Crypto Math

secp256k1, P-256 and P-384 public parameters (p, n, G) from SEC 2, with an on-curve check of a public point.

Generate or validate Luhn and ISO 7064 mod 97-10 (IBAN style) check digits from the string you supply.

Solve x = a1 (mod m1), x = a2 (mod m2), ... with the Chinese remainder theorem, coprime or not.

Convert public bytes between hex, base64 and base64url (RFC 4648) with strict decoding and expected-length check.

Shannon entropy in bits per symbol from a frequency table, plus min-entropy, collision entropy and efficiency.

gcd, lcm and extended-Euclid Bezout coefficients of two integers, plus gcd/lcm of up to 20 more.

Modular multiplicative inverse of a mod m, or the gcd that proves none exists.

Modular exponentiation base^exp mod m for big integers, decimal or 0x hex, up to 4096 bits.

Primality test (deterministic Miller-Rabin below 2^64) plus the next and previous prime, with BigInt.

RSA public key arithmetic: modulus bits, exponent validity, NIST strength; or a labelled toy RSA with tiny primes.