encryption.h

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/*
 ==============================================================================
 Name        : encryption.h
 Author      : polfosol
 Version     : 12
 Copyright   : copyright © 2022 - polfosol
 Description : μAES ™ is a minimalist all-in-one library for AES encryption
 Modified    : Renamed file and public API symbols to SPP_EXTERNAL_ENCRYPTION_*
 ==============================================================================
 */
 
#ifndef SPP_EXTERNAL_ENCRYPTION_H_
#define SPP_EXTERNAL_ENCRYPTION_H_
 
/**----------------------------------------------------------------------------
You can use different AES algorithms by changing this macro. Default is AES-128
 -----------------------------------------------------------------------------*/
#define AES___     128     /* or 256 (or 192; not standardized in some modes) */
 
/**----------------------------------------------------------------------------
AES block-cipher modes of operation. The following modes can be enabled/disabled
 by setting their corresponding macros to TRUE (1) or FALSE (0).
 -----------------------------------------------------------------------------*/
#define BLOCKCIPHERS 1
#define AEAD_MODES   1     /* authenticated encryption with associated data.  */
 
#if BLOCKCIPHERS
#define ECB          1     /* electronic code-book (NIST SP 800-38A)          */
#define CBC          1     /* cipher block chaining (NIST SP 800-38A)         */
#define CFB          1     /* cipher feedback (NIST SP 800-38A)               */
#define OFB          1     /* output feedback (NIST SP 800-38A)               */
#define CTR          1     /* counter-block (NIST SP 800-38A)                 */
#define XEX          1     /* xor-encrypt-xor (NIST SP 800-38E)               */
#define KWA          1     /* key wrap with authentication (NIST SP 800-38F)  */
#define FPE          1     /* format-preserving encryption (NIST SP 800-38G)  */
#endif
 
#if AEAD_MODES
#define CMAC         1     /* message authentication code (NIST SP 800-38B)   */
 
#if CTR
#define CCM          1     /* counter with CBC-MAC (RFC-3610/NIST SP 800-38C) */
#define GCM          1     /* Galois/counter mode with GMAC (NIST SP 800-38D) */
#define EAX          1     /* encrypt-authenticate-translate (ANSI C12.22)    */
#define SIV          1     /* synthetic initialization vector (RFC-5297)      */
#define GCM_SIV      1     /* nonce misuse-resistant AES-GCM (RFC-8452)       */
#endif
 
#if XEX
#define OCB          1     /* offset codebook mode with PMAC (RFC-7253)       */
#endif
 
#define POLY1305     1     /* poly1305-AES mac (https://cr.yp.to/mac.html)    */
#endif
 
#if CBC
#define CTS          1     /* ciphertext stealing (CS3: unconditional swap)   */
#endif
 
#if XEX
#define XTS          1     /* XEX tweaked-codebook with ciphertext stealing   */
#endif
 
#if CTR
#define CTR_NA       1     /* pure counter mode, with no authentication       */
#endif
 
#if EAX
#define EAXP         0     /* EAX-prime, as specified by IEEE Std 1703        */
#endif
 
#define WTF ! (BLOCKCIPHERS | AEAD_MODES)
#define MICRO_RJNDL  WTF   /* none of above; just rijndael API. dude.., why?  */
 
/**----------------------------------------------------------------------------
Parameters and MACROs; refer to the bottom of this document for more info:
 -----------------------------------------------------------------------------*/
 
#if ECB | (CBC && !CTS) | (XEX && !XTS)
#define AES_PADDING     0  /* standard values: (1) PKCS#7  (2) ISO/IEC7816-4  */
#endif
 
#if ECB | CBC | XEX | KWA | MICRO_RJNDL
#define DECRYPTION      1  /* rijndael decryption is NOT required otherwise.  */
#endif
 
#if FPE
#define CUSTOM_ALPHABET 0  /* if disabled, use default alphabet (digits 0..9) */
#define FF_X            1  /* algorithm type:  (1) for FF1, or (3) for FF3-1  */
#endif
 
enum constant_parameters_of_modes
{
#if FF_X == 3
    FF3_TWEAK_LEN    = 7,  /* fixed for FF3-1. (LEN=8 in old version of FF3)  */
#endif
 
#if CTR_NA
    CTR_START_VALUE  = 1,  /* Recommended by the RFC-3686                     */
    CTR_IV_LENGTH    = 12, /* use the last 32 bits of block for counting      */
#define PRESET_COUNTER  0  /* enable this macro if CTR_IV_LENGTH == 16        */
#endif
 
#if CCM
    CCM_NONCE_LEN    = 11, /* for 32-bit count (since one byte is reserved)   */
    CCM_TAG_LEN      = 16, /* must be EVEN. at most 16 bytes in all modes     */
#endif
#if GCM
    GCM_NONCE_LEN    = 12, /* Recommended; but other values are supported.    */
    GCM_TAG_LEN      = 16,
#endif
#if GCM_SIV
    SIVGCM_NONCE_LEN = 12, /* the only valid value recommended by RFC-8452    */
    SIVGCM_TAG_LEN   = 16, /* the only valid value recommended by RFC-8452    */
#endif
#if OCB
    OCB_NONCE_LEN    = 12, /* Recommended; must be positive and less than 16  */
    OCB_TAG_LEN      = 16,
#endif
#if EAX && !EAXP
    EAX_NONCE_LEN    = 16, /* no specified limit; can be arbitrarily large    */
    EAX_TAG_LEN      = 16, /*  more info at the bottom of this document.      */
#endif
 
#if AES___ == 256 || AES___ == 192
    AES_KEYLENGTH    = AES___ / 8
#else
    AES_KEYLENGTH    = 16  /* default value (or fallback for invalid AES___)  */
#endif
};
 
/**----------------------------------------------------------------------------
Since <stdint.h> is not a part of ANSI-C, we may need a 'trick' to use uint8_t
 -----------------------------------------------------------------------------*/
#include <string.h>
#include <limits.h>
#ifdef  LLONG_MAX          /* which means compiler conforms to C99 standard.  */
#include <stdint.h>
#elif   CHAR_BIT == 8
typedef unsigned char  uint8_t;
#if INT_MAX > 200000L
typedef int   int32_t;
#else
typedef long  int32_t;
#endif
#else
#error "YOUR SYSTEM/COMPILER NEITHER SUPPORTS <cstdint> NOR 8-BIT CHARACTERS!!"
#endif
 
#ifdef __SDCC              /* compiler is SDCC (small-device C compiler)      */
#define SDCC_REENT  __reentrant
#else
#define SDCC_REENT
#endif
 
#ifdef __cplusplus
extern "C" {
#endif
 
/**----------------------------------------------------------------------------
Encryption/decryption of a single block with Rijndael
 -----------------------------------------------------------------------------*/
#if MICRO_RJNDL
void SPP_EXTERNAL_ENCRYPTION_cipher( const uint8_t* key,          /* encryption/decryption key    */
                 const char mode,             /* encrypt: 'E', decrypt: 'D'   */
                 const uint8_t x[16],         /* input array (or input block) */
                 uint8_t y[16] );             /* output block                 */
#endif
 
/**----------------------------------------------------------------------------
Main functions for ECB-AES block ciphering
 -----------------------------------------------------------------------------*/
#if ECB
void SPP_EXTERNAL_ENCRYPTION_ecbEncrypt( const uint8_t* key,     /* encryption key               */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext result            */
 
char SPP_EXTERNAL_ENCRYPTION_ecbDecrypt( const uint8_t* key,     /* decryption key               */
                      const void* crtxt,      /* ciphertext input             */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* ECB */
 
/**----------------------------------------------------------------------------
Main functions for CBC-AES block ciphering
 -----------------------------------------------------------------------------*/
#if CBC
char SPP_EXTERNAL_ENCRYPTION_cbcEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t iVec[16], /* initialization vector        */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext result            */
 
char SPP_EXTERNAL_ENCRYPTION_cbcDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t iVec[16], /* initialization vector        */
                      const void* crtxt,      /* ciphertext input             */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* CBC */
 
/**----------------------------------------------------------------------------
Main functions for CFB-AES block ciphering
 -----------------------------------------------------------------------------*/
#if CFB
void SPP_EXTERNAL_ENCRYPTION_cfbEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t iVec[16], /* initialization vector        */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext result            */
 
void SPP_EXTERNAL_ENCRYPTION_cfbDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t iVec[16], /* initialization vector        */
                      const void* crtxt,      /* ciphertext input             */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* CFB */
 
/**----------------------------------------------------------------------------
Main functions for OFB-AES block ciphering
 -----------------------------------------------------------------------------*/
#if OFB
void SPP_EXTERNAL_ENCRYPTION_ofbEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t iVec[16], /* initialization vector        */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext result            */
 
void SPP_EXTERNAL_ENCRYPTION_ofbDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t iVec[16], /* initialization vector        */
                      const void* crtxt,      /* ciphertext input             */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* OFB */
 
/**----------------------------------------------------------------------------
Main functions for XTS-AES block ciphering
 -----------------------------------------------------------------------------*/
#if XTS
char SPP_EXTERNAL_ENCRYPTION_xtsEncrypt( const uint8_t* keys,    /* encryption key pair          */
                      const uint8_t* tweak,   /* tweak value (unit/sector ID) */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext result            */
 
char SPP_EXTERNAL_ENCRYPTION_xtsDecrypt( const uint8_t* keys,    /* decryption key pair          */
                      const uint8_t* tweak,   /* tweak value (unit/sector ID) */
                      const void* crtxt,      /* ciphertext input             */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* XTS */
 
/**----------------------------------------------------------------------------
Main functions for CTR-AES block ciphering
 -----------------------------------------------------------------------------*/
#if CTR_NA
void SPP_EXTERNAL_ENCRYPTION_ctrEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t* iv,      /* initialization vector/ nonce */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext result            */
 
void SPP_EXTERNAL_ENCRYPTION_ctrDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t* iv,      /* initialization vector/ nonce */
                      const void* crtxt,      /* ciphertext input             */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* CTR */
 
/**----------------------------------------------------------------------------
Main functions for SIV-AES block ciphering
 -----------------------------------------------------------------------------*/
#if SIV
void SPP_EXTERNAL_ENCRYPTION_sivEncrypt( const uint8_t* keys,    /* encryption key pair          */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      uint8_t iv[16],         /* synthesized initial-vector   */
                      void* crtxt );          /* ciphertext result            */
 
char SPP_EXTERNAL_ENCRYPTION_sivDecrypt( const uint8_t* keys,    /* decryption key pair          */
                      const uint8_t iv[16],   /* provided initial-vector      */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* crtxt,      /* ciphertext input             */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* SIV */
 
/**----------------------------------------------------------------------------
Main functions for GCM-AES block ciphering
 -----------------------------------------------------------------------------*/
#if GCM
void SPP_EXTERNAL_ENCRYPTION_gcmEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t* nonce,   /* nonce or IV (init. vector)   */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext + auth. tag       */
 
char SPP_EXTERNAL_ENCRYPTION_gcmDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t* nonce,   /* nonce or IV (init. vector)   */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* crtxt,      /* ciphertext + appended tag    */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* GCM */
 
/**----------------------------------------------------------------------------
Main functions for CCM-AES block ciphering
 -----------------------------------------------------------------------------*/
#if CCM
void SPP_EXTERNAL_ENCRYPTION_ccmEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t* nonce,   /* nonce or IV (init. vector)   */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext + auth. tag       */
 
char SPP_EXTERNAL_ENCRYPTION_ccmDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t* nonce,   /* nonce or IV (init. vector)   */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* crtxt,      /* ciphertext + appended tag    */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* CCM */
 
/**----------------------------------------------------------------------------
Main functions for OCB-AES block ciphering
 -----------------------------------------------------------------------------*/
#if OCB
void SPP_EXTERNAL_ENCRYPTION_ocbEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t* nonce,   /* nonce or IV (init. vector)   */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext + auth. tag       */
 
char SPP_EXTERNAL_ENCRYPTION_ocbDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t* nonce,   /* nonce or IV (init. vector)   */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* crtxt,      /* ciphertext + appended tag    */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* OCB */
 
/**----------------------------------------------------------------------------
Main functions for EAX-AES mode; more info at the bottom of this document.
 -----------------------------------------------------------------------------*/
#if EAX
void SPP_EXTERNAL_ENCRYPTION_eaxEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t* nonce,   /* arbitrary-size nonce array   */
#if EAXP
                      const size_t nonceLen,  /* length of provided nonce     */
#else
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
#endif
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext+ appended tag/mac */
 
char SPP_EXTERNAL_ENCRYPTION_eaxDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t* nonce,   /* arbitrary-size nonce array   */
#if EAXP
                      const size_t nonceLen,  /* length of provided nonce     */
#else
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
#endif
                      const void* crtxt,      /* ciphertext+ appended tag/mac */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* EAX */
 
/**----------------------------------------------------------------------------
Main functions for GCM-SIV-AES block ciphering
 -----------------------------------------------------------------------------*/
#if GCM_SIV
void SPP_EXTERNAL_ENCRYPTION_gcmSivEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t* nonce,   /* provided 96-bit nonce        */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* pntxt,      /* plaintext input              */
                      const size_t ptextLen,  /* length of plaintext          */
                      void* crtxt );          /* ciphertext + auth. tag       */
 
char SPP_EXTERNAL_ENCRYPTION_gcmSivDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t* nonce,   /* provided 96-bit nonce        */
                      const void* aData,      /* added authentication data    */
                      const size_t aDataLen,  /* length of AAD (auth. data)   */
                      const void* crtxt,      /* ciphertext + appended tag    */
                      const size_t crtxtLen,  /* length of ciphertext         */
                      void* pntxt );          /* plaintext result             */
#endif /* GCM-SIV */
 
/**----------------------------------------------------------------------------
Main functions for AES key-wrapping
 -----------------------------------------------------------------------------*/
#if KWA
char SPP_EXTERNAL_ENCRYPTION_keyWrap( const uint8_t* kek,        /* key encryption key           */
                   const void* secret,        /* the secret to be wrapped     */
                   const size_t secretLen,    /* length of plaintext secret   */
                   void* wrapped );           /* key-wrapped output           */
 
char SPP_EXTERNAL_ENCRYPTION_keyUnwrap( const uint8_t* kek,      /* key encryption key           */
                     const void* wrapped,     /* key-wrapped secret           */
                     const size_t wrapLen,    /* length of wrapped secret     */
                     void* secret );          /* the unwrapped key result     */
#endif /* KWA */
 
/**----------------------------------------------------------------------------
Main functions for FPE-AES; more info at the bottom of this page.
 -----------------------------------------------------------------------------*/
#if FPE
char SPP_EXTERNAL_ENCRYPTION_fpeEncrypt( const uint8_t* key,     /* encryption key               */
                      const uint8_t* tweak,   /* tweak bytes                  */
#if FF_X != 3
                      const size_t tweakLen,  /* length of tweak array        */
#endif
                      const void* pntxt,      /* input plaintext string       */
                      const size_t ptextLen,  /* length of plaintext string   */
                      void* crtxt );          /* ciphertext result            */
 
char SPP_EXTERNAL_ENCRYPTION_fpeDecrypt( const uint8_t* key,     /* decryption key               */
                      const uint8_t* tweak,   /* tweak bytes                  */
#if FF_X != 3
                      const size_t tweakLen,  /* length of tweak array        */
#endif
                      const void* crtxt,      /* input ciphertext string      */
                      const size_t crtxtLen,  /* length of ciphertext string  */
                      void* pntxt );          /* plaintext result             */
#endif /* FPE */
 
/**----------------------------------------------------------------------------
Main function for Poly1305-AES message authentication code
 -----------------------------------------------------------------------------*/
#if POLY1305
void SPP_EXTERNAL_ENCRYPTION_poly1305( const uint8_t* keys,       /* encryption/mixing key pair   */
                   const uint8_t nonce[16],   /* the 128-bit nonce            */
                   const void* data,          /* input data buffer            */
                   const size_t dataSize,     /* length of data in bytes      */
                   uint8_t mac[16] );         /* poly1305-AES mac of data     */
#endif
 
/**----------------------------------------------------------------------------
Main function for AES Cipher-based Message Authentication Code
 -----------------------------------------------------------------------------*/
#if CMAC
void SPP_EXTERNAL_ENCRYPTION_cmac( const uint8_t* key,            /* encryption or cipher key     */
               const void* data,              /* input data buffer            */
               const size_t dataSize,         /* length of data in bytes      */
               uint8_t mac[16] );             /* CMAC of input data           */
#endif
 
#ifdef __cplusplus
}
#endif
 
/**----------------------------------------------------------------------------
Define some error codes here
 -----------------------------------------------------------------------------*/
enum function_result_codes
{
    M_ENCRYPTION_ERROR     = 'E',
    M_DECRYPTION_ERROR     = 'D',
    M_AUTHENTICATION_ERROR = 'A',
    M_DATALENGTH_ERROR     = 'L',
    M_DATAFORMAT_ERROR     = 'F',
    M_MEMORY_ERROR         = 'M',
    M_SUCCESS              =  0
};
 
#endif /* header guard */
 
/*----------------------------------------------------------------------------*\
¦               Notes and remarks about the above-defined macros               ¦
 ------------------------------------------------------------------------------
 
* The main difference between the standard AES methods is in their key-expansion
    process. So for example, AES-128-GCM and AES-256-GCM are pretty much similar
    except for their key size and a minor change in the KeyExpansion function.
 
* In EBC/CBC/XEX modes, the size of input must be a multiple of block-size.
    Otherwise it needs to be padded. The simplest (default) way of padding is to
    fill the rest of block by zeros. Two standard methods for padding are PKCS#7
    and ISO/IEC 7816-4, which can be enabled by the AES_PADDING macro.
 
* The FPE mode has two distinct NIST-approved algorithms, namely FF1 and FF3-1.
    Use the FF_X macro to change the encryption method, which is FF1 by default.
    The early version of FF3 required 8-byte tweaks. But this turned out to have
    vulnerabilities and so it was reduced to 7 bytes in the FF3-1.
 
    The input and output strings of FPE functions must be consisted of a fixed
    set of characters called "the alphabet". Here, the default alphabet is the
    set of digits {'0'...'9'}. If you want to use a different alphabet, set the
    CUSTOM_ALPHABET macro and refer to the <micro_fpe.h> header. This header is
    required only when a custom alphabet has to be defined, and contains some
    illustrative examples and clear guidelines on how to do so.
 
* Many reference texts may use the terms "nonce" and "initialization vector"
    interchangeably, but technically they are not the same. Sometimes nonce is
    a part of the I.V, which itself can either be a full block or a partial one.
    In CBC, CFB and OFB modes, the provided IV must be a full block. In pure CTR
    (CTR_NA) mode, the IV can either be a full block, or a 96 bit one —which is
    also called nonce. In the latter case, counting begins at CTR_START_VALUE.
 
* In AEAD modes, the size of nonce and tag are considered as constant parameters
    and changing them might affect the results. The GCM and EAX modes support
    arbitrary sizes for nonce. In CCM, the nonce length may vary from 7 to 13
    bytes. Also the tag size is an EVEN number between 4..16. In OCB, the nonce
    size is 1..15 and the tag is 0..16 bytes. Note that the "calculated" tag-
    size is always 16 bytes which is then truncated to the desired values. So in
    encryption functions, the provided buffer for tag must be 16 bytes long.
 
* In most functions, as you may notice, first the entire input data is copied
    to the output and then the encryption process is carried out on its buffer.
    This is a very useful feature especially when the memory is limited, as you
    can perform "in-place encryption" on the input data and there is no need to
    allocate a separate buffer for the output. But please note that the `memcpy`
    function has undefined behavior if its source and destination are the same.
    So in such cases, you can simply delete the memcpy(...); line.
 
* For the EAX mode of operation, the IEEE-1703 standard defines EAX' which is a
    modified version that combines AAD and nonce. Also the tag size is fixed on
    4 bytes. So EAX-prime functions don't need to take additional authentication
    data and its size as separate parameters. It has been proven that EAX' has
    serious vulnerabilities and its usage is not recommended.
 
* In SIV mode, multiple separate units of authentication headers can be provided
    for the nonce synthesis. Here we assume that only one unit of AAD (aData) is
    sufficient, which is practically true.
 
* The key wrapping mode is also denoted by KW. In this mode, the input secret is
    divided into 64bit blocks. Number of blocks is at least 2, and it is assumed
    that no padding is required. For padding, the KWP mode must be used which is
    easily implementable but left as an exercise! In the NIST SP800-38F document
    you may find mentions of TKW which is based on 3DES and irrelevant here.
 
* Here is a technical tip for the keen minds who have managed to read this far:
    Excessive use of macro definitions in code is generally not a good practice,
    especially in large projects. As it may possibly cause some name-conflicts
    and errors for macro redefinition. So you can either delete the unnecessary
    macros and clean up the code, or undef macros at the end of the source file
    (i.e. write: #ifdef MACRO \ #undef MACRO \ #endif). Or replace their names
    with some unique ones that surely won't be used elsewhere. For example,
    rename the CTR macro above to MICRO_AES_CTR_MODE or something like that.
 
* Let me explain three extra options that are defined in the source file. If the
    length of the input cipher/plain text is 'always' less than 4KB, you can
    enable the SMALL_CIPHER macro to save a few bytes in the compiled code. This
    assumption is likely to be valid for some embedded systems and small-scale
    applications. Furthermore, enabling the AVOID_FUNCTION_CALL macro may have a
    positive effect on the speed while increasing the size of compiled code.
    Nonetheless, it is also possible to get different results sometimes.
 
    The INCREASE_SECURITY macro —as its name suggests, is dealing with security
    considerations. For example, since the RoundKey is declared as static array
    it might get exposed to some attacks. By enabling this macro, round-keys are
    wiped out at the end of ciphering operations. However, please keep in mind
    that this is NOT A GUARANTEE against side-channel attacks.
 
*/