The TEA is a very simple encryption algorithm requiring little time and space - perfect for embedded systems.  There are extensions to it, and every version has its flaws (WEP was based on it), but for casual protection it's perfect.Also, the encrypt and decrypt routines are exactly as found from Wikipedia, so I don't expect anyone to check that I'm following the TEA correctly - I'm more interested in the block routines I've added, though if you notice something about the TEA routines it would be good to know.tea.h#ifndef __TEA.H__#define __TEA.H__#include <stdint.h>void encrypt (uint32_t* v, uint32_t* k);void decrypt (uint32_t* v, uint32_t* k);void encryptBlock(uint8_t * data, uint32_t * len, uint32_t * key);void decryptBlock(uint8_t * data, uint32_t * len, uint32_t * key);#endiftea.c#include tea.h/* encryptBlock *   Encrypts byte array data of length len with key key using TEA * Arguments: *   data - pointer to 8 bit data array to be encrypted - SEE NOTES *   len - length of array *   key - Pointer to four integer array (16 bytes) holding TEA key * Returns: *   data - encrypted data held here *   len - size of the new data array * Side effects: *   Modifies data and len * NOTES: * data size must be equal to or larger than ((len + 7) / 8) * 8 + 8 * TEA encrypts in 8 byte blocks, so it must include enough space to  * hold the entire data to pad out to an 8 byte boundary, plus another * 8 bytes at the end to give the length to the decrypt algorithm. * *  - Shortcut - make sure that data is at least len + 15 bytes in size. */void encryptBlock(uint8_t * data, uint32_t * len, uint32_t * key){   uint32_t blocks, i;   uint32_t * data32;   // treat the data as 32 bit unsigned integers   data32 = (uint32_t *) data;   // Find the number of 8 byte blocks, add one for the length   blocks = (((*len) + 7) / 8) + 1;   // Set the last block to the original data length   data32[(blocks*2) - 1] = *len;   // Set the encrypted data length   *len = blocks * 8;   for(i = 0; i< blocks; i++)   {      encrypt(&data32[i*2], key);   }}/* decryptBlock *   Decrypts byte array data of length len with key key using TEA * Arguments: *   data - pointer to 8 bit data array to be decrypted - SEE NOTES *   len - length of array *   key - Pointer to four integer array (16 bytes) holding TEA key * Returns: *   data - decrypted data held here *   len - size of the new data array * Side effects: *   Modifies data and len * NOTES: *   None */void decryptBlock(uint8_t * data, uint32_t * len, uint32_t * key){   uint32_t blocks, i;   uint32_t * data32;   // treat the data as 32 bit unsigned integers   data32 = (uint32_t *) data;   // Find the number of 8 byte blocks   blocks = (*len)/8;   for(i = 0; i< blocks; i++)   {      decrypt(&data32[i*2], key);   }   // Return the length of the original data   *len = data32[(blocks*2) - 1];}/* encrypt *   Encrypt 64 bits with a 128 bit key using TEA *   From http://en.wikipedia.org/wiki/Tiny_Encryption_Algorithm * Arguments: *   v - array of two 32 bit uints to be encoded in place *   k - array of four 32 bit uints to act as key * Returns: *   v - encrypted result * Side effects: *   None */void encrypt (uint32_t* v, uint32_t* k) {    uint32_t v0=v[0], v1=v[1], sum=0, i;           /* set up */    uint32_t delta=0x9e3779b9;                     /* a key schedule constant */    uint32_t k0=k[0], k1=k[1], k2=k[2], k3=k[3];   /* cache key */    for (i=0; i < 32; i++) {                       /* basic cycle start */        sum += delta;        v0 += ((v1<<4) + k0) ^ (v1 + sum) ^ ((v1>>5) + k1);        v1 += ((v0<<4) + k2) ^ (v0 + sum) ^ ((v0>>5) + k3);      }                                              /* end cycle */    v[0]=v0; v[1]=v1;}/* decrypt *   Decrypt 64 bits with a 128 bit key using TEA *   From http://en.wikipedia.org/wiki/Tiny_Encryption_Algorithm * Arguments: *   v - array of two 32 bit uints to be decoded in place *   k - array of four 32 bit uints to act as key * Returns: *   v - decrypted result * Side effects: *   None */void decrypt (uint32_t* v, uint32_t* k) {    uint32_t v0=v[0], v1=v[1], sum=0xC6EF3720, i;  /* set up */    uint32_t delta=0x9e3779b9;                     /* a key schedule constant */    uint32_t k0=k[0], k1=k[1], k2=k[2], k3=k[3];   /* cache key */    for (i=0; i<32; i++) {                         /* basic cycle start */        v1 -= ((v0<<4) + k2) ^ (v0 + sum) ^ ((v0>>5) + k3);        v0 -= ((v1<<4) + k0) ^ (v1 + sum) ^ ((v1>>5) + k1);        sum -= delta;                                       }                                              /* end cycle */    v[0]=v0; v[1]=v1;}Issues so far:CodingUsing magic numbers in encrypt and decrypt routines - use #defines instead - KyleIf the 64 bit encoding functions aren't used outside this module, their prototypes should be in the code, not header - SimonAdd sanity checking to input - RobRequire that input len is a multiple of 8 bytes - making a requirement we can't enforce or check is a recipe for corruption - RobStyleUsing K&R brace style in some places, ANSI in others - consistency - SimonThe long descriptive comment is more useful for the end user if it's in the header file (Header for usage, code for implementation) - RobEDIT:Someone asked for the implementation I indicated in the original post, so here it is.  Note that I've not cleaned it up for presentation.To decode a file use decode inputfilename outputfilename.  To encode a file use decode inputfilename outputfilename e.  ANSI C implementation, so it should work anywhere, although the algorithm itself might be endian dependant.THE DECRYPTED FILE WILL NOT EXACTLY MATCH THE ORIGINAL FILE. This particular implementation leaves a number of null bytes on the end of the decrypted file that were not present in the original.  For my application this was acceptable and simplified my particular usage, but you are likely going to need to modify this for your use.It uses a fixed key on line 4.#include <stdio.h>typedef unsigned long uint32_t;  const uint32_t TEAKey[4] = {0x95a8882c, 0x9d2cc113, 0x815aa0cd, 0xa1c489f7};void encrypt (uint32_t* v, const uint32_t* k);void decrypt (uint32_t* v, const uint32_t* k);void btea(uint32_t *v, int n, uint32_t const k[4]);void simpleencrypt(unsigned char * buffer);void simpledecrypt(unsigned char * buffer);int main(int argc, char **argv){   FILE *fpin, *fpout;   int bytecount;   unsigned char buffer[9], bufferin[9], bufferout[9];   int i;   if(argc < 3)   {      printf(Use: %s [filenameinput] [filenameoutput]\n, argv[0]);      return 0;   }   if( (fpin = fopen(argv[1], rb)) == NULL)   {      printf(Problem opening input file %s.\n, argv[1]);      return 0;   }   if( (fpout = fopen(argv[2], wb)) == NULL)   {      printf(Problem opening output file %s.\n, argv[2]);      return 0;   }   bytecount = 0;   while(fread(buffer, 1, 8, fpin) == 8)   {      if(argc>3)      {            for(i=0;i<8;i++)            {                bufferin[i] = buffer[i];            }          simpleencrypt(buffer);            for(i=0;i<8;i++)            {                bufferout[i] = buffer[i];            }            simpledecrypt(bufferout);            for(i=0;i<8;i++)            {                if(bufferin[i] != bufferout[i])                {                    printf(Internal decode test failed.\n);                }            }      }      else      {          simpledecrypt(buffer);      }      fwrite(buffer, 1, 8, fpout);      bytecount+=8;   }   if (!feof(fpin))   {       printf(Unexpected input file error encountered.\n);   }   fclose(fpin);    fclose(fpout);    printf(%s complete, %i bytes total\n,((argc>3) ? Encrypt : Decrypt), bytecount);   return 0;}void simpleencrypt(unsigned char * buffer){    uint32_t datablock[2];    datablock[0] = (buffer[0] << 24) | (buffer[1] << 16)  | (buffer[2] << 8) | (buffer[3]);    datablock[1] = (buffer[4] << 24) | (buffer[5] << 16)  | (buffer[6] << 8) | (buffer[7]);    encrypt (datablock, TEAKey);    buffer[0] = (char) ((datablock[0] >> 24) & 0xFF);    buffer[1] = (char) ((datablock[0] >> 16) & 0xFF);    buffer[2] = (char) ((datablock[0] >> 8) & 0xFF);    buffer[3] = (char) ((datablock[0]) & 0xFF);    buffer[4] = (char) ((datablock[1] >> 24) & 0xFF);    buffer[5] = (char) ((datablock[1] >> 16) & 0xFF);    buffer[6] = (char) ((datablock[1] >> 8) & 0xFF);    buffer[7] = (char) ((datablock[1]) & 0xFF);}void simpledecrypt(unsigned char * buffer){    uint32_t datablock[2];    datablock[0] = (buffer[0] << 24) | (buffer[1] << 16)  | (buffer[2] << 8) | (buffer[3]);    datablock[1] = (buffer[4] << 24) | (buffer[5] << 16)  | (buffer[6] << 8) | (buffer[7]);    decrypt (datablock, TEAKey);    buffer[0] = (char) ((datablock[0] >> 24) & 0xFF);    buffer[1] = (char) ((datablock[0] >> 16) & 0xFF);    buffer[2] = (char) ((datablock[0] >> 8) & 0xFF);    buffer[3] = (char) ((datablock[0]) & 0xFF);    buffer[4] = (char) ((datablock[1] >> 24) & 0xFF);    buffer[5] = (char) ((datablock[1] >> 16) & 0xFF);    buffer[6] = (char) ((datablock[1] >> 8) & 0xFF);    buffer[7] = (char) ((datablock[1]) & 0xFF);}/* encrypt *   Encrypt 64 bits with a 128 bit key using TEA *   From http://en.wikipedia.org/wiki/Tiny_Encryption_Algorithm * Arguments: *   v - array of two 32 bit uints to be encoded in place *   k - array of four 32 bit uints to act as key * Returns: *   v - encrypted result * Side effects: *   None */void encrypt (uint32_t* v, const uint32_t* k) {    uint32_t v0=v[0], v1=v[1], sum=0, i;           /* set up */    uint32_t delta=0x9e3779b9;                     /* a key schedule constant */    uint32_t k0=k[0], k1=k[1], k2=k[2], k3=k[3];   /* cache key */    for (i=0; i < 32; i++) {                       /* basic cycle start */        sum += delta;        v0 += ((v1<<4) + k0) ^ (v1 + sum) ^ ((v1>>5) + k1);        v1 += ((v0<<4) + k2) ^ (v0 + sum) ^ ((v0>>5) + k3);      }                                              /* end cycle */    v[0]=v0; v[1]=v1;}/* decrypt *   Decrypt 64 bits with a 128 bit key using TEA *   From http://en.wikipedia.org/wiki/Tiny_Encryption_Algorithm * Arguments: *   v - array of two 32 bit uints to be decoded in place *   k - array of four 32 bit uints to act as key * Returns: *   v - decrypted result * Side effects: *   None */void decrypt (uint32_t* v, const uint32_t* k) {    uint32_t v0=v[0], v1=v[1], sum=0xC6EF3720, i;  /* set up */    uint32_t delta=0x9e3779b9;                     /* a key schedule constant */    uint32_t k0=k[0], k1=k[1], k2=k[2], k3=k[3];   /* cache key */    for (i=0; i<32; i++) {                         /* basic cycle start */        v1 -= ((v0<<4) + k2) ^ (v0 + sum) ^ ((v0>>5) + k3);        v0 -= ((v1<<4) + k0) ^ (v1 + sum) ^ ((v1>>5) + k1);        sum -= delta;                                       }                                              /* end cycle */    v[0]=v0; v[1]=v1;}#define DELTA 0x9e3779b9  #define MX ((z>>5^y<<2) + (y>>3^z<<4)) ^ ((sum^y) + (k[(p&3)^e] ^ z));  void btea(uint32_t *v, int n, uint32_t const k[4]) {    uint32_t y, z, sum;    unsigned p, rounds, e;    if (n > 1) {          /* Coding Part */      rounds = 6 + 52/n;      sum = 0;      z = v[n-1];      do {        sum += DELTA;        e = (sum >> 2) & 3;        for (p=0; p<n-1; p++)          y = v[p+1], z = v[p] += MX;        y = v[0];        z = v[n-1] += MX;      } while (--rounds);    } else if (n < -1) {  /* Decoding Part */      n = -n;      rounds = 6 + 52/n;      sum = rounds*DELTA;      y = v[0];      do {        e = (sum >> 2) & 3;        for (p=n-1; p>0; p--)          z = v[p-1], y = v[p] -= MX;        z = v[n-1];        y = v[0] -= MX;      } while ((sum -= DELTA) != 0);    }  }
Tiny Encryption Algorithm (TEA) for arbitrary sized data
c;algorithm;cryptography
You should add some sanity checking on the incoming arguments, especially if you intend this to be library code used from multiple projects.I would recommend you change definition of encryptBlock to require that len is a multiple of 8 bytes.  Requiring that the incoming array is sized to a multiple of 8 bytes, but not enforcing that for len is a sure fire way to get memory corruption when someone forgets (e.g. when they are encrypting strings).Personally I prefer to have the long descriptive comment in the header file rather than the .c file.  Anyone using tea.h will find it more useful there.