Difference between revisions of "Cryptogams AES"

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There are 6 steps to the process. The first step obtains the sources. The second step creates an ASM source file. The third step compiles and assembles the source file into an object file. The fourth steps determines the API. The fifth step creates a C header file. The final step integrates the object file into a program.
 
There are 6 steps to the process. The first step obtains the sources. The second step creates an ASM source file. The third step compiles and assembles the source file into an object file. The fourth steps determines the API. The fifth step creates a C header file. The final step integrates the object file into a program.
  
A few cautions before you begin. First, you are going to examine undocumented features of the OpenSSL library to learn how to work with the Cryptogam's sources. The Cryptogam sources are stable but things could change over time. Second, the ARMv4 implementation encrypts and decrypts full AES blocks. You are responsible for things like padding and side channel counter-measures.
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A few cautions before you begin. First, you are going to examine undocumented features of the OpenSSL library to learn how to work with the Cryptogam's sources. The Cryptogam sources are stable but things could change over time. Second, the ARMv4 implementation operates in ECB mode and encrypts or decrypts full AES blocks. You are responsible for things like padding and side channel counter-measures.
  
 
==Obtain Source Files==
 
==Obtain Source Files==

Revision as of 12:42, 7 July 2018

Cryptogams is Andy Polyakov's project used to develop high speed cryptographic primitives and share them with other developers. This wiki article will show you how to use Cryptogams ARMv4 AES implementation. The ARMv4 implementation runs around 20 to 25 cycles per byte (cpb). Typical C/C++ implementations run around 40 to 80 cpb and Andy's hand tuned ASM should outperform all of them.

Andy's Cryptogam implementations are provided by OpenSSL, but they are also available stand alone under a BSD license. The BSD style license is permissive and allows more developers to use Andy's high speed cryptography without an OpenSSL dependency.

There are 6 steps to the process. The first step obtains the sources. The second step creates an ASM source file. The third step compiles and assembles the source file into an object file. The fourth steps determines the API. The fifth step creates a C header file. The final step integrates the object file into a program.

A few cautions before you begin. First, you are going to examine undocumented features of the OpenSSL library to learn how to work with the Cryptogam's sources. The Cryptogam sources are stable but things could change over time. Second, the ARMv4 implementation operates in ECB mode and encrypts or decrypts full AES blocks. You are responsible for things like padding and side channel counter-measures.

Obtain Source Files

There are two source files you need for Cryptogams AES. The first is arm-xlate.pl and the second is aes-armv4.pl. They are available in the OpenSSL sources. The following commands fetch OpenSSL and then peels off the two Cryptogams files of interest.

# Clone OpenSSL for the latest Cryptogams sources
git clone https://github.com/openssl/openssl.git

mkdir cryptogams/

cp ./openssl/crypto/perlasm/arm-xlate.pl ./cryptogams/
cp ./openssl/crypto/aes/asm/aes-armv4.pl ./cryptogams/

cd cryptogams/
chmod +x *.pl

Create ASM File

The second step is to run aes-armv4.pl to produce an assembly language source file that can be consumed by GCC. aes-armv4.pl internally calls arm-xlate.pl. linux32 is the flavor used by the translate program. aes-armv4.S is the output filename. In the command below note the *.S file extension, which is a capitol S. Do not use a lowercase s because GCC must drive the compile and assemble step.

./aes-armv4.pl linux32 aes-armv4.S

GCC is needed to drive the process because there are C macros in the source file:

$ cat aes-armv4.S
@ Copyright 2007-2018 The OpenSSL Project Authors. All Rights Reserved.
...

#ifndef __KERNEL__
# include "arm_arch.h"
#else
# define __ARM_ARCH__ __LINUX_ARM_ARCH__
#endif
...

At this point there is an ASM file but it needs two small fixups. First, arm_arch.h is an OpenSSL source file so the dependency must be removed. Second, GCC defines __ARM_ARCH instead of __ARM_ARCH__ so a sed is needed.

To fixup the source files executes the following two commands:

# Remove OpenSSL include
sed -i 's/# include "arm_arch.h"//g' aes-armv4.S

# Fix GCC defines
sed -i 's/__ARM_ARCH__/__ARM_ARCH/g' aes-armv4.S

After the two fixups aes-armv4.S is ready to be compiled by GCC.

Compile Source File

The source file is ready to be compiled and assembled. At this point there are two choices. First, you can use ARMv5t or higher which includes Thumb instructions. The following compiles the source file with ARMv5t.

$ gcc -march=armv5t -c aes-armv4.S

The second choice uses ARMv4 and avoids Thumb instructions. If you want to avoid Thumb then add -marm to you compile command.

$ gcc -march=armv4 -marm -c aes-armv4.S

Using ARMv5t as an example we now have an object file with the following symbols. Symbols with a capitol T are public and exported. Symbols with a lower t are private and should not be used.

$ gcc -march=armv5t -c aes-armv4.S
$ nm aes-armv4.o
000011c0 T AES_decrypt
00000540 T AES_encrypt
00000b60 T AES_set_decrypt_key
00000b80 T AES_set_enc2dec_key
00000820 T AES_set_encrypt_key
00000cc0 t AES_Td
00000000 t AES_Te
000012c0 t _armv4_AES_decrypt
00000640 t _armv4_AES_encrypt
00000b80 t _armv4_AES_set_enc2dec_key
00000820 t _armv4_AES_set_encrypt_key

And you can inspect the generated code with objdump.

$ objdump --disassemble aes-armv4.o
aes-armv4.o:     file format elf32-littlearm
...

00000b60 <AES_set_decrypt_key>:
     b60:       e52de004        push    {lr}            ; (str lr, [sp, #-4]!)
     b64:       ebffff2d        bl      820 <AES_set_encrypt_key>
     b68:       e3300000        teq     r0, #0
     b6c:       e49de004        pop     {lr}            ; (ldr lr, [sp], #4)
     b70:       1afffff9        bne     b5c <AES_set_encrypt_key+0x33c>
     b74:       e1a00002        mov     r0, r2
     b78:       e1a01002        mov     r1, r2
     b7c:       eaffffff        b       b80 <AES_set_enc2dec_key>

...

And trace it back to the source code in aes-armv4.S.

.globl  AES_set_decrypt_key
.type   AES_set_decrypt_key,%function
.align  5
AES_set_decrypt_key:
        str     lr,[sp,#-4]!            @ push lr
        bl      _armv4_AES_set_encrypt_key
        teq     r0,#0
        ldr     lr,[sp],#4              @ pop lr
        bne     .Labrt

        mov     r0,r2                   @ AES_set_encrypt_key preserves r2,
        mov     r1,r2                   @ which is AES_KEY *key
        b       _armv4_AES_set_enc2dec_key
.size   AES_set_decrypt_key,.-AES_set_decrypt_key

Determine API

The next step is determine the API so you can call it from a C program. Unfortunately the API is not documented and you have to dig around the OpenSSL sources. The functions of interest are AES_set_encrypt_key, AES_set_decrypt_key, AES_encrypt and AES_decrypt.

A quick grep of OpenSSL sources reveals the following for AES_set_encrypt_key.

openssl$ grep -nIR AES_set_encrypt_key | grep '\.c'
...
crypto/aes/aes_core.c:632:int AES_set_encrypt_key(const unsigned char *userKey, const int bits,

Examining aes_core.c:632 reveals the following.

openssl$ cat -n crypto/aes/aes_core.c
...
   632  int AES_set_encrypt_key(const unsigned char *userKey, const int bits,
   633                          AES_KEY *key)
   634  {
            ...
   728      return 0;
   729  }

The next piece of information to discover is AES_KEY. Again a quick grep leads you to aes_key_st.

openssl$ grep -nIR AES_KEY | grep typedef
include/openssl/aes.h:39:typedef struct aes_key_st AES_KEY;

openssl$ cat -n include/openssl/aes.h
...

    31  struct aes_key_st {
    32  # ifdef AES_LONG
    33      unsigned long rd_key[4 * (AES_MAXNR + 1)];
    34  # else
    35      unsigned int rd_key[4 * (AES_MAXNR + 1)];
    36  # endif
    37      int rounds;
    38  };
    39  typedef struct aes_key_st AES_KEY;

Finally, we need AES_MAXNR from aes.h.

openssl$ grep -IR AES_MAXNR | grep define
include/openssl/aes.h:# define AES_MAXNR 14

Lather, rinse repeat for AES_set_decrypt_key, AES_encrypt and AES_decrypt. AES_encrypt can be found at crypto/aes/aes_core.c:787.

openssl$ grep -nIR AES_encrypt | grep '\.c'
...
crypto/aes/aes_core.c:787:void AES_encrypt(...)


openssl$ cat -n crypto/aes/aes_core.c
...
   783  /*
   784   * Encrypt a single block
   785   * in and out can overlap
   786   */
   787  void AES_encrypt(const unsigned char *in, unsigned char *out,
   788                   const AES_KEY *key) {

And AES_decrypt can be found at aes_core.c:978.

openssl$ grep -nIR AES_decrypt | grep '\.c'
...
crypto/aes/aes_core.c:978:void AES_decrypt(...)


openssl$ cat -n crypto/aes/aes_core.c

   974  /*
   975   * Decrypt a single block
   976   * in and out can overlap
   977   */
   978  void AES_decrypt(const unsigned char *in, unsigned char *out,
   979                   const AES_KEY *key)
   980  {

Create C Header

The fifth step creates a C header file based on information from Determine API. The header file is needed for two reasons. First, it removes the OpenSSL dependency from your project. Second, it avoids OpenSSL licensing violations.

Below is the C Header file you can use.

/* Header file for use with Cryptogam's ARMv4 AES.     */
/* Also see http://www.openssl.org/~appro/cryptogams/  */
/* https://wiki.openssl.org/index.php/Cryptogams_AES.  */

#ifndef CRYPTOGAMS_AES_ARMV4_H
#define CRYPTOGAMS_AES_ARMV4_H

#define AES_MAXNR 14

typedef struct AES_KEY_st {
    unsigned int rd_key[4 * (AES_MAXNR + 1)];
    int rounds;
} AES_KEY;

int AES_set_encrypt_key(const unsigned char *userKey, const int bits, AES_KEY *key);
int AES_set_decrypt_key(const unsigned char *userKey, const int bits, AES_KEY *key);
void AES_encrypt(const unsigned char *in, unsigned char *out, const AES_KEY *key);
void AES_decrypt(const unsigned char *in, unsigned char *out, const AES_KEY *key);

#endif  /* CRYPTOGAMS_AES_ARMV4_H */

Test Program

The final step is to test the integration of Cryptogam's AES with your program. The program below was executed on a BananaPi with a Cortex-A7 running at 950 MHz.

$ gcc -std=c99 aes-armv4-test.c ./aes-armv4.o -o aes-armv4-test.exe
$ ./aes-armv4-test.exe
Encrypted plaintext!
Decrypted ciphertext!

And the test program is shown below.

#include <stdio.h>
#include <string.h>
#include <assert.h>
#include "aes-armv4.h"

typedef unsigned char byte;

int main(int argc, char* argv[])
{
    /* Test key from FIPS 197 */
    const byte kb[] = {
        0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6,
        0xab, 0xf7, 0x15, 0x88, 0x09, 0xcf, 0x4f, 0x3c
    };

    const byte pb[] = {
        0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96,
        0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a
    };

    const byte cb[] = {
        0x3a, 0xd7, 0x7b, 0xb4, 0x0d, 0x7a, 0x36, 0x60, 
        0xa8, 0x9e, 0xca, 0xf3, 0x24, 0x66, 0xef, 0x97
    };

    /* Scratch */
    byte buf[16];
    int result;
    
    /********************************************/

    AES_KEY ekey;
    result = AES_set_encrypt_key(kb, sizeof(kb)*8, & ekey);
    assert(result == 0);

    AES_encrypt(pb, buf, &ekey);
    if (memcmp(cb, buf, 16) == 0)
        printf("Encrypted plaintext!\n");
    else
        printf("Failed to encrypt plaintext!\n");

    /********************************************/

    AES_KEY dkey;
    result = AES_set_decrypt_key(kb, sizeof(kb)*8, & dkey);
    assert(result == 0);

    AES_decrypt(cb, buf, &dkey);
    if (memcmp(pb, buf, 16) == 0)
        printf("Decrypted ciphertext!\n");
    else
        printf("Failed to decrypt ciphertext!\n");
    
    return 0;
}