# Is it safe to use same private key in two or more EC signature algorithms?

Given two (or more) algorithms:

• ed25519
• private key is random blob of 32 bytes
• public key is encoded point on Edwards 25519 curve.
• ECDSA with secp256k1
• private key is random blob of 32 bytes
• public key is encoded point on secp256k1 curve.

Lets say, we have a message M and we sign it with both algorithms, with the same private key (public keys are calculated deterministically based on private key). Adversary is able to see both signatures, collect history of such signatures for different messages, messages are unencrypted.

Are there any security implications for this scheme?

• Note that you could also store 128 bits of data or more and derive the two keys from that using a KDF with two labels. – Maarten Bodewes Jan 13 '18 at 21:55
• Why do you want to do that? Reusing a key-pair on the same curve can be useful. But on different curves, I don't see any advantage at all over using different private keys (since the public key is already different). – CodesInChaos Jan 14 '18 at 15:35
• @CodesInChaos I am building 2-way-peg and considering usage of the same private key for different blockchains. I think I will stick to Maarten Bodewes's answer. Thanks for your help! – warchantua Jan 22 '18 at 14:56