The RSA public-key cryptosystem provides a digital signature scheme (sign + verify), based on the math of the modular exponentiations and discrete logarithms and the computational difficulty of the RSA problem (and its related integer factorization problem). The RSA sign / verify algorithm works as described below.

Key Generation

The RSA algorithm uses keys of size 1024, 2048, 4096, ..., 16384 bits. RSA supports also longer keys (e.g. 65536 bits), but the performance is too slow for practical use (some operations may take several minutes or even hours). For 128-bit security level, a 3072-bit key is required.

The RSA key-pair consists of:

• public key {n, e}

• private key {n, d}

The numbers n and d are typically big integers (e.g. 3072 bits), while e is small, typically 65537.

By definition, the RSA key-pairs has the following property:

​$(m^e)^d \equiv (m^d)^e \equiv m \pmod n$ for all m in the range [0...n)

RSA Sign

Signing a message msg with the private key exponent d:

1. Calculate the message hash: h = hash(msg)

2. Encrypt h to calculate the signature: $s = h^d \pmod n$​

The hash h should be in the range [0...n). The obtained signature s is an integer in the range [0...n).

RSA Verify Signature

Verifying a signature s for the message msg with the public key exponent e:

1. Calculate the message hash: h = hash(msg)

2. Decrypt the signature: $h' = s^e \pmod n$​

3. Compare h with h' to find whether the signature is valid or not

If the signature is correct, then the following will be true:

​$h' = s^e \pmod n = (h^d)^e \pmod n = h$​

The RSA sign / verify algorithm is pretty simple. Let's implement it with some code.