RSA CLI Tool with Custom BigInt Library

A command-line RSA tool written in C, using a custom-built arbitrary-precision arithmetic (BigInt) library. The RSA implementation is based on the original RSA paper.

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Disclaimer

This implementation is strictly for educational use.
It is not cryptographically secure and should not be used in production systems.
Use of this code in real-world applications may lead to serious security vulnerabilities.

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Project Structure

.
├── src/              # RSA logic and CLI interface
├── include/          # Header files
├── lib/
│   └── bigint/       # Custom BigInt arithmetic
├── Makefile          # Build rules
├── .gitignore
└── README.md

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Features

• RSA key generation with configurable bit sizes
• CLI interface for generation & encryption/decryption
• Fully custom BigInt math

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Building

make

To clean build artifacts:

make clean

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Usage

Key Generation

./rsa-simple keygen --bits BITS --pub PUBFILE --priv PRIVFILE  //filenames are optional

Example:

./rsa-simple keygen 1024

Output Format

• Keys are output in PEM like format (Base64 with header)

Sample Output

Keys generated successfully and stored in pubkey.txt and privkey.txt

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Encryption

./rsa-simple encrypt --pub PUBKEY -i INFILE -o OUTFILE

Example:

./rsa-simple encrypt --pub pubkey.txt -i message.txt -o encrypted.txt

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Decryption

./rsa-simple decrypt --priv PRIVKEY -i INFILE -o OUTFILE

Example:

./rsa-simple decrypt --priv privkey.txt -i encrypted.txt -o decrypted.txt

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BigInt API

BigInt big_int_constructor(short sign, size_t size, ...);
int big_int_safe_realloc(BigInt *a, size_t new_size_words);
void big_int_destructor(BigInt * bignum);

BigInt big_int_from_uint64_t(uint64_t num);
BigInt big_int_from_uint32_t(uint32_t num);
BigInt big_int_from_byte_array_le(unsigned char * buff,size_t buff_len);
BigInt big_int_from_byte_array_be(unsigned char * buff,size_t buff_len);

void big_int_uadd(BigInt *a, BigInt *b,BigInt *c);
void big_int_add(BigInt *a, BigInt *b,BigInt *c);
void big_int_usub(BigInt *a, BigInt *b,BigInt *c);
void big_int_sub(BigInt *a, BigInt *b,BigInt *c);
void big_int_mult(BigInt *a, BigInt *b,BigInt *c);
int big_int_div(BigInt *u, BigInt *v,BigInt *q, BigInt *r);		//divmod
int big_int_mod(BigInt *a,BigInt *b, BigInt * c);
int big_int_modpow(BigInt * a,BigInt *b, BigInt * c, BigInt * d);

void big_int_gcd(BigInt *a, BigInt *b, BigInt *c);
void big_int_xgcd(BigInt *a, BigInt *b, BigInt *gcd, BigInt *x_out, BigInt *y_out);
void big_int_modinv(BigInt *a, BigInt *b, BigInt *c);

void big_int_inc(BigInt *a);
void big_int_bit_shift_r(BigInt *a,size_t s);
void big_int_bit_shift_l(BigInt *a,size_t s);
void big_int_word_shift_r(BigInt *a,size_t s);
void big_int_word_shift_l(BigInt *a,size_t s);

int big_int_is_zero(BigInt *a);
int big_int_compare(BigInt *a,BigInt *b, int modulus);

void big_int_print(BigInt * a,int mode);
void big_int_swap(BigInt * a, BigInt * b);
int big_int_count_leading_zeros(BigInt *a);
BigInt big_int_copy(BigInt a);

size_t big_int_bit_length(BigInt *a);
unsigned char * big_int_to_byte_array_be(BigInt * a, size_t * len);

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TODO

• Optimize BigInt prime generation
• Add test suite with edge cases

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References

• RSA implementation based on the original RSA paper:
  Rivest, Shamir, and Adleman (1978) - "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems"

• Arbitrary-precision arithmetic inspired by tiny-bignum-c:
  https://github.com/kokke/tiny-bignum-c

• Knuth's division (Algorithm D) based on Hacker's Delight:
  https://github.com/hcs0/Hackers-Delight

• Primality testing using the Miller–Rabin algorithm:
  https://en.wikipedia.org/wiki/Miller%E2%80%93Rabin_primality_test

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License

This project is licensed under the MIT License.

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Made with ❤️ and a lot of malloc() + free() debugging.