This tool, written in Ruby, allows to guess the private key of the ECDSA when there is a flaw into the signature function. In particular, if the value k of this function is not random and it is used more then one time, the private key can be retrieved.

S = k^-1 (z + dA *R)

• S = Signature
• k = Random number
• z = Message hash
• dA = Private key
• R = Group

If you want go deeper, I suggest you to read this.

This tool uses ecdsa library and can be installed with:

gem install ecdsa

You need to edit 4 variables before running the script:

• msghash1_hex and msghash2_hex are the SHA2 hashes of the plaintext.
• sig1_base64 and sig2_base64 are the signatures of the messages encoded in base64.
• group is the elliptic curve group

Thanks to DavidEGrayson for his ruby_ecdsa repository where I found some code snippets and other useful things for this script.