Why Use uint8_t Instead of int in C (Especially in Crypto)

When writing C programs that deal with AES encryption, hashing, file I/O,network packets, or any low-level binary processing, you will frequentlysee the use of "uint8_t" instead of the plain "int" type.

Here we discuss why "uint8_t" is the correct and standard type for such code.

"int" IS NOT A FIXED SIZE

The C standard allows "int" to vary in size depending on the platform:

• On some systems: 16 bits

• On most modern systems: 32 bits

• On some architectures: 64 bits

This means:

int x;   // size is system-dependent

If you need EXACTLY 1 byte, "int" is the wrong choice.

AES AND CRYPTOGRAPHY REQUIRE EXACT BYTE SIZES

AES keys and IVs must be:

• AES-128 key: 16 bytes

• AES-192 key: 24 bytes

• AES-256 key: 32 bytes

• AES block size: 16 bytes

• AES IV size: 16 bytes

These values must be exact. Using "int" will not give you 1-byteelements. Using "uint8_t" will.

uint8_t key[32];   // exactly 32 bytes

BINARY FILES ARE BYTE-ORIENTED

Binary files store data byte-by-byte.

If you read a binary file into an array of int:

int buffer[100];

Each element is 4 bytes (on Windows, Linux, etc.). That means you arenot representing the file as raw bytes anymore. Your encryption outputwill be incorrect.

Binary data must use a byte-sized type. That type is "uint8_t".

uint8_t IS UNSIGNED AND SAFE FOR BYTE MATH

AES operations involve values from 0 to 255.

"int" can hold negative values. This introduces possible problems withsign-extension when performing operations like XOR, table lookups, orcasting.

Example:

uint8_t a = 0xFF;   // 255
int b = a;
int c = b ^ 0x10;   // may cause undesired sign behavior later

Crypto avoids these problems by using "uint8_t", which always holds unsigned 8-bit values.

uint8_t IS THE STANDARD TYPE FOR CRYPTO

All major cryptography libraries use uint8_t for byte buffers:

• OpenSSL

• mbedTLS

• libsodium

• WolfSSL

• TinyAES

• BoringSSL

• Linux kernel crypto API

This is because cryptography always operates on raw bytes.

CONCLUSION

If the data represents bytes, use uint8_t.If the data is binary, use uint8_t.If the data is an AES key, IV, block, buffer, or piece of ciphertext,use uint8_t.

"int" is for counters, flags, and arithmetic."uint8_t" is for real binary data.

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