jmp RDSEED_GenerateBlock_Top
+
+ ;; 1,2,3,4,5,6,7 bytes remain
+RDSEED_Partial_Machine_Word:
+
+ ;; Test bit 2 to see if size is at least 4
+ test bsize, 4
+ jz RDSEED_Bit_2_Not_Set
+
+ mov DWORD PTR [buffer], eax
+ shr rax, 32
+ add buffer, 4
+
+RDSEED_Bit_2_Not_Set:
+
+ ;; Test bit 1 to see if size is at least 2
+ test bsize, 2
+ jz RDSEED_Bit_1_Not_Set
+
+ mov WORD PTR [buffer], ax
+ shr eax, 16
+ add buffer, 2
+
+RDSEED_Bit_1_Not_Set:
+
+ ;; Test bit 0 to see if size is at least 1
+ test bsize, 1
+ jz RDSEED_Bit_0_Not_Set
+
+ mov BYTE PTR [buffer], al
+ ;; shr ax, 8
+ ;; add buffer, 1
+
+RDSEED_Bit_0_Not_Set:
+
+ ;; We've hit all the bits
+
+RDSEED_GenerateBlock_Return:
+
+ ;; Clear artifacts
+ xor rax, rax
+ ret
+
+MASM_RDSEED_GenerateBlock ENDP
+
+ENDIF ;; _M_X64
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+END