david wong

Hey! I'm David, cofounder of zkSecurity and the author of the Real-World Cryptography book. I was previously a crypto architect at O(1) Labs (working on the Mina cryptocurrency), before that I was the security lead for Diem (formerly Libra) at Novi (Facebook), and a security consultant for the Cryptography Services of NCC Group. This is my blog about cryptography and security and other related topics that I find interesting.

Signature forgeries in Golang ECDSA library? posted March 2021

Take a look at the following program that you can run in Golang's playground.

// sign a message
hash, _ := hex.DecodeString("ffffffff00000000ffffffffffffffffbce6faada7179e84f3b9cac2fc632552")
r, s, err := ecdsa.Sign(rand.Reader, privateKey, hash[:])
if err != nil {
    panic(err)
}

// print the signature
signature := r.Bytes()
signature = append(signature, s.Bytes()...)
fmt.Println("signature:", hex.EncodeToString(signature))

// verify the signature
if !ecdsa.Verify(&privateKey.PublicKey, hash[:], r, s) {
    panic("wrong signature")
} else {
    fmt.Println("signature valid for", hex.EncodeToString(hash[:]))
}

// I modify the message, this should invalidate the signature
var hash2 [32]byte
hash2[31] = 1 
if !ecdsa.Verify(&privateKey.PublicKey, hash2[:], r, s) {
    panic("wrong signature")
} else {
    fmt.Println("signature valid for", hex.EncodeToString(hash2[:]))
}

this should print out:

signature: 4f3e60dc53ab470d23e82567909f01557f01d521a0b2ae96a111d107741d8ebb885332d790f0691bdc900661bf40c595a07750fa21946ed6b88c61c43fbfc1f3
signature valid for ffffffff00000000ffffffffffffffffbce6faada7179e84f3b9cac2fc632552
signature valid for 0000000000000000000000000000000000000000000000000000000000000001

Can you tell what's the problem? Is ECDSA broken? Is Golang's standard library broken? Is everything fine?

Well done! You've reached the end of my post. Now you can leave a comment or read something else.

Comments

AnomalRoil

Yeah, both ECDSA and DSA have fun edge cases on verification. Thankfully these require generally crafted hashes which are difficult to craft from valid messages :)

leave a comment...